Potential wiring problem

Gliderider said:

Temp gauge redlines and motor shuts off,

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Make sure battery cables are good & snug. (both ends)- Sounds like a possible ground issue if it started after a very rough road.

FWIW, I just rewired my entire engine harness (which includes the temp sender wire - Dark Green, and usually there is two that are wired in parallel) and swapped my sending unit out and my temp gauge does the exact same thing. It's a cheap electrical gauge from the 80s, I wouldn't trust it even if it was correct. But I also know that seeing the gauge pegged is not comforting.

Is installing a mechanical gauge (for your sanity) out of the question?

It is worth considering. I’ve actually been having a problem with the temp gauge and sensor for several months. My sensor quit working and personally I don’t like not knowing how hot or cool my engine is operating, so I replaced the sensor. When I did that the truck would start normal , run a few seconds then the gauge would redline and truck would quit. I figured faulty sensor so got another one. Truck would start normal, run for about a minute then redline again and truck would shut off. So I just unhooked the temp wire from the sensor and have been driving it like that. Now the gauge is redlining without even being hooked up and truck won’t start. There has to be a hot short somewhere I’m thinking

Gliderider said:

It is worth considering. I’ve actually been having a problem with the temp gauge and sensor for several months. My sensor quit working and personally I don’t like not knowing how hot or cool my engine is operating, so I replaced the sensor. When I did that the truck would start normal , run a few seconds then the gauge would redline and truck would quit. I figured faulty sensor so got another one. Truck would start normal, run for about a minute then redline again and truck would shut off. So I just unhooked the temp wire from the sensor and have been driving it like that. Now the gauge is redlining without even being hooked up and truck won’t start. There has to be a hot short somewhere I’m thinking

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I completely agree on wanting to know the actual engine temp. But I am not aware (someone correct me if I am wrong) of an ignition cutoff based on engine temp in these trucks. Oil pressure cutoff is a thing, but as far as I know, you could theoretically overheat your engine until it got so hot that the engine locked up. So, I think it may be coincidental that this problem is accompanied by a no start symptom.

That being said - you can easily test that the sender is working correctly. Remove the sender from the cylinder head. If you have a multimeter, simply set it to the resistance setting, connect one lead to the body of the sender, the other lead to the spade connector (where you hook up the wire), and check the resistance. Then let the sender soak in hot water and watch the resistance change, will work the same way in reverse if you use cold water.

These senders have a range they work in, something like 200(low)-2k(hot) ohms, or something - not sure the resistance range, but it can be found online. I may also have that backwards, but it doesn't matter.

If the sender is doing its job, then the next weakest link is the gauge itself. As I stated above, my gauge is the same way. It's a lot easier to slap a mechanical gauge in than rip apart the cluster to swap to a new gauge. Plus the mechanical is going to be more accurate than an electrical resistive sender from the 80s. Hell, a lot of these trucks simply came with a dummy light(warning light) that would just come on when the engine was above a certain temp.

If you need assistance with wiring let me know. I just got done doing all this a couple months ago (minus swapping my gauge for a new one)

Try tapping on the gauge, I'm willing to bet you can make it move.

Your sensor and gauge should not have anything to do with the engine shutting off.
Disconnect your battery and check ALL of the connections at the starter solenoid AND the power block on the firewall. You are likely losing voltage at one of those two places OR losing the power to your dizzy, which will come from your ignition switch.

I’m in the process of essentially checking connections on everything. So far I’ve found one broken vacuum line, I found that when I disconnect the tach line into the HEI distributor the temp gauge goes to 0. And also found a mystery connector hanging down behind my motor right beside the bell housing. It’s not exactly a plug, but it is some kind of plastic connector. Inside of it is a wire that is just a loop. I’ve sent a pic of it to a mechanic friend, he has no idea what it is for

Gliderider said:

Yeah that was a poor choice of words on my part. I was just trying to figure out what would cause the motor to quit and the temp gauge to redline at the same time. I keep thinking it’s a ground issue but so far I’ve found nothing

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lol…

There’s something I can think of that would result in the temp gauge pegging and the engine not running at the same time.

To clarify, does the engine turn over when trying to start?

Yes it does

I'd check that repair you made. Also, you can always run a new wire - of course getting to that cluster sucks.

My response to a couple of weird electrical gremlins on my '84 K20 was to weigh the true costs. In other words, did I want to spend a lot of time during the next years trying to track down inevitable 40-year-old electrical problems (that could potentially leave me stranded), or just rip it all out and start over. I took option #2, used a harness from Painless and upgraded a couple of other things while the work was in process (like a new starter and a 140-amp alternator). Time-consuming but absolutely worth it, as it's like having a new truck. Everything works exactly like it's supposed to. A side benefit is that you learn a whole lot about your truck.

I feel your pain, but the scorched earth approach can be very liberating.

Completely replacing the wiring harness would be my preference, as it seems I’m constantly chasing faulty wiring. On top of dealing with this problem I’m also trying to find out why my right taillight isn’t working either. But as of right now it’s not possible. I recently went on disability so I don’t have the money to spend, but got a few small spools of different gauge wire, so for now I’m just patching what I have to

Only a few things that will cause your issue
1) temp sensor is bad ( you said its new ) if you bought a new part and its from standard electric i have gone thru 4 once to get one good one. (2) the wire from the sensor to the guage is grounded out to negative could be the body or some other ground. (3) the gauge itself went bad. Pull the cluster disconnect the 18 pin connector at the rear of the cluster remove the single wire ( my service manual says its yellow and black it may not be my book is from 1986 ) from the connector reinstall connector and see what the guage does. You will have your answer

I have considered the temp sensor being bad. This is actually the second new one I’ve bought and neither has worked. I think I’m better off checking the wire itself and the gauge like you said. To me, all the symptoms point to something in that system grounding out