Farringdon RCA50 Series User manual

Farringdon Instruments Limited .Unit 9 Oriel Court . Omega Park . ALTON .
Hampshire GU34 2YT. Telephone [phone].
www.farringdoninstruments.co.uk
The RCA50 Series of Tachometers
Installation and Operating Instructions
Revision 6
Farringdon Instruments Limited .Unit 9 Oriel Court . Omega Park . ALTON .
Hampshire GU34 2YT. Telephone [phone].
www.farringdoninstruments.co.uk
The RCA50 Series of Tachometers
Installation and Operating Instructions
Revision 6

Contents
1. Introduction and Outline Specification
2. Fitting Your Tacho
2.1 Mounting the Tacho
2.2 Connecting the Tacho
2.3 The Push Button
2.4 Shift Lights
3. Setting Up Your Tacho
3.1 Using the Push Button
3.2 Number of Cylinders
3.3 Tell-Tale and Other Modes
3.4 Ignition Pulse Filter
3.5 Shift Light Settings
3.6 Resetting the tell-tale.
4. Installing the Filter Unit
4.1 Purpose
4.2 Fitting
4.3 Commissioning
5. Troubleshooting
Contents
1. Introduction and Outline Specification
2. Fitting Your Tacho
2.1 Mounting the Tacho
2.2 Connecting the Tacho
2.3 The Push Button
2.4 Shift Lights
3. Setting Up Your Tacho
3.1 Using the Push Button
3.2 Number of Cylinders
3.3 Tell-Tale and Other Modes
3.4 Ignition Pulse Filter
3.5 Shift Light Settings
3.6 Resetting the tell-tale.
4. Installing the Filter Unit
4.1 Purpose
4.2 Fitting
4.3 Commissioning
5. Troubleshooting

Notes
1. Introduction
The Farringdon RCA50 Series of tachometers are believed to be
the first available with stepper motor driven main and tell-tale
hands. The tell-tale hand is a very useful feature especially
when used in its auto-reset mode and not just an extra hand to
make it look more like historic chronometric types.
Please read these instructions carefully and make sure that the
tacho displays the correct rpm—if set up incorrectly, it is likely
to show double or half the rpm but it is possible to set it up so
that it shows 4/5th of the true rpm which might be very expen-
sive!
Outline Specification
Supply Voltage 7 to 16 volts DC. Below 10 volts hand speed
is reduced
Trigger Pulse 5 to 100 volts. Pulse width greater than 10
microseconds
Supply Current (mean) Less than 250 mA without shift lights, less
than 300 mA with shift lights
Accuracy +/- 100 rpm or 1%
Notes
1. Introduction
The Farringdon RCA50 Series of tachometers are believed to be
the first available with stepper motor driven main and tell-tale
hands. The tell-tale hand is a very useful feature especially
when used in its auto-reset mode and not just an extra hand to
make it look more like historic chronometric types.
Please read these instructions carefully and make sure that the
tacho displays the correct rpm—if set up incorrectly, it is likely
to show double or half the rpm but it is possible to set it up so
that it shows 4/5th of the true rpm which might be very expen-
sive!
Outline Specification
Supply Voltage 7 to 16 volts DC. Below 10 volts hand speed
is reduced
Trigger Pulse 5 to 100 volts. Pulse width greater than 10
microseconds
Supply Current (mean) Less than 250 mA without shift lights, less
than 300 mA with shift lights
Accuracy +/- 100 rpm or 1%

