Webasto Thermo 90 S-ADR Instruction manual

Water heaters
09/2005
Workshop manual
Thermo 90 S
Thermo 90 S-ADR
(Transport of hazardous goods)
Thermo 90 ST
Thermo 90 ST-ADR
(Transport of hazardous goods)

Thermo 90 S / Thermo 90 ST Table of contents
I
Table of contents
1 Introduction
1.1 Contents and purpose ........................................................................................................................... 101
1.2 Meaning of signal words........................................................................................................................ 101
1.3 Additional documentation to be used..................................................................................................... 101
1.4 Safety instructions and regulations........................................................................................................ 101
1.4.1 General safety regulations........................................................................................................... 101
1.5 Statutory regulations governing installation............................................................................................102
2 General description
2.1 Combustion air fan................................................................................................................................. 202
2.2 Heat exchanger ..................................................................................................................................... 202
2.3 Temperature sensor .............................................................................................................................. 203
2.4 Temperature limiter................................................................................................................................ 203
2.5 Burner insert.......................................................................................................................................... 203
2.6 Glow plug............................................................................................................................................... 203
2.7 Flame monitor........................................................................................................................................ 203
2.8 Combustion pipe.................................................................................................................................... 203
2.9 Circulating pump.................................................................................................................................... 203
2.10 Control unit ............................................................................................................................................ 203
2.11 Metering pump....................................................................................................................................... 203
3 Function description
3.1 Switching on .......................................................................................................................................... 301
3.2 Heating mode ........................................................................................................................................ 301
3.3 Control mode......................................................................................................................................... 301
3.4 Switching off .......................................................................................................................................... 302
3.5 Heater functions when installed in ADR vehicles .................................................................................. 302
3.6 Malfunctions........................................................................................................................................... 303
3.6.1 Fault lock-out............................................................................................................................... 303
3.6.2 Diagnosis after fault lock-out, Thermo 90 S and Thermo 90 ST ................................................. 304
4 Technical data................................................................................................................................................ 401
5 Troubleshooting
5.1 General.................................................................................................................................................. 501
5.2 General fault indications........................................................................................................................ 501
5.3 Fault indications on fault lock-out .......................................................................................................... 502
5.4 Visual check for assessing the burner................................................................................................... 503
5.4.1 Burner housing.............................................................................................................................503
5.4.2 Back wall with metal evaporator.................................................................................................. 504
5.4.3 Combustion chamber .................................................................................................................. 504
5.4.4 Complete burner.......................................................................................................................... 504

Table of contents Thermo 90 S / Thermo 90 ST
II
6 Function tests
6.1 General................................................................................................................................................. 601
6.2 Settings................................................................................................................................................. 601
6.2.1 Setting the CO2content.............................................................................................................. 601
6.3 Testing individual components.............................................................................................................. 601
6.3.1 Resistance test of the temperature sensor................................................................................. 601
6.3.2 Resistance test of the glow plug................................................................................................. 601
6.3.3 Resistance test of the flame monitor .......................................................................................... 601
6.3.4 Testing the combustion air fan.................................................................................................... 601
7 Circuit diagrams
7.1 General................................................................................................................................................. 701
8 Service work
8.1 General................................................................................................................................................. 801
8.2 Work on the heater ............................................................................................................................... 801
8.3 Work on the vehicle .............................................................................................................................. 801
8.4 Heater trial run...................................................................................................................................... 802
8.5 Service work ......................................................................................................................................... 802
8.6 Visual inspections and installation instructions..................................................................................... 804
8.6.1 Connection to the vehicle cooling system................................................................................... 804
8.6.2 Connection to the vehicle fuel system........................................................................................ 804
8.6.3 Metering pump with damper....................................................................................................... 807
8.6.4 Fuel filter..................................................................................................................................... 807
8.6.5 Combustion air supply................................................................................................................ 807
8.6.6 Exhaust line................................................................................................................................ 808
8.7 Removal and installation....................................................................................................................... 808
8.7.1 Heater, removal and installation................................................................................................. 808
8.7.2 Changing the circulating pump................................................................................................... 809
8.7.3 Changing the temperature limiter ............................................................................................... 809
8.7.4 Changing the temperature sensor.............................................................................................. 809
8.7.5 Changing the combustion air fan................................................................................................ 809
8.8 Start-up................................................................................................................................................. 809

