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.
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
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).
Latest Classic Car REV counters on eBay
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.