Car is an 87 350 TPI dual fan set-up. The temperature gauge is pegged to the absolute bottom and does not move ever.. I believe the gauge is controlled by the green wire on the drivers side manifold (connects to the *temp switch?* need to clarify that), Or would it have something to do with the coolant temp sensor? The green wire looks to be in good shape, not melted, etc..

Fan issues to come later..

 

Temp sender--(?)

The one that tell the gauge what to say-- has that been replaced?

IMHO, it is important to know the temp so it is known if the fan(s)
work & at what temp comes on..??...

 

As in where the green wire connects to on the drivers side, below the exhaust manifold? then no it has not been replaced (yet)

I also ground out the green wire and the gauge still does not move. I'm beginning to think the gauge is bad.

 

 

i have similar prob in 91 305 tbi except green wire is not present (see new posting). i am wondering if wire is broke and where it traces back to in wire loom. My gauge does properly peg on startup too.

 

 

grounding the green wire will not cause the gauge to move...merely read zero.
To move the gauge requires resistance. Try a resistor between green wire and ground. Gauge movement tells you that the sender has malfunctioned with this test. 50 ohms or so should give a reading...I'm unsure of the low and high values right now. Of course no reading would indicate a problem with the wire itself.
Hope this can be helpful.

 

Grounding the gauge causes it to peg. NINÅ already explained the easy/correct test above.

The ignition switch grounds the sending lead on the gauge causing the gauge to do a full sweep test during start (crank). Grounding the lead to the block should produce the same result.

That's how it is. It's worked that way on GM gauges since at least the late '60's.

 

Practically every tempurature sensor used in any automobile today is a "negative coefficient" thermistor. Negative coefficient means that as temp increases, resistance decreases. Therefore less resistance will cause a higher temp reading and grounding the sender circuit will peg the gauge. Careful with this test though as it can damage some gauges.

 

yeah my bad...got confused with the fuel gauge...been a while since I played with them. Been working on some warning circuits but on the shelf at the moment.
Apologies for my misinformation....wasn't thinking right.

 

Turns out my gauge was stuck! Had to remove the plastic gauge cover and move the needle. Good deal!

 

Im having a similar problem except that my needled goes all the way to to red zone but im certin is not overheating, i just changed the temp sensor, the fan switch,and the coolant temp sensor. any idea what else could be wrong???

 

 

that's the gm label for the circuit or, contact point.

f5 is in the firewall harness that NINÅ stated. pin 4 is the cluster plug (seen w/cluster removed). may be mildly different between cam & bird.