Dash temperature gauge / coolant temp sending sensor

[SA_trucker]

I'm stumped here. 1998 K1500 with 294,000 on the odometer. I don't know exactly when, but at some point my gauge that tells me the engine temp has stopped working. Done some research and zeroed in on the sensor that screws into the engine block between two spark plugs - thought that had gone bad. Got a new one, swapped it out, nothing. I had left the teflon tape on there (not knowing I should have removed it), so after some more research I decided that was the issue so I pulled it and removed the tape, plugged her back in and still, nothing on the dash - gage is pegged pretty much to the left at 100.

I have tried bypassing the sensor by hooking up a wire to the negative battery terminal and into the plug (truck side) and the gage goes all the way to hot (260). I even tried tonight to plug the old sensor into the truck side (so that I didn't have to catch the coolant coming out of the engine) - grounded it back to the negative terminal since it wasn't screwed into the block, and put that in some hot water. Gage moves up to about the 1/3 mark, but I was excited that the old sensor actually MOVED the gage.

However, that leaves me confused more than ever. I don't think there is any residual teflon tape on the threads of the engine block (possibly blocking my ground) because before I ever put the new sensor in with tape on it. The old one didn't have any teflon tape on it that I could tell, and the gage used to work - I just don't recall how long its been since it quit reading.

Thoughts? Help? I don't know where the ground hooks up to my engine block - will need to consult google on that one. The truck starts and runs just fine. Just trying to troubleshoot weird little things like this to keep her running regular! Thanks!

 

[stutaeng]

Possible you have bad sensors. Or your cluster is bad.

The both coolant temperatures sensors can be checked. Haynes manual has a table with temperatures and corresponding resistance. Hook up your DVOM and dunk the sensor on a cup with iced water. Or heat it with a heat gun and watch the resistance.

Don't know how you would check your cluster though.

And, you can look at the other sensor that sends to your ECU with a hand held scanner that has live data. Hopefully that one is reporting accurately.

 

[kenh]

I think IIRC the temp gauges in these trucks is problematic. I put teflon on my sending unit without issue. Not a ton of tape just two wraps. Had to replace my temp gauge. My new to me gauge now reads correctly.

Ken

 

[mtl111]

Possibly a bad air core motor on the guage itself? I've had issues with the motors on the fuel guages and bought new replacements on ebay. Works like a champ now. Just a thought...

Sent from my SAMSUNG-SM-G930A using Tapatalk

 

[Schurkey]

zeroed in on the sensor that screws into the engine block between two spark plugs

Goes into the HEAD between two spark plugs.

- gage is pegged pretty much to the left at 100.

Low temperature/no ground

I have tried bypassing the sensor by hooking up a wire to the negative battery terminal and into the plug (truck side) and the gage goes all the way to hot (260). I even tried tonight to plug the old sensor into the truck side (so that I didn't have to catch the coolant coming out of the engine) - grounded it back to the negative terminal since it wasn't screwed into the block, and put that in some hot water. Gage moves up to about the 1/3 mark, but I was excited that the old sensor actually MOVED the gage.

You've proven than the gauge works, and the wire harness doesn't have an open circuit.

You've proven that the original sending unit works, but you haven't proven that it's accurate.

However, that leaves me confused more than ever. I don't think there is any residual teflon tape on the threads of the engine block (possibly blocking my ground) because before I ever put the new sensor in with tape on it. The old one didn't have any teflon tape on it that I could tell, and the gage used to work - I just don't recall how long its been since it quit reading.

Use a wire brush to clean the threads in the head. Liquid thread sealer is far better than Teflon/PTFE "thread tape".

I don't know where the ground hooks up to my engine block - will need to consult google on that one. The truck starts and runs just fine.

Mine is much older than yours, the ground cable connects to a stud on what used to be the fuel pump mount on the block.

Wouldn't hurt to run a 10- or 12-gauge wire from the driver's cylinder head to where ever the main ground wire from the battery connects.

Does the molded-plastic connector on the wire plug into the sensor securely?

Any chance they sold you a SWITCH (for an idiot light) instead of a sending unit for a guage?

 

[PlayingWithTBI]

Any chance they sold you a SWITCH (for an idiot light) instead of a sending unit for a guage?

^^^X2 - that'd be my guess.

 

[SA_trucker]

Schurkey-
I appreciate your analysis of what I’ve run through so far! My thinking is that I’m not getting a ground some how, so it isn’t reading.

This is what I purchased: (tried to upload from my phone but it says the image is too large). Part is a Standard, TS375 and it says “sender” as the part name. It looks exactly like the original - kind of spark plug like with one single wire coming out of the back which is well seated.

 

[Schurkey]

Seems to be the correct part.

Clean the threads in the head where it screws in. Run the ground wire from the head to either the alternator housing or the main negative battery cable. See what happens.

 

[east302]

GM part number is 12556263 so you can cross reference (1998 with 350).

Normal operating position on these is slightly more than halfway between 100 and 210.

You must be registered for see images attach

You must be registered for see images attach

Sent from my iPhone using Tapatalk

 

[thinger2]

Check the ground strap from the back of the passenger head to the firewall and from the firewall to the frame.
This is often overlooked yet is also the cause of many instrument issues.
If they are bad, and they are.
Get a couple of cheap 4 ga battery cables and replace those grounds.
Then you can chase the problem for about 10 bucks of investment.
No grounds, no input, tail chasing..