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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.