This guide captures ‘bygone’ advice on HOW TO FIT A REV COUNTER.

Useful information for Classic Car and Retro Car enthusiasts.

The tachometer or rev counter is an extremely useful addition to any car which does not already have one fitted as standard. By skilful reading of the instrument it is possible to tell at what point your engine is developing maximum power and also when it is at its most efficient in relation to the amount of fuel con­sumed. For a relatively modest outlay and it enables you to get the most from your power plant without risking the serious damage caused by over-revving.

Two basic kinds of tachometer where generally available on the accessory market, the ‘pod’ type, which is mounted in its own casing on top of the fascia and the more conven­tional dial type designed to be mounted in the fascia panel itself. Both are electronic in operation and work by detecting the impulses in the car’s ignition system which occur when there is a spark at one of the plugs. The number of sparks per second is then converted into the number of engine revolutions per minute or rpm.

Siting the instrument

To get the most use from a tachometer, you should be able to keep an eye on it in the same way you would a speedo­ meter-that is, you should be able to see it out of the corner of your eye while you are driving. Siting it is there­fore of prime importance. You may be tempted to mount it on a little panel under the fascia, but here you will not be able to see it and keep your eyes safely on the road. The pod type of tachometer is the easiest to fit because it does not require any hole cutting in the fascia panel and can be transferred to another car without difficulty. The instrument should be positioned just to the left or right of the center line of the steering, so that it is in the field of vision while driving, but not a source of distraction.

If you are going to fit the larger dial type, find a site where it can be seen clearly with your hands on the steering wheel and in their normal straight ahead position.

If it is located in the fascia panel, there must be sufficient space behind it for wiring, and in practice this is often hard to find on modern production saloons. Therefore the first task when buying a tachometer is to investigate possible locations and any obvious fitting problems before choosing a parti­cular type of instrument.

Other Considerations

Before buying a tachometer, make sure that the one of your choice is suitable for use with your engine and electrical system. Some instruments are designed to operate only with four-cylinder engines; others are specifically for sixes and eights. Make sure yours is compatible, or that it has an adaptor switch on the back like the Sanpet S-S00A as illustrated

This tachometer will suit either four- or six-cylinder cars. It
is important to set the switch at the back before you start

The tachometer must also match the polarity of the car to which it is fitted.

Check whether your car is positive or negative earth by noting which battery lead runs straight to the car body, this is the earth and buy a tachometer to match.

If your car has a 6-volt electrical system rather than the usual 12-volt, you must buy a 6-volt tachometer. The wires provided with accessory instruments are often not long enough, so be prepared for any extension work with extra wire and a supply of screw connectors.

If you have electronic ignition, you must buy a tacho­ meter which is compatible with it (see below).

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Whichever type of tachometer you have chosen, it is important at this stage to familiarize yourself with the wires at the back of the instrument by matching their colours with the manufacturer’s individual instructions.

One of these will have to be wired into the panel light circuit to provide night-time illumination; find out from the instruction leaflet which one this is.

Many later cars have printed circuits to which it is difficult or impossible to add an extra wire. So connect the illumination wire instead to the sidelamp / headlamp switch which has a separate panel light wire.

Loosen the switch so that the terminals at the back are accessible and the wires visible.

Loosen the light switch from the dashboard. Some switches just
pull out, while others have a securing clamp behind the panel
The wires to the switch are now exposed. In this case they are
fixed to a terminal block which clips on to the switch itself

Disconnect each wire in turn and try the switch until the side and headlamps go on but the panel lights do not, that is the one you want. Connect the illumination wire to the same terminal as this wire, using a spade connector if necessary.

After you have found out which wire runs to the panel lighting
circuit, connect the illumination wire from the pod to it

Alternatively, the circuit may be broken into in the same way at the fusebox if this should prove more readily accessible. If a wiring diagram is available, you may be to find the correct colour wire under the fascia and splice the new wire with a Scotchlok connector.

If the light switch proves to be inaccessible, you must attach
the illumination wire to the appropriate fusebox terminal

You will now be left with three connections to make into the ignition circuit-one to earth, one (the ‘triggering’ lead) to the distributor side of the coil, and one to a live voltage supply.

Wiring into the ignition

Begin by rechecking which wire is which in the manufacturer’s instructions as any mistake could damage the instrument. Take the earth lead first. This must be fixed to any metal part of the car in contact with its main body­ usually a fascia panel fixing bolt is ideal. Make sure the connection is clean and tight.

Fix the earth lead to any metal part of the bodywork. If there
are no convenient bolts to use, drill a hole for a new screw

The next step is to connect the triggering lead, which is the one that actually provides information on the number of engine revolutions per minute. To do this, trace the low tension wire running from the side of the distributor back to the coil-even if it disappears into the wiring loom, it should still be easy to match the colour up when it reappears; this will be at the ‘distributor side’ of the coil.

