Farringdon RCA5 Series User manual

The RCA50 Series of Tachometers

Installation and Operating Instructions

Revision 8

Contents

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 (Older Models Only)

4.1 Purpose

4.2 Fitting

4.3 Commissioning

5. Troubleshooting

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 expensive!

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 100 mA without

shift lights, less than 150 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, the RCA51 is 100 mm in diameter and the RCA52

120 mm and the hole in your dash should accept this diameter. It will require 60 mm behind the

dash to accommodate the depth of the instrument 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 compliance 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 latest tachos have a small M8 connector and the back surface and are supplied with a

moulded mating connector with flying leads. These are connected as follows:

White

Ground or Earth

Connect this to a good chassis ground

Red

12 volt Supply

Connect to a fused 12 volt supply from master switch

(preferred) or ignition switch

Brown

Trigger

Negative side of coil or to dedicated tacho output from

engine ECU

Green

Shift Light 1 or Tell-Tale

Enable

Supplied shift light ONLY positive or 52 mm gauge tell-tale

enable.

Yellow

Shift Light 2

Supplied shift light positive ONLY

Grey

Push button

Connect to one side of push button

Pink

Push Button

Connect to other side of push button

Blue

Back lighting

Lighting 12 volts

Older tachometers are supplied with flying leads so that you canfit the connector of your choiceto

the car loom. The table lists the functions of the five free wires.

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/White 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/Brown Wire

The trigger input can be connected to the switched end (-) of the coil directly or through the

supplied Filter Box for older tachos 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 isindependent from the ignition. (Please 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 an 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.

Blue and Grey Wires or Green and Yellow

Connect these wires to the red wire of the shift light. Connect the black or blue wire of the shift

light to ground. The shift light outputs are positive 5 volt, 50 mA signals that are designed to

drive the LED shift lights supplied by Farringdon. Do not connect filament type lights to these

outputs—it will not drive them!

Orange/Blue Wire

Connect this to the lighting supply - a supply that switches to 12 volts when the lights are on.

2.3 The Push Button (Grey and Pink wires)

The push button should be mounted where you can reach it while being able to readthe tacho. If

other people are going to drive the car you may wish to put it somewhere hidden 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 lightsshould be fittedwhere they will catch your eye while youare 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.

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 the rpm at which shift light 2 comes on and

both lights flash.

To revert to factory setup, enter setup mode by turning 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 digit 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)

In order to allow for uneven firing engines, Farringdon uses thenumber of trigger pulsesin 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

Auto reset

Negative

Manual

Negative

Auto reset

Positive

Manual

Positive

Auto reset

Negative

Yes

Manual

Negative

Yes

Auto

Positive

Yes

Manual

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 it to positive.

Chronometric Action

If this option is selected with settings 4,5,6 or 7, the tacho moves the main needle once every half

second in the manner 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 wiring, 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 setting.

After setting the filter, push the button for 5 seconds to save the setting

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.

Important Notes

Further steps in the set up process can be skipped at any time by switching off the tacho

but only the settings that have been terminated with the 5 second long push button press

will be saved.

If you increase a setting so that it reaches the maximum on the scale, it will “wrap” round

to 1 if the setting is increased again. This allows you to set up a 16 cylinder engine on a 0

to 8000 rpm dial. When this wrap occurs, the tell-tale hand steps on half a division to

indicate that this has happened.

****** To set the tacho back to the factory settings (4 cylinders, mode 4, filter 4, shift

lights 7000 and 8000 rpm), enter set up mode and press button for 20 seconds. ******

4. The Filter Unit. (not applicable to newer models)

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 negative 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 flywheel.

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 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

filtering than the green so start with the green wire.

Temporarily insulate the unused blue or green wire.

4.3 Commissioning

Set the tachofilter setting to 8. See Section 3.4for details of howthis is done. Try the tachoand

check that the needles do not jump about (+ or –500 rpm or so) when the 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 showing the correct rpm..

5. Troubleshooting

To set the tacho back to the factory settings (4 cylinders, mode 4, filter 4, shift lights 7000

and 8000 rpm), enter set up mode and press button for 20 seconds.

Farringdon Instruments Limited .The Old Stables, Manor Farm,

West Worldham. ALTON .

Hampshire GU34 3BD. Telephone 01420 541591.

www.farringdoninstruments.com

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.

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