2. Fitting Your Tachometer
2.1 Mounting the Tachometer
The RCA50 tachometer is 80 mm in diameter and the hole
in your dash should accept this diameter. It will require 60
mm behind the dash to accommodate the depth of the in-
strument and the clamping studs.
An ‘O’ ring is supplied that may reduce vibration a little if
it is fitted behind the bezel. Also it will provide a compli-
ance for the clamp to work against. If the dash is wood,
then it is probably unnecessary and will prevent the bezel
from seating on the dash neatly.
In order to keep the overall depth of the instrument as small
as possible, the mounting studs are relatively short and
therefore the U shaped clamp may have to be trimmed to
accommodate thick dash boards. The U clamp is steel but
is easy to cut.
2. Fitting Your Tachometer
2.1 Mounting the Tachometer
The RCA50 tachometer is 80 mm in diameter and the hole
in your dash should accept this diameter. It will require 60
mm behind the dash to accommodate the depth of the in-
strument and the clamping studs.
An ‘O’ ring is supplied that may reduce vibration a little if
it is fitted behind the bezel. Also it will provide a compli-
ance for the clamp to work against. If the dash is wood,
then it is probably unnecessary and will prevent the bezel
from seating on the dash neatly.
In order to keep the overall depth of the instrument as small
as possible, the mounting studs are relatively short and
therefore the U shaped clamp may have to be trimmed to
accommodate thick dash boards. The U clamp is steel but
is easy to cut.

2.2 Connecting the Tachometer
The tachometer is supplied with flying leads so that you can fit
the connector of your choice to the car loom. The table lists the
functions of the five free wires.
Wire Colour Function Connect To
Black Ground or Earth Ground or Chassis
Red 12 volt Supply A fused 12 volt supply from
the master switch or Ignition
if there is no master switch
Violet Trigger input Negative side of coil or to
dedicated tacho output from
engine ECU
Grey Shift Light 1
Output
Supplied shift light ONLY or
52 mm gauge tell-tale enable
Blue Shift Light 2
Output
Supplied shift light ONLY
Orange Backlighting Lighting 12 volts
2.2 Connecting the Tachometer
The tachometer is supplied with flying leads so that you can fit
the connector of your choice to the car loom. The table lists the
functions of the five free wires.
Wire Colour Function Connect To
Black Ground or Earth Ground or Chassis
Red 12 volt Supply A fused 12 volt supply from
the master switch or Ignition
if there is no master switch
Violet Trigger input Negative side of coil or to
dedicated tacho output from
engine ECU
Grey Shift Light 1
Output
Supplied shift light ONLY or
52 mm gauge tell-tale enable
Blue Shift Light 2
Output
Supplied shift light ONLY
Orange Backlighting Lighting 12 volts

Black and Red Wires
Connect the black wire to a good earth point and the red to a
suitable switched and fused 12 volt supply. A fuse of 1 amp is
sufficient. The tacho requires a few seconds to power up so
connecting it to the master switch is preferable to the ignition
supply.
On powering up, the tacho drives both hands forwards and then
backwards into stops. After this the tell-tale hand moves to the
maximum rpm recorded since it was last reset.
Violet Wire
The trigger input can be connected to the switched end (-) of the
coil through the supplied Filter Box but if there is a dedicated
tacho pulse from the ignition system this would be better. If
you are using a modern engine management system, you may
need a 3.3 K resistor between this wire and +12 volts. If you
have a rev. limiter fitted, then it is much better to provide a
separate trigger that is independent from the ignition. (Please
Symptom Possible Cause Fix
No movement of tacho
hands when turned on
No power Check for +12 volts between red
and black wire.
Tacho runs against stop
on power on but shows no
rpm
No or incorrect trigger
pulse
Make sure that the trigger pulse
is greater than 5 volts and falls
to less than 0.2 volts. Set trigger
pulse filter to 1
Tacho does not show
correct rpm
Trigger pulse setting is
incorrect
Check set up. Set to factory
setup (start of section 3) and
start again. Call Farringdon for
help.
Main hand drops to 0
above a certain rpm
Filter setting is too high Reduce filter setting
Main hand is unsteady. Irregular trigger or too
short trigger pulses.
Increase filter setting or fit Filter
Unit
Shift Lights do not work Lights wired incorrectly.
Shift RPM setting wrong
Make sure the shift lights are
wired correctly Black to black.
Check RPM settings for Shift
Lights.
5. Troubleshooting
Black and Red Wires
Connect the black wire to a good earth point and the red to a
suitable switched and fused 12 volt supply. A fuse of 1 amp is
sufficient. The tacho requires a few seconds to power up so
connecting it to the master switch is preferable to the ignition
supply.
On powering up, the tacho drives both hands forwards and then
backwards into stops. After this the tell-tale hand moves to the
maximum rpm recorded since it was last reset.
Violet Wire
The trigger input can be connected to the switched end (-) of the
coil through the supplied Filter Box but if there is a dedicated
tacho pulse from the ignition system this would be better. If
you are using a modern engine management system, you may
need a 3.3 K resistor between this wire and +12 volts. If you
have a rev. limiter fitted, then it is much better to provide a
separate trigger that is independent from the ignition. (Please
Symptom Possible Cause Fix
No movement of tacho
hands when turned on
No power Check for +12 volts between red
and black wire.
Tacho runs against stop
on power on but shows no
rpm
No or incorrect trigger
pulse
Make sure that the trigger pulse
is greater than 5 volts and falls
to less than 0.2 volts. Set trigger
pulse filter to 1
Tacho does not show
correct rpm
Trigger pulse setting is
incorrect
Check set up. Set to factory
setup (start of section 3) and
start again. Call Farringdon for
help.
Main hand drops to 0
above a certain rpm
Filter setting is too high Reduce filter setting
Main hand is unsteady. Irregular trigger or too
short trigger pulses.
Increase filter setting or fit Filter
Unit
Shift Lights do not work Lights wired incorrectly.
Shift RPM setting wrong
Make sure the shift lights are
wired correctly Black to black.
Check RPM settings for Shift
Lights.
5. Troubleshooting

