86 C10 Temp Gauge Pegs Hot

[josh454]

Hello,

Having an issue with the temp gauge on my 86 C10. It seems that I have inherited the issue from a previous owner.

The gauge pegs to full hot the instant the truck is started or keyed on. I have verified with a mechanical gauge it is not overheating. I have tried two different temp sensors for two different applications (also different brands). I have also measured the temp senders with an ohm meter and they are reading 3,000 ohms cold and 700 while running. I have swapped the temp gauge and cluster from an old one that's been sitting around for a long time. I have also replaced the temp gauge with a new one with no change. I took the original cluster apart and cleaned the contacts on the printed circuit board.

The previous owner cut the green wire at the cluster connector and ran a wire straight from the connector straight to the temp sender, bypassing the bulkhead connector. I have pulled the wire out of the loom and there are no pinches/shorts. I have also ran a ground strap from the firewall to the engine and the engine from the frame to make sure everything is grounded.

With the engine running I have 12V at the gauge measured with the ground terminal. I ran a redundant ground right to the gauge peg with no change. I also measured 7V on the sensing peg of the gauge.

So wondering having tried 2 different sending units, running a new wire, trying a different cluster, replacing the temp gauge why it would be pegging out like this. One of the big mysteries I have found online is that the schematic shows the green wire running through the ignition switch. If this is correct, is there additional resistance being applied through the ignition switch? Is having it bypassed causing the issue. Hoping someone could verify that the ignition switch routing is correct and what it would do...

Surely GM didn't engineer that way without some reason.

Appreciate any thoughts or suggestions!

 

[Snoots]

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[josh454]

Isn't that a 67-72 style cluster? is the layout the same? I only seem to have 3 clips in the back of my plastic cluster.

Thanks,
Josh

 

[WP29P4A]

If you disconnect the wire at the sender and leave it hanging, (not touching anything) and then turn the key on, do you get the same results?

 

[75gmck25]

IIRC, with the ignition key on you should measure approximately 12 volts on the wire running to the sending unit. And if the wire is disconnected from the sending unit, the gauge reading should be at its lowest point.

 

[josh454]

Update if I ground the 4th terminal on picture from Snoots I am getting a normal reading. I don’t have a 4th clip in my cluster though…
If I unplug the sending unit it reads on C and if I short the lead to ground it pegs out

 

[Snoots]

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[Matt69olds]

Cars/trucks with idiot light have a green wire that goes to the ignition switch. There is a position on the switch between “run” and “start” called bulb test (there is no detent, you won’t feel it in the key rotation) that provides a ground for the hot lamp. You can test the oil and charge lights by just turning on the ignition. GM figures there needs to be an easy way to test the hot lamp as well.