Thermo 90 S / Thermo 90 ST Table of contents
III
9Repair
9.1 General.................................................................................................................................................. 901
9.1.1 Work on stripped-down components........................................................................................... 901
9.2 Dismantling and assembling.................................................................................................................. 902
9.2.1 Electrical connections (Thermo 90 S).......................................................................................... 902
9.2.2 Changing the circulating pump.................................................................................................... 903
9.2.3 Changing the temperature limiter................................................................................................ 904
9.2.4 Changing the temperature sensor............................................................................................... 904
9.2.5 Changing the combustion air fan................................................................................................. 906
9.2.6 Changing the burner, flame monitor and glow plug..................................................................... 907
9.2.7 Changing the burner head........................................................................................................... 909
9.2.8 Changing the heat exchanger ..................................................................................................... 910
10 Packing, storage and shipment
10.1 General................................................................................................................................................ 1001

List of illustrations Thermo 90 S / Thermo 90 ST
IV
List of illustrations
301 Sequence of functions ............................................................................................................................... 302
501 General fault indications ............................................................................................................................ 501
502 Fault indications on fault lock-out............................................................................................................... 502
503 Visual check, burner back wall................................................................................................................... 503
504 Visual check, pilot flame outlet opening..................................................................................................... 504
505 Visual check, complete burner................................................................................................................... 504
701 System circuit for Thermo 90 S, 12 and 24 V, with standard clock............................................................ 702
702 System circuit for Thermo 90 S, 12 and 24 V, with standard clock and separate
circulating pump control............................................................................................................................. 703
703 System circuit for Thermo 90 S-ADR, 24 V, with switch............................................................................ 704
704 System circuit for Thermo 90 S-ADR, 24 V, with switch without power take-off........................................ 705
705 System circuit for Thermo 90 ST, 12 and 24 V, with standard digital timer ............................................... 706
706 System circuit for Thermo 90 ST, 12 and 24 V, with standard digital timer and separate
circulating pump control............................................................................................................................. 707
707 System circuit for Thermo 90 ST-ADR, 24 V, with switch.......................................................................... 708
708 System circuit for Thermo 90 ST-ADR, 24 V, with switch without power take-off...................................... 709
801 Specimen installations for heater in a truck ............................................................................................... 803
802 Fuel supply................................................................................................................................................. 804
803 Webasto fuel extractor............................................................................................................................... 805
804 Fuel extraction from the plastic tank (extraction through tank drain screw)............................................... 805
805 Fuel extraction from the plastic tank (extraction through tank fitting)......................................................... 805
806 Webasto fuel extractor............................................................................................................................... 806
807 Pipe/hose connection................................................................................................................................. 806
808 Metering pump, installation position and attachment ................................................................................ 807
809 Exhaust pipe opening, installation position................................................................................................ 808
901 Electrical connections (Thermo 90 S)........................................................................................................ 902
902 Changing the circulating pump .................................................................................................................. 903
903 Changing the temperature limiter and temperature sensor ....................................................................... 905
904 Changing the combustion air fan ............................................................................................................... 906
905 Changing the burner, flame monitor and glow plug ................................................................................... 908
906 Changing the burner head......................................................................................................................... 909

