Problems with vdo gauges

[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?

[PrestonH22]

Update: After googling my way to becoming an electrician I learned that you can not measure resistance when the circuit has power going through it. That is why my readings were wrong when the car was running.

I am still lost as to why my gauge is reading much lower than it should be

[Dave at Pelican Parts]

The sender may not match the gauge. There are different senders with different temperature/resistance curves, and the gauge will be expecting one in particular.

Are you running the oil temp sender wire to anything but the one new gauge? If so, then that will mess up the gauge readings. If, for instance, you run the signal wire to two gauges they will both read identically, but wrong.

If you want to check the resistance range that the gauge sees, you can unplug the wire from the back of the gauge and measure from there to ground. Since there won't be a circuit, power going through the wire won't mess up your resistance reading.

--DD

[PrestonH22]

Finally figured it out. I was running 12 volt power to temp and pressure gauges from the same source. The power source must not have had enough juice to power both gauges. After disconnecting the pressure gauge the temp started to read normally. I guess I will have to find a separate 12v source for the pressure gauge