engine is held at a steady 3000 rpm. If the needle is steady,
try at full speed under load.
If either needle makes jumps to higher rpm when the engine
is running at a constant speed, increase the filtering by
connecting the trigger wire to the blue instead of the green
filter wire.
If the needles are steady, then try changing the tacho filter
setting to 7. This will make the tacho more responsive.
Warning DO NOT rely of the tacho reading until you
are certain that you have set it up correctly and that it is
reading the true RPM.
contact Farringdon for advice on sensors that you can fit to your
engine for this purpose. When the engine is running faster than
the rev. limiter should allow (on a change down at too higher
road speed for example), the ignition pulses are very irregular
and will cause the tell-tale to record a incorrect (usually off the
scale!) figure. Apart from alarming you, you will not be able to
make sensible decisions about any damage that may have been
caused to the engine. A new Farringdon Rev. Limiter is availa-
ble for certain ignition systems that provides a clean tacho trig-
ger pulse and rpm and engine life logging.
Blue and Grey Wires
Connect these wires to the red wire of the shift light. Connect
the black or blue wire to ground. The shift light outputs are
positive 5 volt, 50 mA signals that are designed to drive the
LED shift lights supplied with the instrument. Do not connect
filament type lights to these outputs—it will not drive them!
Orange Wire
Connect this to the lighting supply. This should be a supply that
engine is held at a steady 3000 rpm. If the needle is steady,
try at full speed under load.
If either needle makes jumps to higher rpm when the engine
is running at a constant speed, increase the filtering by
connecting the trigger wire to the blue instead of the green
filter wire.
If the needles are steady, then try changing the tacho filter
setting to 7. This will make the tacho more responsive.
Warning DO NOT rely of the tacho reading until you
are certain that you have set it up correctly and that it is
reading the true RPM.
contact Farringdon for advice on sensors that you can fit to your
engine for this purpose. When the engine is running faster than
the rev. limiter should allow (on a change down at too higher
road speed for example), the ignition pulses are very irregular
and will cause the tell-tale to record a incorrect (usually off the
scale!) figure. Apart from alarming you, you will not be able to
make sensible decisions about any damage that may have been
caused to the engine. A new Farringdon Rev. Limiter is availa-
ble for certain ignition systems that provides a clean tacho trig-
ger pulse and rpm and engine life logging.
Blue and Grey Wires
Connect these wires to the red wire of the shift light. Connect
the black or blue wire to ground. The shift light outputs are
positive 5 volt, 50 mA signals that are designed to drive the
LED shift lights supplied with the instrument. Do not connect
filament type lights to these outputs—it will not drive them!
Orange Wire
Connect this to the lighting supply. This should be a supply that