Thermo 90 S / Thermo 90 ST 1Introduction
101
1Introduction
1.1 Contents and purpose
This workshop manual is designed to assist trained
personnel with repairing both the petrol and the diesel
versions of the Thermo 90 S und Thermo 90 ST water
heaters.
The heater is only allowed to be operated with the
intended fuel (diesel or also with heating oil EL) and only
in the particular prescribed electrical connection type.
1.2 Meaning of signal words
Throughout this manual, the signal words WARNING,
CAUTION and NOTE have the following meanings:
WARNING
This heading is used to highlight operating instructions or
procedures which, if not or not correctly followed, may
result in personal injury or fatal accidents.
CAUTION
This heading is used to highlight operating instructions or
procedures which, if not or not correctly followed, may
result in damage to the equipment or its components.
NOTE
This heading is used to direct your attention to a special
feature deemed essential to highlight.
1.3 Additional documentation to be used
This workshop manual contains all necessary information
andinstructionsforrepairingThermo90SandThermo90ST
water heaters.
Normally, there is no need to use additional
documentation.
If required, the installation instructions, the vehicle-
specific installation instructions and the operating
instructions can be used as well.
1.4 Safety instructions and regulations
In principle, the general accident prevention regulations
and current works safety instructions are applicable.
The “General safety regulations” that go beyond the
scope of the above regulations are stated below.
Any special safety regulations relevant to this instruction
manual will be highlighted in the relevant sections or text
passages of the procedures.
1.4.1 General safety regulations
The year of first start-up must be indelibly identified
on the rating plate by removing the corresponding
year number.
The heaters are only licensed for heating the motor
vehicle engine and vehicle cabin, not however for heating
the hazardous material transport space.
Theheaterisonly allowed tobeinstalledin motorvehicles
or in independent heating systems with a minimum
coolant volume of 6 litres.
The heater is not allowed to be installed in the driver’s cab
or passenger compartment of vehicles. If the heater is
nevertheless installed in such a place, the installation box
must be tightly sealed against the vehicle interior. The
installation box must have sufficient external ventilation to
ensure that a maximum temperature of 40 °C is not
exceeded in the box. Malfunctions may occur if the
temperature exceeds this level.
WARNING
The heater must not be operated in enclosed areas,
such as garages or workshops without an exhaust
emissions extraction system, not even using the
timer or Telestart, because of the risk of asphyxiation
and poisoning.
The heater must be switched off at filling stations and
fuel tank farms because of the risk of explosion.
CAUTION
Wherever inflammable vapours or dust may form (for
example near fuel, coal, wooddust or cereal stores or
the like), the heater must be switched off because of
the risk of explosion.
Atemperatureof85 °C (storagetemperature)mustnot be
exceeded in the vicinity of the control unit (for example
when completing painting work on the vehicle).
If this temperature is exceeded, the electronic systems
may suffer permanent damage.
Follow the instructions supplied by the vehicle
manufacturer to check the cooling water level. The water
in the heating circuit of the heater must contain at least
10% branded anti-freeze.
Non-compliance with the installation instructions and the
warnings contained therein will lead to the exclusion of all
liability by Webasto. The same applies if repairs are
carried out incorrectly or with the use of parts other than
genuine spare parts. This will result in the revocation of
the type licence for the heater and therefore the general
operating licence for the vehicle.

1Introduction Thermo 90 S / Thermo 90 ST
102
1.5 Statutory regulations governing
installation
Refer to the installation instructions for the statutory
regulations if necessary.

Thermo 90 S / Thermo 90 ST 2 General description
201
2 General description
Thermo 90 S and Thermo 90 ST water heaters operate in
conjunction with the vehicle’s own heating system and
serve
– to heat the cab,
– to defrost the vehicle windows and
– to preheat water-cooled engines.
The water heater operates independently of the engine
and is connected to the cooling system, the fuel system
and the electrical system of the vehicle.
The concept of the heater is based on the evaporator
principle. It is controlled by the temperature sensor and
operates intermittently.
Depending on the deviation between the current coolant
temperature and the nominal value measured at the
temperature sensor, the burner power is regulated
between the limits of 1.8 to 7.6 kW in diesel heaters and
1.8 to 7.6 kW in petrol heaters. When the heating
requirement is particularly high (preheating), diesel
heaters provide a maximum power of 9.1 kW for up to
2 hours after the heater is switched on.
The Thermo 90 S and Thermo 90 ST heater is principally
composed of
– the combustion air fan
– the heat exchanger
– the burner insert with combustion pipe
– the circulating pump
To control and monitor it, the heater has
– a control unit (external)
– a flame monitor
– a glow plug
– a temperature sensor
– a temperature limiter
inside it.
Inthe case of the Thermo 90S heater,the control unit can
also be flange-mounted on the combustion air fan.
TheThermo 90 ST heateris delivered with the control unit
already flange-mounted on the combustion air fan.
Thermo 90 S heater
Thermo 90 S
1
2
3
4
5
6
7
8
9
10
1 Combustion air fan
2 Fuel connection
3 Temperature sensor
4 Temperature limiter
5 Heat exchanger
6 Inlet, coolant
7 Circulating pump
8 Outlet, coolant
9 Outlet, exhaust gas
10 Inlet, combustion air
11 Control unit (can also be
arrangedexternallyinthe
vehicle)
11

