VW Instrument Panels - a bodger's guide
Back in the late 1970s, Volkswagen was an early pioneer in the use of electronics in vehicle instrumentation. The systems used were fairly primitive - just a handful of transistors and a few diodes - but meant that instrument wiring could be greatly simplified, with additional functions (rev counter, economy gauge) able to be added without making any changes to the vehicle wiring loom or connectors. These instrument packs first appeared on the Mk1 Polo and Golf, and can easily be identified because they use coloured LEDs rather than bulbs for the warning lights.
Unfortunately these instrument packs tend to decay with old age, resulting in several irritating faults. Although you can still find replacement packs easily enough in scrapyards, there is no guarantee that these will be free of faults, and you also need to be sure that the gearbox final drive ratio of the donor car is the same as your own, otherwise the speedometer will be inaccurate. So here's a short guide to fixing the most common faults, which will hopefully save you some money.
Oil warning light / buzzer faults
The oil pressure warning system on most 1980s VWs is quite complex. There is a standard warning light (actually an LED) connected to a pressure sender in the cylinder head, just like any other car. However, there is also a secondary system. This uses a second pressure sender, usually mounted near the first, and also senses engine speed via a connection to the ignition coil. The second pressure sender operates at a much higher pressure than the first (around 30 psi I think). If the engine speed is above 2000 rpm, and the oil pressure drops below 30 psi, the oil warning light flashes and a buzzer sounds. This is a good system in theory, as it gives early warning of a drop in oil pressure and might just give you time to shut down the engine before it is damaged.
However, the system can play up on older vehicles. Either the control circuitry fails so that the buzzer sounds continuously or, on a moderately worn engine, the oil pressure at 2000 rpm is not quite sufficient to open the pressure sender. I once had a high mileage Audi 80 (same system) where, with the engine hot, the buzzer would sound between 2000 and 2200 rpm which was annoying in heavy traffic. If the system is giving trouble, there are two lines of attack. Firstly, pull the spade connector off the secondary pressure sender. If you can't work out from the Haynes manual which is the primary sender and which the secondary, disconnect each in turn with the ignition on but the engine not running. When you disconnect the primary sender the oil pressure light will go out; disconnecting the secondary sender should make no difference.
If the buzzer still sounds with the sender disconnected, the fault is in the circuitry. However, provided the buzzer only sounds when the engine is running, you can fool the circuitry into thinking that the engine is not running. Look at the ignition coil, specifically the negative side of the low tension circuit. There should be two wires - one going to the distributor, the other (black and red on Polos) goes to the instrument panel and provides the signal which tells the oil pressure circuit whether the engine is running, and if so at what speed. Disconnect this wire and the buzzer should stay silent permanently. If this doesn't work, you will need to replace the instrument pack. Note that disconnecting this wire will also kill the rev counter if you have one fitted.
With the circuit disabled, you still have the conventional oil pressure warning light, as fitted to every other car on the planet. So I wouldn't worry too much about losing the extra protection of the VW system - total loss of oil pressure is very rare in engines these days.
Temperature warning light flashes constantly
The warning light is located within the temperature gauge, and has a self-check function whereby it flashes when you switch the ignition on, then goes out when you start the engine. Except sometimes it doesn't go out, and flashes even when you know the engine is stone cold. It's easy to get rid of, although you need to remove the instrument pack from the dashboard. Behind the temperature gauge are three brass nuts. The centre one is the earth for the temperature warning light. Remove it, and use a thin screwdriver or similar to ensure that there is no contact between the brass post it was screwed to, and the surrounding flexible circuit board. Job done, and the temperature gauge itself will still function as before.
Temperature and / or fuel gauges non-functioning or intermittent
Assuming you've checked the wiring and connections at the sender units, the problem is likely to be with the brass nuts which connect the gauges to the flexible circuit 'board' on the back of the instrument pack. There are two for each instrument. Remove each in turn and clean any corrosion from the nut, the board and the washer between the two. This will usually restore the gauges to working order.
Warning lights fail intermittently
A non-working indicator or main beam warning light means MoT failure. The cause is usually dirty or corroded contacts on the multi-way connector which plugs into the back of the instrument pack. Clean the contacts very carefully with fine abrasive, then spray with contact cleaner / lubricant.