I am trouble shooting a non-functioning temp gauge. It always reads low. I have put in a new sender and get the same results. When I ground the sender wire the gauge reads full hot as it should.

My question is - What should the sender resistance read when eangine is cold and what should it be when normal?

 

I did some experimentation on this a while back. Althought he full data set is at home, here is an except from a post I posted long ago...
This should help you.

I am using a stock replacement sending unit (borg warner WT203) and it reads 100 Ohms (disconnected) and 6.11V (connected) at normal operating temp. I let it cool and took measurements along the way. The gauge middle line is 130 Ohms and 7.03V. A tad above 'C' was 320 Ohms and 9.4V. Completely cold (60*) was over 1K Ohms. It does not seem to be a linear scale.

The auto parts store probably sold you a switch for the idiot light. (Common issue...)
You have to specify it is for the gauges (rally pac).

 

Thanks Rob, They sold me a gauge sending unit but it reads 6000 ohms cold and about 2000 ohms normal. I think I have either a bad sending unit or the wrong unit.

I am no where near the numbers you are quoting. I will put some resistors together and see if the gauge responds with say a 100 ohms. I know it goes full sacale when grounded.

 

According to my data, 100 Ohm occured at 195*F.

78.5 Ohm was 212F and was the H line of the gauge scale
134 Ohm was 174*F and was the middle of the gauge scale.

You have a bad or wrong unit...

 

Rob thanks, this gives me a basis to work forward on. I will use some resistors to simulate the sender and make sure that the gauge is functioning as well as getting a new sender.

Thanks again.

 

Rob, after further research and discussion with the Tachman repair I find I was on the wrong scale on the ohm meter.

I am reading 600 ohms at 70F and 190 ohm at 140F. So the sender is good but the gauge will not respond with down to 50 ohm connected to test the gauge.

I am going to get the gauge repaired.

Thanks

 

50 Ohm should have it pegging the H side...
Did you try measuring voltage at the sender with the gauge wire attached? Some of my readings are above. I am curious to your results...