Puzzling electrical fault, overdrive, fuel and temp gauges
 

Facing a very puzzling electrical fault on a 1990 240 estate automatic (2L, 114k miles)
The fault ties up the fuel and temp gauges with the overdrive lock lamp. Any advice or guidance greatly appreciated.

Car is stored when not in use with the battery disconnected. (Always starts first time when taken out :-).
BUT after 5 mins or so the fuel and temp guages suddenly drop to zero and stay there plus the overdrive Lock indicator lamp (orange arrow up) comes on and stays on.
When the button is pressed on the gear shift the light now goes out (instead of on !).

So far I've tired
Replacing the gearbox relay (the one down left of the steering wheel)
Replacing the power transitor on the back of the instrument panel that feeds the fuel and temperature sensor.
Testing out the fuel guage sender in the tank (current and voltage ok).
Taking out the radio circuit fuse (original radio finally died last year - awaiting replacement).

Just wondered if any of the really experienced 240 pros have any idea whats going before I start a full 'foresenzic' electrical diagonstic job.

Any insights or comment geatly appreciated.

Thankyou.

Check the small wire that comes from the alternator direct. I had a problem like this recently.

probably an earthing problem on the instrument panel , does the tachometer work? add a separate earth wire to the black wire which goes to the 3 way plug on the back of the Tachometer ...

We occasionally get some strange electrical faults. The fix for us is always to check the ignition key has returned to its proper position after starting.

Chances are it's a dry joint somewhere on the PCB on the back of the instrument cluster. If i don't use my 760 for longer journeys for a while, i get the OD warning light coming on by itself, using the button in the side of the shifter sorts it out though.

If it performs ok after a short period of activity, i'd suggest the dry joint is getting moisture in from the atmosphere, once dried out with some use everything is back to normal. Could also be slight corrosion on one of the instrument cluster plug connectors.

Hello

Has this been solved/resolved please?

Stephen

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