Oil Pressure Gauge Problem

[Brendo169]

Ive recently installed an oil pressure gauge and water temperature gauge in my 5L Clubby
I think the oil pressure gauge is reading wrong due to interference of some kind
Say im on the freeway the oil pressure might read 20 psi...if i then switch my headlights and stereo on the pressure needle will drop dramatically to say 5 or 10

My question is could this be a problem with the power supply to the gauge or the sender.
On the 5L v8's the oil pressure sender is located very close to the alternator (see photo)...could this be causing the fluctuating readings?

also where is the best place to earth out the sender
at the moment its earthing to the engine block (see photo 2). would i better off earthing it to the chassis.
I do not believe the earth is the problem as my temp gauge is earthed at the same point and is working fine

Would the next step be to get hold of a multimeter and check the gauge power supply with the lights on and off. Should the voltage remain the same regardless?

[jamessharks]

It sounds like an earth problem ... the block is not a good place for it you can see there is rust so that will be interfering with it ... the chassis would be better but best would be the negative battery terminal.

[Brendo169]

i changed the earth just now to a point on the chassis. Problem SOLVED!

cheers mate