2 General description Thermo 90 S / Thermo 90 ST
202
Thermo 90 ST heater
2.1 Combustion air fan
The combustion air fan supplies the air required for the
combustion process from the combustion air inlet to the
burner insert.
Combustion air fan
2.2 Heat exchanger
In the heat exchanger, the heat generated by combustion
is transferred to the coolant circuit.
Heat exchanger
12
3
4
5
6
7
8
9
10
11
1 Combustion air fan
2 Fuel connection
3 Temperature sensor
4 Temperature limiter
5 Heat exchanger
6 Inlet, coolant
7 Circulating pump
8 Outlet, coolant
9 Outlet, exhaust gas
10 Inlet, combustion air
11 Control unit
Temperature limiter
Temperature sensor

Thermo 90 S / Thermo 90 ST 2 General description
203
2.3 Temperature sensor
The temperature sensor records the coolant temperature
at the coolant outlet of the heater as an electrical
resistance. This signal is fed to the control unit where it is
processed.
2.4 Temperature limiter
The overheating guard circuit (bimetallic) protects the
heater against impermissibly high operating
temperatures. The overheating guard circuit trips if the
heat exchanger temperature exceeds 127 ±7 °C and
switches off the heater.
2.5 Burner insert
The fuel is distributed over the burner cross-section in the
combustion pipe in the burner insert.
2.6 Glow plug
Themixtureoffuel and airisignitedbythe glowplugwhen
the heater is started. The glow plug takes the form of an
electrical resistor and is positioned in the burner insert on
the side away from the flame.
2.7 Flame monitor
The flame monitor is a low-ohm PTC resistor, which
changes its resistance depending on the heat emitted by
the flame. The signals are fed to the control unit where
they are processed. The flame status is monitored by the
flame monitor whenever the heater is operating.
2.8 Combustion pipe
The mixture of fuel and air burns in the combustion pipe,
thus causing the heat exchanger to become hot.
Combustion pipe
2.9 Circulating pump
The circulating pump ensures that coolant is pumped
through the vehicle and heater circuits. The pump is
switched on by the control unit and it runs throughout
operation of the heater (including in the control pause).
Circulating pump
2.10 Control unit
The control unit is the central component. It guarantees
the sequence of functions and monitoring of combustion
mode. It is flange-mounted on the combustion air fan,
although in the case of the Thermo 90 S it may be
installed externally in the vehicle.
2.11 Metering pump
The fuel supply and metering for the heater is assured by
an external metering pump. It addition, it serves as a shut-
off element when the heater is switched off.
Thesolenoid coil of themeteringpump receivesits pulses
from the microprocessor in the control unit.
Burner insert
Glow plug
Flame monitor

2 General description Thermo 90 S / Thermo 90 ST
204
Page for notes

Thermo 90 S / Thermo 90 ST 3 Function description
301
3 Function description
(Fig. 301)
3.1 Switching on
The “operating indicator” on the digital timer lights up
when the “immediate heating” button is pressed.
or
Whenswitching on withthe switch, the operating indicator
in the switch lights up.
The circulating pump, glow plug and combustion air fan
start operating.
3.2 Heating mode
Combustion starts after approx. 60 to 140 seconds.
Automaticallycontrolled heating operationrunsat fullload
for a maximum of 2 hours.
The vehicle’s own heater fan does not switch on until the
heat carrier (coolant) has heated up sufficiently. Once the
preset nominal temperature has been reached (see table
of regulating temperatures) the heating power is reduced
to the lowest part load in small individual steps.
3.3 Control mode
The heater switches to the control pause if the
temperature of the coolant continues to rise to the
threshold of the control pause. The circulating pump, the
vehicle’s own heater fan and the operating indicator
continue to operate during the control pause.
The heater restarts when the coolant cools to the preset
temperature for switching back on.
Diesel heaters
If the water temperature in the heater is higher than the
nominal temperature but has not yet reached the
thresholdforthecontrolpauseandif thetemperaturethen
falls back to the nominal temperature within 10 minutes
(after reaching this temperature for the first time), the
heater switches back to the 9.0 kW heating stage.
When the nominal temperature is reached, the heater is
only switched back on again up to the 7.6 kW heating
stage.
Petrol heaters
The maximum heating power in petrol heaters is
always 7.6 kW.

