Factory Temp Gauge quit working. Where should I start?

[72skytop]

I am working on a basically stock 1971 Chevrolet Cheyenne Super 10 with a big block 402. It has dual temp sensors with one in each head. The gauge has work fine up until this point and I haven't worked on anything recently to have given it reason to quit...I looked in the 1971 Service manual but that book is kinda different than later model GM service manuals in that there is really no trouble shooting procedures to find the problem on anything...just disassembly and reassembly instructions....But anyway... the only reason I am starting here is because I drove the truck no less than three years without an amp gauge working only to find out that all it was, was one of the little fuses was blown that are located on each side of the radiator and that was all it took to fix it. I just wanted to check with you to see if there is a simple, almost always that's what it is, fix before I start tearing into it.

Thanks in advance for taking the time to type in all your suggestions.

Chevrolet truckingly yours,

Kevin Korff

[4tiresngas]

I'd like to know also. My guage just stays in the center. I'm not sure it is doing anything. I have a new sending unit and a new wiring harness on the truck.

[72skytop]

My needle is all the way to the left. It acts like it never warms up. It moved up off of the left side before....It probably moved up about a quarter of the way over...unless it was hotter and it idled in traffic...then it went to the half way point....Wish us BOTH luck....

[pjmoreland]

It could be one of three things.

1) Gauge is bad
2) Wiring is bad
3) Sender is bad

Turn your ignition on (engine doesn't have to be running), disconnect the green wire from the temperature sender located on the driver head, and check the needle. It should move all the way left. Then touch and hold the green wire to something that is grounded, like the head. The needle should peg all the way to the right. If the needle doesn't move at all, then there is either something wrong with your wiring or your gauge. If the gauge does move, then there is probably something wrong with the sending unit.

[4tiresngas]

Hmmm, my needle did not move. I have a new harness. The dash cluster is refurbished. Probably the gauge, huh?

[pjmoreland]

Seems like it would either be the gauge or the wiring. I would inspect the wiring first. I cannot seem to locate a schematic that shows the wiring for the temperature gauge. All of my schematics have idiot lights on them. I'm guessing that the green wire from the sender runs to one of the two terminals on the back of the temperature gauge, and the other terminal is probably connected to 12 volts. If this is correct, you should be able to disconnect the green wire from the sender and measure 12 volts on one of the terminals (ignition on). You should be able to measure continuity between the other terminal and the end of the green wire where it connects to the sender (again, with the green wire disconnected from the sender).

Also with the ignition off and the green wire disconnected from the sender, you should not have continuity between the green wire and ground. If you do, then the green wire is probably shorted somewhere.

[Derrickbwg]

where does that green wire come out of the harness? Is it on the heater side or the brake side of the engine compartment? Mine currently has an aftermarket unit that is plugged into the intake, I can't seem to find the factory wires from the harness. And finally, does it terminate in some type of plug on the engine side?

[4tiresngas]

Derrickbwg, the green wire comes out of the engine compartment wiring up by the distributor. Mine has a puch-n connector. My sending unit is located on the driver's side head near the front. My green wire has a heat shielding material on it also.

[VetteVet]

The wiring schematic in the Haynes manual shows the gauges are all fed by a pink wire from the fuse block which would be hot with the key on. Each gauge has a different color wire on the other side depending on the application.
VV

[4tiresngas]

If you are looking at the front of your dash. The 12v is on the right-hand side of the gauge. I did not get continuity between the sending wire (green) and the left side of the gauge. I did get continuity from the end of the sending unit wire to where it goes into the fuse box (under the hood, left side plug, upper right wire (green)). I'll chase the rest down tomorrow...getting dark here.

[pjmoreland]

Sounds like you're getting close.

[byrd]

I was going to say I think you have a problem with the sending unit wire under the hood, it is probably broken and not grounded out. quik check to see if the wiring and gauge is good is too short out the sending unit wire to the block of the engine,then turn on the key the temp gauge should peg all the way hot, if not then it is probably the wiring between the sending unit and the fuse box. You may want to check the resistance of the resistor to see if it is ok it should be around 52ohms iirc and may be off a decimal piont, but that should be close enough to make the gauge work. Randy

[72skytop]

TO PJMORELAND:

Thanks for the troubleshooting agenda. It worked. It was a bad connection onto the sending unit itself...The metal clip inside the black plastic cover on the end got jammed up into the black plastic cover and was not contacting the sending unit terminal. Thanks for your help. I am just glad it was something simple for a change. I wish everybody on "the board" good luck in all your repair endeavors, and may F#*d's be a stranger to you.

Thanks again.

[4tiresngas]

Okay, I found my problem also. I checked continuity of the sending unit wire to the fuse block..it was good. I checked continuity from the back of the fuse block to the #6 pin on the dash plug in...it was good. I checked continuity from the #6 pin to the left nut on the back of the gauge...bad. Across the gauge was good. And grom the right-hand nut back up to the dash plug-in (Pink- Hot) was good. So, bad printed circuit. I have another one on the way.