Coolant temp gauge.

AquaBeef

New member
Hello!
I just purchased a Syclone that has been sitting for 17 years. It runs and drives great but the coolant temp gauge is having issues. I'll run the truck and take it to work and back but the gauge will raise a small amount then drop down and not read. I have replaced the coolant temp sensor, but still has the same issue. I tested the voltage but it reads at 1v when I back probe it and when unplugged it reads at 3v, from my basic understanding these sensors have a 5v reference. Any help or advice on possible causes would be appreciated thanks!
 
Not sure about the voltage reference needs but what thermostat do you have in your truck? I run a 160 in mine and the gauge barely moves off of cold so I read my temp with my scanmaster. Is the rise of your needle slow like the truck is warming up or is it a quick rise followed by a quick fall? If the rise and fall is quick I would suspect an electrical problem either with the gauge or the wiring. If the sensor is bad it usually doesn't move at all or it will go straight to hot. If it is a slow rise like it is working properly and then a fairly rapid drop of the needle then it might just be some air still trapped in the block. The sensor will read the temp when it is in contact with the water but will cool off if it gets an air pocket around it.
 

AquaBeef

New member
Thanks for the response. All good info. It will show like it's not reading for a good 30 minutes at idle then it will show likes it's slowly climbing not even to the first peg on the gauge then back to nothing. I have drained and vacuum filled the cooling system. I'm starting to think it's more wiring related, I was looking into lowering the engine ecu and running an overlay to the sensor.
 

wildphil

I Love My Ty's
Dave will come on and be able to help you out on this. He is great with wiring on these trucks. At the top of this section is a thread with wiring diagram it may be useful to you. Here is a link.

 

DaveP's Ghost

Active member
I've never back-probed the temp gauge wire for voltage, so I don't know what it is. If there's voltage, the wiring is intact.

What temperature is the engine running at, and is it stable. What you describe is more of a thermostat or cooling system not completely filled scenario, not a gauge problem.

Scanmaster, or any scann tool will tell you what the engine ECM sensor is seeing, and you can compare the gauge to the actual temp.
 

graham1524

Member
None of the stock ones I've ever seen move much on the dial, if it isn't overheating I think it may just be normal
 

RealFastV6

@jb_and_his_coffee
Hello!
I just purchased a Syclone that has been sitting for 17 years. It runs and drives great but the coolant temp gauge is having issues. I'll run the truck and take it to work and back but the gauge will raise a small amount then drop down and not read. I have replaced the coolant temp sensor, but still has the same issue. I tested the voltage but it reads at 1v when I back probe it and when unplugged it reads at 3v, from my basic understanding these sensors have a 5v reference. Any help or advice on possible causes would be appreciated thanks!
There are two temp sensors, one for the ECM and one for the gauge.

The 2 wire coolant temp sensor for the ECM uses a 5V reference and is located in the intake manifold. This is probably the one you replaced.

The 1 wire temp sensor for the dash gauge is a single (green) wire. There is no 5V reference - just straight resistance to ground. It's located on the driver's side head. GMPN#25037332. You can't back probe it for voltage, you'd have to back probe it against ground for resistance with a 20k ohm meter.
 

AquaBeef

New member
There are two temp sensors, one for the ECM and one for the gauge.

The 2 wire coolant temp sensor for the ECM uses a 5V reference and is located in the intake manifold. This is probably the one you replaced.

The 1 wire temp sensor for the dash gauge is a single (green) wire. There is no 5V reference - just straight resistance to ground. It's located on the driver's side head. GMPN#25037332. You can't back probe it for voltage, you'd have to back probe it against ground for resistance with a 20k ohm meter.

Thank you for the info. Im leaning more toward the cluster side of this mystery... I found temp gauge needle will move now when I hit a bump... It will begin working after being jostled... May be a connector backing out or a ground issue. Ill let you know about my findings when I unsolved it.

Thanks
 
Also check the wire going to the sensor. It's in a pretty harsh place as far as heat and if your truck is like mine was, the lead is pretty short and takes a lot of stress. My wire had an internal break in it. I just clipped it above the break and extended the lead while I was at it.
 
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