3 Function description Thermo 90 S / Thermo 90 ST
302
Fig. 301 Sequence of functions
3.4 Switching off
The operating indicator on the digital timer/switch goes
out when the heater is switched off. The combustion
process will be ended and the run-on phase will start.
The circulating pump and the combustion air fan continue
to run, however, in order to cool down the heater (run-on)
and are switched off automatically after approx.
105 seconds. The heater may be switched on again
during the run-on time.
The temperature in the cabin can also be controlled using
the vehicle’s own heater fan with a cabin thermostat.
3.5 Heater functions
when installed in ADR vehicles
The heater is started using the switch. An ADR case
(constrained shut-off) is triggered when
– the vehicle’s engine is switched off,
– a pumping device is started,
in which case the short run-on is terminated after
20 seconds (Thermo 90 S) and after 40 seconds
(Thermo 90 ST).
Following this, the control unit is left in the “fault lock-out”
status. The on/off switch must be moved to “Off” before it
can be restarted.
The disconnecting switch (emergency off switch) is only
allowed to be used in case of danger, because the heater
is switched off without any run-on (overheating possible).
Sequence of functions for Thermo 90 S and ST
Switch on
Component interrogation
Pre-glow 40 s (pulsed)
Metering pump supply 5 – 7 s (1)
Metering pump / part load (1/4)
Flame monitor take-over
Stabilisation time
Full load
Vehicle fan “On”
Control range
Control pause
Flame monitor “Cold” (0)
Run-on ended
Pre-glow 15 – 20 s (pulsed)
Metering pump supply 5 – 7 s (1)
Metering pump / part load (1/4)
Flame monitor take-over
Stabilisation time
Coolant temperature reduced
Full load
Switch off (run-on)
Flame monitor “Cold” (0)
Run-on ended
A
B
C
D
E
F
G
1
0
1
0
0
0
0
0
0
1
1
1
1
1
1/4
1/2
1/4
*
30 80 90 75 70 °C
23
22
212019171614131110
181512
986431
2 5 7
Safety time
❏
*If no flame is formed, the
start is automatically
repeated (20 s pulse pre-
glow, 90 s safety time).
oPossible regulating
temperatures that can be
set, see table below
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23

Thermo 90 S / Thermo 90 ST 3 Function description
303
A Glow plug
B Metering pump
C Combustion air fan
D Flame monitor
E Operation indicator light
F Circulating pump
G Vehicle fan
3.6 Malfunctions
3.6.1 Fault lock-out
Fuel is supplied for max. 240 seconds if the flame does
not start to burn.
Fuel is supplied for max. 240 seconds if the flame goes
out during operation.
The fuel supply is shut off if the system overheats
(temperature limiter is tripped).
Once the cause of the fault has been eliminated, the fault
lock-out is cancelled by switching the heater off and on
again.
If the undervoltage guard switches off the system
for longer than 20 seconds, the fuel supply is interrupted.
Thermo 90 S and Thermo 90 ST
12 volts 10.5 V - 0.5 V
24 volts 21 V -1 V