switches to 12 volts when the lights are on.
2.3 The Push Button
The push button should be mounted where you can reach it
while being able to read the tacho. If other people are go-
ing to drive the car you may wish to put it somewhere hid-
den so that the tell-tale cannot be reset easily! The push
button requires a 12 mm diameter hole. Fit the sealing cap
to keep moisture out of the switch.
2.4 The Shift Lights
The supplied shift lights should be fitted where they will
catch your eye while you are looking at the track or road.
Try various places (holding them in place with tape) before
deciding where to mount them. The black plastic bezels
push into the dash from the front and are held in place when
the LEDs are inserted from the back. The hole size for the
bezels is 14 mm diameter.
mounted in the engine bay.
a) Connect the violet wire of the filter unit to the
violet wire of the tacho.
b) Connect the black wire of the Filter Unit to
ground.
c) Connect the negative of the coil to the green or
blue wire. The blue input provides more filter-
ing than the green so start with the green wire.
d) Temporarily insulate the unused blue or green
wire.
4.3 Commissioning
Set the tacho filter setting to 8. See Section 3.4 for details
of how this is done. Try the tacho and check that the nee-
dles do not jump about (+ or – 500 rpm or so) when the
switches to 12 volts when the lights are on.
2.3 The Push Button
The push button should be mounted where you can reach it
while being able to read the tacho. If other people are go-
ing to drive the car you may wish to put it somewhere hid-
den so that the tell-tale cannot be reset easily! The push
button requires a 12 mm diameter hole. Fit the sealing cap
to keep moisture out of the switch.
2.4 The Shift Lights
The supplied shift lights should be fitted where they will
catch your eye while you are looking at the track or road.
Try various places (holding them in place with tape) before
deciding where to mount them. The black plastic bezels
push into the dash from the front and are held in place when
the LEDs are inserted from the back. The hole size for the
bezels is 14 mm diameter.
mounted in the engine bay.
a) Connect the violet wire of the filter unit to the
violet wire of the tacho.
b) Connect the black wire of the Filter Unit to
ground.
c) Connect the negative of the coil to the green or
blue wire. The blue input provides more filter-
ing than the green so start with the green wire.
d) Temporarily insulate the unused blue or green
wire.
4.3 Commissioning
Set the tacho filter setting to 8. See Section 3.4 for details
of how this is done. Try the tacho and check that the nee-
dles do not jump about (+ or – 500 rpm or so) when the

4. The Filter Unit.
4.1 Purpose
The Tacho Filter Unit is a simple filter contained in a small
plastic box that may be required when using a Farringdon
RCA5x Tacho with contact points ignition systems. It may
be of use with some electronic systems as well. Essentially
the unit filters the signals taken from the negative (on nega-
tive earth systems) side of the coil. These signals carry
considerable noise from the arcing inside the distributor
cap. Many other tachos have this filtering included but this
prevents them from working with very short pulses that can
come from directly triggering from a sensor on the fly-
wheel.
4.2 Fitting
Mount the unit at least 25cms away from the distributor.
The components in the plastic box are “potted” so it can be
3. Setting Up Your Tachometer
There are five settings that must be adjusted before you can use
the tacho. These are
1. the number of cylinders or pulses form the engine for
every two turns of the engine
2. the mode of operation. This is mainly the tell-tale reset
mode
3. the trigger pulse filter. This allows you to use a wide
range of pulse widths for the trigger input.
4. the rpm at which shift light 1 comes on and
5. the rpm at which shift light 2 comes on and both lights
flash.
To revert to factory setup, enter setup mode by turn-
ing on the tacho with the button pressed, release the
button and then press it again for 20 seconds.
4. The Filter Unit.
4.1 Purpose
The Tacho Filter Unit is a simple filter contained in a small
plastic box that may be required when using a Farringdon
RCA5x Tacho with contact points ignition systems. It may
be of use with some electronic systems as well. Essentially
the unit filters the signals taken from the negative (on nega-
tive earth systems) side of the coil. These signals carry
considerable noise from the arcing inside the distributor
cap. Many other tachos have this filtering included but this
prevents them from working with very short pulses that can
come from directly triggering from a sensor on the fly-
wheel.
4.2 Fitting
Mount the unit at least 25cms away from the distributor.
The components in the plastic box are “potted” so it can be
3. Setting Up Your Tachometer
There are five settings that must be adjusted before you can use
the tacho. These are
1. the number of cylinders or pulses form the engine for
every two turns of the engine
2. the mode of operation. This is mainly the tell-tale reset
mode
3. the trigger pulse filter. This allows you to use a wide
range of pulse widths for the trigger input.
4. the rpm at which shift light 1 comes on and
5. the rpm at which shift light 2 comes on and both lights
flash.
To revert to factory setup, enter setup mode by turn-
ing on the tacho with the button pressed, release the
button and then press it again for 20 seconds.

