temp gauge pegged hot

[blue72k10]

I have a 72 K10 that I'm redoing. Finally got around to putting in a tach cluster I've had for 20+ yrs in the truck. The orig one the temp gauge never did work as I thought it should so I replaced the sender with one from LMC before I installed the tach cluster. Did an op check on new cluster before install to make sure it was good. Tach worked as it should and temp gauge pegged hot when wire was grounded and fuel gauge worked. Now when key is turned on the temp gauge pegs hot and stays there. Unplug sender and gauge goes cold. Tried the ground wire trick someone suggested that didn't help. Any thoughts or suggestions appreciated.

[pjmoreland]

The good news is that your new gauge demonstrates the ability to move in both directions. Also, the wiring seems to be functional due to the change in gauge position when you disconnect the wire.

What is the part number of the sender you ordered? I just want to make sure you didn't end up with one intended for a warning light instead of a gauge. It should have a sideways connector on it and not a straight blade connector.

I wonder if you got a dud temp sender from LMC. Would be good to measure the sender resistance. Unplug the wire first, and then measure between the post and body. There's a graph that shows resistance vs. temp on post #10 of the following thread.

https://67-72chevytrucks.com/vboard/...d.php?t=725468

Also, there should be no sealant used, such as Teflon tape, on the sender threads.

[72SB]

agree the sender is either bad or one for idiot light vs gauge

[blue72k10]

Thanks for the thoughts. The sender is 36-0634 from LMC which shows to be for gauges 68-72. The other interesting thing is that this one is in the rt front of the left head for gauge but there's another in the rear of the rt head which I guess is for a light? On the dash nothing lights up when the keys turned to start. I'm gonna call LMC and see if I can get the specs for that sender. Thanks again and have a great day.

[pjmoreland]

Based on the description, it should be the correct sender. Does it look like #1 in the image on the LMC website?

[blue72k10]

Yes it does look like #1. I don't know if a broken plastic connector is an issue as long as the connector doesn't touch any metal.

[blue72k10]

#2 is in the right head as well.

[pjmoreland]

Originally Posted by blue72k10

Yes it does look like #1. I don't know if a broken plastic connector is an issue as long as the connector doesn't touch any metal.

Shouldn't be an issue.

On a side note, your Brake warning light should illuminate when cranking the engine.

[blue72k10]

That's not happening as well. Wonder if that's related?

[pjmoreland]

Originally Posted by blue72k10

#2 is in the right head as well.

That other sender on the passenger side might have been for a TCS (Transmission Controlled Spark Advance) system.

[blue72k10]

I have no idea what that is and don't remember any electrical stuff on the turbo 350. Truck was a 4spd from factory.

[pjmoreland]

Originally Posted by blue72k10

That's not happening as well. Wonder if that's related?

The brake warning light and the temp gauge get their power from the same place. Your temp gauge appears to be getting power since the needle is moving. There's a wire that comes from the ignition switch that completes the circuit on the brake warning light when cranking the engine. Maybe it's as simple as a bad bulb or corrosion on the bulb contacts.

[blue72k10]

Thank you for that info. I'll look into that issue after I get the temp gauge handled. According to LMC the ohms reading should be 220 ohms @ 100 and 51 ohms @ 250. I'll go try your suggestion at the sender first.

[blue72k10]

Just looking at the graph it appears that the sender may not be the correct resistance.

[pjmoreland]

Originally Posted by blue72k10

Just looking at the graph it appears that the sender may not be the correct resistance.

It does look a bit off at the low end, but those specs aren't far enough off to make the gauge peg hot. It would just cause it to read slightly above cold prematurely.

[blue72k10]

I'm not very good electronically. You might say I'm very challenged electrically. Thanks for your help and thoughts. I find this forum very helpful when I run into issues such as this. I don't have the resources here that I had up north in Idaho.

[pjmoreland]

I measured the resistance at room temperature on three senders I've got, and here are the results:

718 AC Delco
740 Napa
525 Factory? (old & crusty)

[blue72k10]

interesting to say the least. I'll go out and try the ones I've collected over the years. Thank you

[pjmoreland]

Based on that graph, your LMC sender would have to be below 260 Ohms at room temperature to cause the needle to peg hot. Maybe it has an internal short. You could try out one of your other senders on your truck without actually mounting it in the head. You'd just need to connect the signal wire to it and then press the base of the sender to a good clean ground. Might need an assistant to view the gauge while you do this or take a video of the gauge with your phone.

[blue72k10]

I'll go give that a try after I get a few errands taken care of. Thanks alot.

[blue72k10]

Ok just got my homework done. Took my trusty multimeter set it at 2000 and went to testing. The new one in block reads 002 new one in box from LMC ordered two great to be old reads 536 old one from motor 1240 What's all that mean?

[pjmoreland]

Sounds to me like you found the problem. That new one in the block shouldn't be reading 2 Ohms. It's gotta be shorted inside. I would try installing the other new one from LMC that reads 536.

[pjmoreland]

Just to double check, did you unplug the wire from the installed sender before measuring it?

[blue72k10]

Ok so if I understand what you said about testing a new one Take the connector hook it up and ground the body to frame/engine and then see what the gauge does correct?

[pjmoreland]

Originally Posted by blue72k10

Ok so if I understand what you said about testing a new one Take the connector hook it up and ground the body to frame/engine and then see what the gauge does correct?

Yep