After picking up my new truck (1988 isuzu na) and driving it an hour and a half home without incident and then around the hills that night I realised that the temp gauge needle was at the wrong end of the gauge. I drove the last km home, worried that it would creep higher and something expensive would need fixing/rebuilding. I also noticed that I had a whole lot of extra fuel in the tank as I drove down the driveway. A check this morning showed that the coolant was full, and the fuel gauge gains another quarter tank when I turn the headlights on
I'm guessing the temp gauge was pulling the same trick as it was reading under half for the whole day.

Where should I be pointing the multi-meter to find the problem? Or are the gauges stuffed and need replacing? My old man mentioned something about the voltage regulator in them dieing.

I would check the chassis to body and chassis to engine earth cables. Read the voltage between these points under load and replace cables as necessary. Also the instrument cluster, dash bulb current may change the reference voltage at the gauge earth. I have replaced the temp gauge earth on some cars with a wire direct to the engine block.

The problem is the earthing of the gauges. Because the instrument lights use the same earth, when they are on the "earth" will be at a voltage above earth and this will affect the readings (and this effect can be used to confirm you have fixed the problem, just remember it takes a minute or so for the gauges to reach their reading).

There are several places where the problem can be. One is between body and the chassis, but the most likely place, in my experience, is in the instrument cluster itself. All the earth leads are gathered under one of the knurled nuts holding one of the gauges - it is likely that this nut has come loose. Remove the instrument cluster to check. The other place is where this is earthed to the bulkhead, from memory under the front screw on the bottom half of the binnacle behind the steering wheel.

John

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JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol

Thanks for that. There are a few small electrical gremlins in the car. This morning I was regretting the purchase and thinking i had a dud one, but that cleared up after an hour. The battery went flat overnight and I discovered there is a 3.5 amp current draw (short) through the interior lights when everything is off. I have pulling the fuse until I can trace it, and that fixed a fair bit of the problem, but it is still there. I'll pull the gauge cluster out and see what I can find. The battery leads are new but I didn't check where the ground lead connects yet.

The battery should earth onto the chassis. There is an engine/chassis link across the LH front engine mount. This is likely to have bad connections, which will result in starting problems, engine gauge problems, and fusing of the earth link on the regulator.

In my case I have changed the battery earth to the transfer case and put a link from there to the chassis.

Body earth is by bolted connections. As there are many of these, particularly if the vehicle has been used in salty conditions you can expect all sorts of problems with earthing.

John

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JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol

The car starts fine, so I believe the main earth leads are in good nick, and judging by the lack of rust it hasn't seen much salt. So I'll check inside the gauge cluster.