[PrestonH22]

I am trying to install a vdo oil temp gauge. The gauge is reading much lower than the actual temp of the car.

These senders are variable resistance based so at different temps they give off different resistance values. So at room temp it should be about 700 ohms and when e fine is up to temp about 80 ish ohms.

When I test the sending unit on the engine case it reads 700 ish as room temp and when engine is hot it reads about 80 ohms so the sender appears to be working fine.

When I test the resistance at the gauge I get ohm readings that are way too high. Such as many thousands instead for couple hundred. This happens when I measure the resistance across the wire( coming from sender) to a chassis ground under the dash. I want to note that this only happens when the car is running. So when the car is off I get the values I should at the dash. But with the car running, I get the values that are way to high.

Can anyone explain to me why the car being on would change the resistance value when grounding to the dash?