Coolant Temp Gauge Question

[Kilian]

The coolant temp gauge on my 83 GMC C2500 is pegged at max temp, I'm assuming the problem is in the gauge and not the sender. What do I need to replace to get the temp back working? Can I just replace the gauge or do I have to swap out the whole instrument cluster. Thanks!

 

[Blue Ox]

A maximum reading can be caused by a faulty sender or a short to ground. Find the sender and pull the wire to see if the reading changes. It should show minimum temp with the wire disconnected. If it does the sender is probably NG. If it remains pegged the wire may still be shorted to ground somewhere.

If the gauge turns out to be bad you can just replace the gauge.

*Sorry, said pressure, meant temp. Fixed it.

 

[78C10BigTen]

If the gauge turns out to be bad you can just replace the gauge.

But to do that you still need to pull the cluster and disassemble it... correct?

 

[Blue Ox]

Yes sir. You are correct. I wasn't going to get in to typing a novel. But it's been sitting unanswered all day so I thought I'd give it a boost. Also, I don't think it's common for the gauges themselves to fail, but that's only my limited experience.

 

[78C10BigTen]

Yes sir. You are correct. I wasn't going to get in to typing a novel. But it's been sitting unanswered all day so I thought I'd give it a boost. Also, I don't think it's common for the gauges themselves to fail, but that's only my limited experience.

Well to my knowledge (that they havent been replaced) all my 40 year old gauges still work so.... (even the clock)

 

[Kilian]

Thanks for the replies. I will check out the sender to see if that is the issue.

 

[Kilian]

Checked the sender wire tonight and I have 12 volts at the connector. No change in the gauge when sender wire is disconnected. I plan on checking out the gauge next.

 

[chengny]

Checked the sender wire tonight and I have 12 volts at the connector. No change in the gauge when sender wire is disconnected. I plan on checking out the gauge next.

You are pegged high correct? What you need to do is look for is a short to ground in the DK GRN lead that runs from the gauge to the sender. It comes out of the loom behind the distributor and runs along the LH valve cover to up near the front. Then it drops down to the sender. Start at the sender end and inspect the wire all the way back to where it goes into the loom. Look closely for breaks/chafing in the insulation. If there is a short to ground in this lead, it is generally going to be in the engine compartment. Even if a ground exists in the DK GRN lead, you will still read 12 VDC at the sender. Also, disconnecting the lead from the sender - when a short to ground exists - won't cause the gauge to come off the high end.

 

[Snoots]

You can 'Ohm out' the sender.

For Truck Years: 1979 - 1990

The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 1,365 Ohms
Middle Line when sender resistance = 96 Ohms
Right Line (Hot) when sender resistance = 55 Ohms

 

[NICK K]

I have the same problem on my 85 GMC. I disconnected the wire from sender and gauge went to zero. replaced the sender and the gauge read correctly for a little while but now is pegged to the hot side again

 

[chengny]

I have the same problem on my 85 GMC. I disconnected the wire from sender and gauge went to zero. replaced the sender and the gauge read correctly for a little while but now is pegged to the hot side again

A false high reading is usually an indication that the gauge's sensing leg is shorted to ground. Trace the DK GRN lead that runs along the valve cover and connects to the sender.

This is just a WAG but it might be the case that, the original high reading was caused by a short to ground in the sensing lead rather than a faulty sender. But, during the sender replacement procedure you moved the wire in such a way that the short circuit was temporarily broken. Then after a while, the conductor again came in contact with the engine - and now you are showing high again.

 

[NICK K]

thanks
previous owner has left me a few wiring puzzles