3 Function description Thermo 90 S / Thermo 90 ST
304
3.6.2 Diagnosis after fault lock-out
Thermo 90 S and Thermo 90 ST
Check the fuses and plug connectors.
3.6.2.1 Version with digital timer
If the system is equipped with a standard clock, a fault
message appears on the display of the digital timer after
a fault occurs:
F 01 No start (after 2 attempts to start)
F 02 Flame failure
F 03 Undervoltage or overvoltage
F 04 Premature flame recognition
F 05 Flame monitor interrupt or flame monitor short
circuit
F 06 Temperature sensor interrupt or temperature
sensor short circuit
F 07 Meteringpump interrupt ormetering pumpshort
circuit
F 08 Fan motor interrupt or fan motor short circuit or
fan motor incorrect speed
F 09 Glow plug interrupt or glow plug short circuit
F10 Overheating
F 11 Circulating pump interrupt or circulating pump
short circuit
3.6.2.2 Version with switch
If the system is operated with a switch, the nature of the
fault is indicated by a flashing code on an operating
indicator light during the run-on time of the heater.
After five short signals, count the long flashes:
1x No start (after 2 attempts to start)
2x Flame failure
3x Undervoltage or overvoltage
4x Premature flame recognition
5x Flame monitor interrupt or flame monitor short
circuit
6x Temperature sensor interrupt or temperature
sensor short circuit
7x Metering pump interrupt or metering pump short
circuit
8x Fan motor interrupt or fan motor short circuit or fan
motor incorrect speed
9x Glow plug interrupt or glow plug short circuit
10x Overheating
11x Circulatingpumpinterruptor circulatingpumpshort
circuit

Thermo 90 S / Thermo 90 ST 3 Function description
305
Page for notes

Thermo 90 S / Thermo 90 ST 4 Technical data
401
4 Technical data
Exceptwherelimit valuesare specified,the technical data
below refer to the usual heater tolerances of ±10% at an
ambient temperature of +20 °C and at the rated voltage.
Electrical components
The control unit, motors for combustion air fan and
circulating pump, glow plug, switch and digital timer (no
timer for ADR mode) are designed for either 12 volts or
24 volts.
The temperature limiter, temperature sensor and flame
monitor are identical on 12 V and 24 V heaters.
Fuel for Thermo 90 S / Thermo 90 ST petrol:
The fuel specified by the manufacturer must be used.
Both leaded and unleaded fuel may be used.
Fuel for Thermo 90 S / Thermo 90 ST and Thermo 90
S-ADR / Thermo 90 ST-ADR (diesel):
The diesel fuel specified by the manufacturer must be
used.
Weknow of no negative influences due to additives. If fuel
is extracted from the vehicle’s tank, follow the additive
instructions issued by the vehicle manufacturer.
If you change to low-temperature fuel, the heater must be
operated for approx. 15 minutes so that the fuel line and
fuel pump are filled with the new fuel.
Heater Operation Thermo 90 S
Petrol Thermo 90 ST Petrol Thermo 90 S diesel Thermo 90 S-ADR Thermo 90 ST diesel Thermo 90 ST-ADR
EC licensing symbol ~S299 ~S298
Model Water heater with Ferro-Tec technology
Heat output Max.
regulating
range 2.0 kW – 7.6 kW 9.1 kW
1.8 kW – 7.6 kW
Fuel Petrol Diesel
Fuel consumption Max.
regulating
range 0.25 l/h – 1.0 l/h 1.1 l/h
0.19 l/h – 0.9 l/h
Rated voltage 12 volts 12 or 24 volts
Operating voltage range 10 ... 15 volts 10 ... 15 or 20 ... 30 volts
Nominal power consumption with circulating
pump
(without vehicle fan)
Max.
regulating
range 37 W – 83 W 90 W
37 W – 83 W
Max. ambient temperature:
Heater: - Operation -40 ... +110 °C (90 °C with control unit installed on heater)
- Storage -40 ... +110 °C (90 °C with control unit installed on heater)
Control unit: - Operation -40 ... +75 °C -40 ... +85 °C -40 ... +75 °C -40 ... +85 °C
- Storage -40 ... +85 °C
Metering pump:- Operation -40 ... +20 °C -40 ... +40 °C
- Storage -40 ... +85 °C
Permitted operating pressure (heat carrier) Max. 2.0 bar
Capacity of the heat exchanger 0.15 l
Max. combustion air intake temperature +40 °C
Minimum capacity of the circuit 6.00 l
Delivery rate of the circulating pump
against 0.15 bar 1650 l/h
CO2in exhaust fumes (permitted function
range) Max. 10 ... 12.0% by volume
CO2adjustment values at approx. +20 °C
and geographic altitude above sea level Max. 0 m 500 m 1000 m 10% 10.6% 11.3%
Heater dimensions (tolerance ±3 mm)
* Control unit installed on the heater L 310 (355*) mm W 131 mm
H 232 mm
L 307 (352*) mm
W 131 mm
H 232 mm
L 310 (355*) mm
W 131 mm
H 232 mm
L 307 (352*) mm
W 131 mm
H 232 mm
Weight 4.8 kg