Identify the low tension lead running from the distributor to
the coil. Do not confuse it with the thick black HT lead

Feed the two remaining wires from the tachometer throughthe engine bulkhead,  remembering to fit a rubber grommet to prevent them fraying if a fresh hole has to be drilled.

Feed the wires from the pod inside the car through the engine
bulkhead. Make use of any existing holes where possible

If the wires have to be extended, use proper screw connectors and make sure that you can still identify the wires correctly at the other end. Solder or crimp a spade connector to the triggering lead, and attach it to the distributor side of the coil using a double spade connector.

If you have to extend the wires, use a proper screw connector or crimp
to do it. Do not simply twist the bared ends together.

On some cars, the coil terminals will be marked CB and SW, in which case connect the triggering lead to the CB one. On later negative earth vehicles they maybe marked + and – . If so connect it to the side.

Wiring the live connection

The final wire is the live one which provides power to the circuit when the ignition is switched on.

Different manufacturers recommend different points at which to make the connection, but there are basically only three, the switch (SW) side of the coil; a spare terminal on the fusebox connected to any circuit which becomes live when the ignition is switched on; and a spare terminal behind the ignition switch itself.

With the Sanpet S-S00A, for example, it is a simple matter of connecting the live lead to the opposite side of the coil to the distributor. In other cases follow instructions carefully and ensure the terminals needed are accessible by unscrewing the ignition switch or removing any trim around it.

Attach the triggering lead to the terminal on the distributor
side of the coi I. Do not forget to replace the low tension lead

There will almost certainly be a spare terminal provided, specifically for such accessories, and the lead an be attached by means of a standard spade connector. If the ignition switch is impossible to get at then the fusebox is a good alternative. The correct terminal should be identified using a wiring diagram.

The finished job, showing the live supply lead attached to the
other terminal on the coil, in the same way as the trigger lead
One way to connect up a tachometer, with the live supply as well as·the triggering lead conected to the coil. The makers’ instructions will tell you which method to use
The alternative method. The live supply is connected further down the ignition circuit either to the ignition switch side of the fusebox or to the ignition switch itself

Electronic ignition

Some earlier types of tachometer, such as the Smiths RVI work by sensing complete breaks in the current through the coil instead of the minute voltage variations sensed by later units.

These are not compatible with certain types of electronic ignition, and require an addi­tional adaptor to be fitted. If you already have an electronic system, check with its manufacturer that your tachometer will be compatible. If considering buying one do not forget to get the appropriate adaptor as well if you already have a tachometer.

Best use of your REV Counter

All tachometers are calibrated per thousand rpm. Some­ times they run 1, 2, 3 etc. with a small motif on the dial face saying ‘X 1,000’. Others go 20, 30, 40 etc. with ‘X 100’ underneath.

Most of the instruments on the accessory market are provided with an extra movable needle which enables you to set on the dial the upper limit at which your engine should be revved.

On tachometers fitted as standard equipment there is usually a red sector on the dial which ends at the maximum rev limit. This indicates that the driver can keep his engine turning at such high speeds only for short bursts of time; the needle must not pass this point.

In practice the upper rev limit may be taken as that at which the engine develops maximum power and this information can always be found in the data section of the car’s handbook. It must be strictly adhered to.

Far more useful in the course of normal motoring is the rpm figure at which the engine develops maximum torque­, that is when its ‘twisting’ power is at its peak in relation to the force imposed against it by the weight of the car.

The engine output and torque curves of a VW Derby. The engine output curve climbs steeply to its peak at 5500 rpm, leaving the torque curve much flatter with a slight peak at 3500 rpm

At lower or higher revs the relationship becomes less favour­able but at this figure (again obtainable from the car’s handbook) you can be sure that you are getting the most from your engine in terms of speed for fuel consumed.

It is also the optimum engine speed at which to climb hills (especially important if you are towing a caravan). Some tachometers have warning lights which come on when the engine reaches a certain speed. The Sanpet S-S00A, for example, is provided with an automatic warning light which comes on when the indicator needle reaches the red hand. Set the hand at the maximum rev limit and the light will warn of over-revving.

Alternatively, set it at the maximum torque figure and the red light will indicate optimum engine speed.

The tachometer dial, showing the rev limit indicator adjusted
to 5500 rpm. At this speed the red warning light below comes on

If you know the tick-over speed of your engine (usually around 800rpm), a tachometer is an invaluable aid when adjusting the carburettor and can warn of over-fast running.

Finally, remember that it is often possible to rev your car excessively with detrimental effects, so you should pay special attention to the tachometer when accelerating hard or overtaking.