3.1 Using the push button
Having the tell-tale hand makes setting up very easy as the
red tell-tale hand shows what you are setting up and the
white main hand shows the value of the setting. The button
is used to increase the value of the setting, with a quick
push, to decrease the value with a push of about 2 seconds
and to go on to the next setting with a push of 5 seconds.
You will find this easier than it sounds!
In the following paragraphs, numbers refer to the first num-
ber on the tacho scale; e.g. when setting the cylinders to 4,
the white hand has to be set to 4000 rpm however this is
marked, maybe 4 or 40.
To enter set up mode, hold the push button down and then
apply the power. The tell-tale needle will move to 1 (the
first set up item) and the main hand will show the currently
set no of cylinders. When the needles have stopped moving,
release the button.
3.2 Number of Cylinders (tell-tale at 1)
3.5 Shift Light Settings (tell-tale at 4 and 5)
The next two settings are for the shift lights. Use the push
button to set the rpm at which you would like the first and
second shift lights to illuminate. Remember to hold the
push button pressed for 5 seconds after each adjustment to
move on to the next. After the last shift light is set, the
tacho will initialise as if it had been switched on.
3.6 Resetting the Tell-Tale
In auto reset mode, the maximum rpm recorded is shown
after switch on and the hands have been driven into the
stops. To reset this value, or the tell-tale hand when in
manual mode, simply press the push button after the tell-
tale reading has been shown.
3.1 Using the push button
Having the tell-tale hand makes setting up very easy as the
red tell-tale hand shows what you are setting up and the
white main hand shows the value of the setting. The button
is used to increase the value of the setting, with a quick
push, to decrease the value with a push of about 2 seconds
and to go on to the next setting with a push of 5 seconds.
You will find this easier than it sounds!
In the following paragraphs, numbers refer to the first num-
ber on the tacho scale; e.g. when setting the cylinders to 4,
the white hand has to be set to 4000 rpm however this is
marked, maybe 4 or 40.
To enter set up mode, hold the push button down and then
apply the power. The tell-tale needle will move to 1 (the
first set up item) and the main hand will show the currently
set no of cylinders. When the needles have stopped moving,
release the button.
3.2 Number of Cylinders (tell-tale at 1)
3.5 Shift Light Settings (tell-tale at 4 and 5)
The next two settings are for the shift lights. Use the push
button to set the rpm at which you would like the first and
second shift lights to illuminate. Remember to hold the
push button pressed for 5 seconds after each adjustment to
move on to the next. After the last shift light is set, the
tacho will initialise as if it had been switched on.
3.6 Resetting the Tell-Tale
In auto reset mode, the maximum rpm recorded is shown
after switch on and the hands have been driven into the
stops. To reset this value, or the tell-tale hand when in
manual mode, simply press the push button after the tell-
tale reading has been shown.

