1978 GMC Sierra Temperature Gauge

J_TAG

MEMBER
1978 GMC SIERRA
5.7L
V8
4WD
AUTOMATIC
105,000 MILES

The temperature gauge is not working. It was working prior to the engine swap, but I put the same exact model of engine in it. I have pulled the wire from the temperature sensor and grounded it to the block to see if the gauge would "ping" and it does, so that tells me that the gauge should be working properly. So then I replaced the temperature sensor switch 3 separate times and still the gauge does not even budge. Am I missing something else here?

5 Replies

HMAC300

MECHANIC
48,601 POSTS

If you put silicone on the threads of temp sensor it won't work you need pipe sealant or Teflon tape. Your wire leading to temp sender should blink on and off with low voltage and when you ground it out the temp gauge should go to hot.

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Thursday, July 30th, 2015 AT 4:02 PM

CARADIODOC

MECHANIC
33,916 POSTS

Measure the resistance of the sensor from its terminal to ground. You should expect to find around perhaps 500 ohms. The exact value isn't critical. What's critical is you have something and it's not open.

If you find an open circuit, you either have the switch for use with a dash warning light, not a gauge, or you used teflon tape on the threads to provide a better seal. That tape will insulate the ground and prevent a solid connection.

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Thursday, July 30th, 2015 AT 4:41 PM

J_TAG

MEMBER
3 POSTS

I used Teflon tape. When I ground it it does go all the way to hot. I'm not sure what you mean by "the wire should blink with low voltage"

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Thursday, July 30th, 2015 AT 6:40 PM

CARADIODOC

MECHANIC
33,916 POSTS

We were typing at the same time, and sometimes you get answers from two of us. We don't mean to double up and confuse the issue. What Hmac300 is referring to is the sensor is supposed to ground a signal voltage coming from the dash gauge, and you can measure that. Regardless, it sounds like you already know the problem is the sensor is insulated where it screws into the engine. Plug the wire in like normal, then use a piece of wire to touch the metal body of the sensor to a paint and rust-free point on the engine, and you'll see the gauge start to work.

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Thursday, July 30th, 2015 AT 9:30 PM

J_TAG

MEMBER
3 POSTS

Just tried that today with no luck still. So I removed the Teflon tape and tried it again and it didn't work. I left it like that, threw my hands in the air, and purchased a mechanical temp gauge. There is also something draining the battery every couple of days if I don't start it a couple of times a week. Those problems could be connected somehow. I get frustrated with electrical problems, so instead of sifting through hundreds of wires to find a faulty ground I just put a "kill" switch. It's just a farm truck but that should kee my battery going and give me an accurate temp reading, I may return to the problem once the frustration of it all goes away a little