Instruments

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While instrumentation varies considerably between classics, Rob Marshall gives an overview of the different types, plus repair and upgrade options

Gauges have come a long way, since King Edward VII retrofitted Samuel Smith and Sons’ first mass-produced speedometer to his Daimler 6hp in 1904. Like many ensuing speedometers, the instrument’s head was driven by a cable, the eddy currents produced within which would draw the instrument’s pointer across a scale. As progress marched on, the cable was replaced by a hall sensor. Tachometers (‘rev-counters’) on most pre-1990’s classics can employ either cables, or electronics.

Early temperature gauges worked by measuring the expansion of fluid, sealed within a capillary tube. Electrically-operated instruments replaced them, although mechanical aftermarket oil temperature gauges can still be bought from specialists. While some early instruments were made redundant by improved technology, such as those that measured fuel tank pressure and inlet manifold vacuum, useful developments included fuel gauges that superseded the crude in-tank dipstick. Carmakers also saw marketing benefits in fitting extra instruments to more expensive models, including those that indicated engine oil pressure, oil and ambient temperatures, engine speed, the health of the electrical system and even the humble time clock.

WHAT GOES WRONG?

Instrument heads tend to be very reliable; bad wiring and faulty sensors are more probable roots of ailments. An inoperative fuel gauge, therefore, is more likely to be caused by an issue with the sender unit, within the petrol tank. A voltage stabiliser might be employed, to provide a consistent output, so that certain instruments, most commonly those that measure fuel level and coolant temperature, remain unaffected by voltage fluctuations. A failing stabiliser will provide odd readouts across several instruments.

An annoying rattling can be caused by a glass frontage striking the gauge’s metal bezel, often caused by a disintegrated foam/rubber gasket. Replacements tend to be available from classic instrumentation specialists, some of which can even supply replacement dial faces, to either original, or customised, specifications.

Electronically-controlled instrument clusters tend to be more vulnerable to loose electrical contacts and moisture. Oxidised circuit board soldered joints can cause either erratic readings, or complete cluster shut-down. Long-term exposure to heat, stemming from both electronic chips and light bulbs, also results in broken tracks and dry joints. Replacing any OE-specification halogen bulbs with modern LEDs might help with this, although the light output tends to be starker. Excessive low (or high) voltage situations, caused by an owner eking out every last drop of life from an ailing battery, can also cause data corruption within the board’s chips and the software for many modern classics is longsince obsolete. Fortunately, several companies can assist with cluster repairs, including more advanced diagnostics, although a competent DIYer should be able to conduct continuity tests with a multimeter.

While an electronic speedometer can be plagued by a faulty sensor, an ailing stepper motor, or deteriorated circuit boards, mechanical types can suffer from weakened magnetic heads and even oil contamination from a failed speedometer drive seal. Needle waver tends to be caused by a worn, or kinked, speedometer cable. The MoT Test not only requires a working main beam telltale but also the speedometer must be backlit, when the headlamps are switched on. Its needle must also not be obstructed from making a clean sweep across the scale.

UPGRADE OPTIONS

Tearing out your original specification instrumentation and plonking in a dashboard from a higher-specified model will not only affect your classic’s originality (potentially making it less saleable) but altering the original wiring can also be tricky. Yet, you can still buy instrument kits and group them together in either a separate pod, or bracket, without making any permanent alterations.

Many enthusiasts prefer to fit an oil pressure gauge, with both electric and mechanical types being available. If your engine’s oil gallery cannot be tapped into by any other means, a kit might include a T-piece adapter that screws into the oil pressure warning lamp sender’s aperture. Oil temperature gauges are useful for engines that are either renowned for overheating their lubricant (such as Triumph’s longstroke 1500cc and 2500cc units), or those that are subjected to sustained high engine speeds. Some enthusiasts fit two temperature gauges to monitor the performance of retro-fitted oil coolers, one connected to the inlet, the other to the outlet. Otherwise, many people install them to the sump.

It is possible to install an ammeter, to a car equipped with an alternator, but be wary that all electrical loads must pass through the instrument, which necessitates that you use cabling that is heavyduty enough for the job. It is also wise to fit the circuit with its own dedicated fuse. Voltmeters are simpler to install but ensure that it is wired into the ignition switch circuit, so that it is not connected permanently to the battery. A clock, on the other hand, will need a constant power feed. If your classic lacks a tachometer, most electricallypowered units for older cars measure the pulse reading from the ignition coil but be wary that the gauge must be compatible with the engine’s number of cylinders.

Vacuum gauges can be used as a guide for fuel consumption and may also help to indicate mechanical wear. As the feed is taken from the inlet manifold, you might have to drill and tap a thread into the casting. To prevent swarf from damaging the engine, remove the part from the car, before working on it.

WATCH THE WIRING

If fitting electrically-operated gauges, always use wiring of the appropriate current rating and never cut into the existing wiring loom, use piggy-back terminals but never be tempted to use Scotchloks. Original-looking wiring components are available through specialists but always solder any bullet connectors to the wire ends.

Protect any extra wiring from heat and abrasion with sheaths and pass them though grommets, so that their insulation is not chafed by the edges of metal holes. Finally, if your car has a positive earth system, or 6-volt electrics, ask your parts supplier if the new gauges might be compatible. And can you understand what the dials are indicating? That’s for another issue!