Ping Ray O- 4AC update

I have not seen the circuit, so this wild-ass guess may be 100% wrong. But it smells like a 5-volt stabiliser chip is not doing its job. Either it has blown (and AFAIK semiconductors usually fail short-circuit, meaning the unregulated input is being sent straight through to what should be the 5v regulated output) or the ground connection has become open-circuit (which could be caused by (eg) a mechanical break or corrosion).

The physical form of stabilisers depends on the power handled: either what looks like a low power transistor (three wire leads going into a plastic blob) or a medium-power transistor (three robust wires going into a flattened package, whose other edge has a metal tab screwed onto metalwork, commonly with some insulation between the tab and metalwork).

The voltage at the connector for the temp sender should not be fluctuating. If the voltage going to the temp sender is fluctuating and the impedance of the temp sender changes with temp, then the gauge will be inaccurate.

There are 2 possible causes for the voltage fluctuation, and Andrew Stephenson's is probably the most likely, with a damage wire between the back of the instrument cluster and the sender being second most likely.

The repair manual has specifications for checking impedance at the back of the gauge, and I'd bet that the impedance is incorrect. If it is within specs, then you can run a new wire between the gauge and the sender.

Thanks for the help/suggestion.

Cheers

Yep, that'll be the plan!

Many thanks

You're welcome! Let us know what you find.