I have noticed in Cessna 152 that as i switch on landing,taxi or beacon light oil temperature indicator goes high.
unable to get the possible cause.
It seems either a ground or positive supply unit. Check to see if you also have this when you switch on pitot heat, likely it will
A loose (free) wire to our starter engaged warning light caused our oil pressure to drop to near zero every time it earthed against the starter, but did not affect the oil temp side of the dual gauge.
Something to do with voltage or earth return resistance???
Yes; I would bet on a faulty earthing connection somewhere.
Presumably it happens with the engine not running 
so it should be easy to find with a voltmeter and a wiring diagram.
This is almost certainly a grounding issue with a possibility of a supply issue.
