New guy needs with my temp gauge.

[Customdeluxe81]

Hi all, it’s my first time here and I’m not sure where to post this. So here it goes. I have my dads 1981 customdeluxe that he bought new. It’s a C10 variety with the I 6 250 and a three on the tree, I’m working on restoring her. The problem I have at the moment is the temp gauge is pegged at hot on the right and comes off just a touch when you turn the key off. So here’s what I tried, with the temp cold and the gauge still on hot I tried pulling the wire off the sending unit, turned key on and no change. The other gauges work as well as the dash lights. My question is this is there a way to bench test the temp gauge? Thanks I’m advance
Bill

[bd]

Welcome to the forum!

As the engine temperature increases the sender resistance decreases.  An open sender wire forces a COLD gauge indication, a grounded sender wire forces a HOT gauge indication.  Check the sender wire for being pinched, abraded, melted, or otherwise grounded.  The gauge dropping slightly when the ignition is switched off is immaterial since the gauge has no 'return to zero' mechanism in its design.

[Customdeluxe81]

Thanks, the wire is only exposed for about a foot then it goes into a hard plastic loom the kind that has a split so wires can be inspected. I followed it as best I could and couldn’t find anything and the loom remains in good shape and not pinched any place so as near as I can tell everything in the engine compartment is ok so if it’s a shorted wire then it must be under the dash. I do know how the sender signals the gauge as to temp. I’m looking for the process to bench test the gauge to determine if the gauge is working correctly or if it’s faulty.
Thanks

[dusterdude]

Welcome

Sent from my LM-X420 using Tapatalk

[bd]

Read through Functional Tests of Factory GM Electric Gauges and refer to page 8C-60 toward the back of the 1981 Service Manual

If you remove the sender wire from the IP cluster connector will the needle swing to COLD?

[Customdeluxe81]

Bd, that’s a lot of reading! And in the end I felt like I needed an electrical engineering degree to grasp it all. I’m an AC tech and I diagnose low and high voltage wiring issues every day but I didn’t grasp how to bench test the temp gauge from this . As for your question I have not started to diagnose from inside the cab yet.
Thanks
Bill

[bd]

Study page 8C-60 (and associated pages as needed) in the linked factory manual.  Use the wiring diagrams and illustration of the PCB on the backside of the instrument cluster to ID which gauge pins connect to B+, B- and Sender.  Using fuse-protected jumpers with the gauge on the bench, connect the gauge pins accordingly.  Then refer to...

For Truck Years:  1979 - 1987 (91)

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

...and substitute the given resistances or close approximations, in turn, between the Sender and B- pins while the B- and B+ pins are connected to battery power.  If the gauge responds as noted, it is within factory specs.

Simply disconnecting and then grounding the sender pin with the gauge still in the cluster is a fast, grassroots approach to verifying gauge function and determining whether the issue resides in the gauge or external wiring.

Note that a common problem is oxidation between the gauge pins, the pin contact clips, and the PCB, as well as, between the PCB and the IP harness-to-cluster connector, leading to poor electrical connections.  Often, burnishing those connections will resolve such connection issues.  However, the trouble you're experiencing points more to a grounded sender circuit.

[Customdeluxe81]

Alright I have the manual as well as the component manual but they are where the truck is right now and that’s a few hundred miles from me right now, but I  Stumbled into a used temp gauge and was just trying to save time when I get to the truck by checking this gauge out.
Thanks
Bill

[Customdeluxe81]

Thanks to all that helped with this. After I got the fuel system sorted out and the ol six banger running again, I spent some quality time with the service manual looking over the wiring diagrams and discovered that there are two wires in the engine bay that feed from the same terminal and are both the same color. And I found both were hooked up one to the sender and other to the idle solenoid! So I disconnected the wire on the solenoid  and found the solenoid wire and put that where it should be. And turned the key and gauge went to cold and after idling about 10 minutes the gauge was just a little above cold (I remember it always running cold, maybe the thermostat I think dad would run a colder stat then factory. So that’s one more thing fixed on the old Girl!
Bill