moves the main needle once every half second in the man-
ner of the original mechanical chronometric movements.
This action is switched off after reaching 5000 rpm for 0 to
8,000 and after reaching 6,000 rpm for the 0 to 10,000 and
12,000 versions.
Once you have adjusted the mode setting, hold the push
button pressed for five seconds.
3.4 Trigger Pulse Filter (tell-tale at 3)
The trigger pulse filter sets the shortest electrical pulse to
which the tacho will respond. For ignition connected wir-
ing, set this to 7 and only reduce it if the tacho does not
read over a certain rpm. For special sensor systems, set this
to 1 as a first try. If the needle starts to jump up when the
engine is running under load, try increasing the filter set-
ting.
After setting the filter, push the button for 5 seconds to save
the setting
In order to allow for uneven firing engines, Farringdon uses
the number of trigger pulses in two full turns of the engine
to calculate the rpm. Unless you have a special sensor, this
is likely to be the number of cylinders connected to the coil
you are using.
Use the push button to increase or decrease the number
shown by the main hand until it is correct. Then hold the
push button pressed for 5 seconds to move on to the next
set up item.
Note. If the red hand indicated 1500 rpm, this shows that
the number of cylinders has been set to the max and has
wrapped over. E.g If the red hand is on 1500 rpm and the
white on 4000 rpm, then the tacho has been set to 8+4 (for a
0 to 8000 tacho) cylinders. Press the button for 20 seconds
to start again from the factory setting.
moves the main needle once every half second in the man-
ner of the original mechanical chronometric movements.
This action is switched off after reaching 5000 rpm for 0 to
8,000 and after reaching 6,000 rpm for the 0 to 10,000 and
12,000 versions.
Once you have adjusted the mode setting, hold the push
button pressed for five seconds.
3.4 Trigger Pulse Filter (tell-tale at 3)
The trigger pulse filter sets the shortest electrical pulse to
which the tacho will respond. For ignition connected wir-
ing, set this to 7 and only reduce it if the tacho does not
read over a certain rpm. For special sensor systems, set this
to 1 as a first try. If the needle starts to jump up when the
engine is running under load, try increasing the filter set-
ting.
After setting the filter, push the button for 5 seconds to save
the setting
In order to allow for uneven firing engines, Farringdon uses
the number of trigger pulses in two full turns of the engine
to calculate the rpm. Unless you have a special sensor, this
is likely to be the number of cylinders connected to the coil
you are using.
Use the push button to increase or decrease the number
shown by the main hand until it is correct. Then hold the
push button pressed for 5 seconds to move on to the next
set up item.
Note. If the red hand indicated 1500 rpm, this shows that
the number of cylinders has been set to the max and has
wrapped over. E.g If the red hand is on 1500 rpm and the
white on 4000 rpm, then the tacho has been set to 8+4 (for a
0 to 8000 tacho) cylinders. Press the button for 20 seconds
to start again from the factory setting.

3.3 Mode (tell-tale at 2)
The following table shows the value to set for the mode:
Mode Tell-Tale Trigger Pulse Chromometric
0Auto reset Negative No
1Manual Negative No
2Auto reset Positive No
3Manual Positive No
4Auto reset Negative Yes
5Manual Negative Yes
6Auto Positive Yes
7Manual Positive Yes
Tell-Tale Modes
The tell-tale needle on original chronometric tachometers
was mechanically pushed to the highest reading reached by
the main hand. The tell-tale could be reset by pushing a
small button on the side of the instrument. This is copied in
the manual reset mode. The auto reset mode resets the tell-
tale hand after about two seconds back to follow the main
hand. This is very useful as it gives the driver time to see
what the engine was revved to before or during each gear
change.
Trigger Pulse Polarity
This may need to be changed to match the pulse from a
special sensor. For ignition coil connected wiring set this
as negative.
Chronometric Action
If this option is selected with settings 4,5,6 or 7, the tacho
3.3 Mode (tell-tale at 2)
The following table shows the value to set for the mode:
Mode Tell-Tale Trigger Pulse Chromometric
0Auto reset Negative No
1Manual Negative No
2Auto reset Positive No
3Manual Positive No
4Auto reset Negative Yes
5Manual Negative Yes
6Auto Positive Yes
7Manual Positive Yes
Tell-Tale Modes
The tell-tale needle on original chronometric tachometers
was mechanically pushed to the highest reading reached by
the main hand. The tell-tale could be reset by pushing a
small button on the side of the instrument. This is copied in
the manual reset mode. The auto reset mode resets the tell-
tale hand after about two seconds back to follow the main
hand. This is very useful as it gives the driver time to see
what the engine was revved to before or during each gear
change.
Trigger Pulse Polarity
This may need to be changed to match the pulse from a
special sensor. For ignition coil connected wiring set this
as negative.
Chronometric Action
If this option is selected with settings 4,5,6 or 7, the tacho