4 Technical data Thermo 90 S / Thermo 90 ST
402
Page for notes

Thermo 90 S / Thermo 90 ST 5 Troubleshooting
501
5 Troubleshooting
5.1 General
This section describes how to identify and remedy faults
on the Thermo 90 S und Thermo 90 ST heater.
CAUTION
Troubleshooting work demands precise knowledge of the
structure and theory of operation of the various
components and must be carriedout by trained personnel
only.
If in doubt, refer to sections 2 and 3 for a description of
how the functions interact.
CAUTION
As a rule, fault detection is restricted to the localisation of
defective components.
The following potential sources of malfunctions are not
taken into account and should always be checked so that
they can then be excluded as the cause of the particular
fault:
• Corroded plugs
• Loose plug contacts
• Poor crimp contacts on plugs
• Corroded cables and fuses
• Corroded battery terminals
• Impermissibly high ambient temperature
Conduct a function test in the vehicle after rectifying each
fault; switch the heater off and back on again first.
5.2 General fault indications
The following table (Fig. 501) lists the possible general
fault indications.
Fig. 501 General fault indications
NOTE
In many cases, the burner may be one possible cause.
Perform a visual check in accordance with 5.4.
Fault indication Possible cause Remedy
Heater switches off
automatically No combustion after start and restart Control unit enters fault lock-out status. Switch the heater off and back on again.
Flame extinguishes during
operation Contact a Webasto service centre if heater operation does not start again
Heater overheats due to lack/
loss of coolant Top up coolant.
Voltage failure
longer than 20 seconds Check fuses, plug connectors and charge condition of the battery
No flame recognition on start Check flame monitor and connections

5 Troubleshooting Thermo 90 S / Thermo 90 ST
502
5.3 Fault indications on fault lock-out
NOTE
If the system is operated with a switch, the nature of the
fault is indicated by a flashing code on an indicator light
during the run-on time of the heater.
After five short signals, count the long flashes.
In the Thermo 90 S heater witha standard clock, the fault
is output on the clock display (see 3.6.3). The following
table can be used accordingly.
Fig. 502 Fault indications on fault lock-out (page 1 of 2)
Fault indication Possible cause Remedy
No function Electrical cabling,
fuses Check fuses Check battery connections:
+ on 12 / – on 9 / + on 3, plug X12
(Thermo 90 S)
+ on 12 / – on 9 / + on 3, plug X8
(Thermo 90 ST)
Control unit defective Replace control unit
1 Flashing pulse
(start did not occur) Fuel system Check fuel level Check fuel filter
Bleed fuel system
Combustion air/exhaust line Check combustion air/exhaust line for
foreign bodies and clean if necessary
Burner Clean burner and change if necessary
2 Flashing pulses
(flame failure during burner
operation)
Fuel supply Check fuel level
Check fuel filter
Bleed fuel system
Burner Clean burner and change if necessary
3 Flashing pulses
(undervoltage) Electrical power supply Check battery Check electrical connections
4 Flashing pulses
(flame monitor continuously
hot)
Flame monitor defective Replace flame monitor
5 Flashing pulses
(flame monitor defective) Cabling Check cabling for damage, discontinuity and short-circuit
Flame monitor defective Replace flame monitor
6 Flashing pulses
(temperature sensor
defective)
Cabling Check cabling for damage, discontinuity and
short-circuit
Temperature sensor defective Replace temperature sensor
7 Flashing pulses
(metering pump defective/
overheating guard circuit
defective)
Coolant circuit Check coolant level
Bleed coolant circuit
Cabling Check cabling for damage, discontinuity and
short-circuit
Metering pump defective Replace metering pump
8 Flashing pulses
(combustion air fan
defective)
Cabling Check cabling for damage, discontinuity and
short-circuit
Combustion air fan defective Replace combustion air fan