I have a 1986 GT. When I got it a couple of months ago, the temp gauge was not working. I tried a bunch of things. Finally, I replace the coolant temp gauge sensor, unhooked battery, switched wire positions at the coolant sensor. Hooked up battery and the gauge now works nicely. One post said to take off instrument cluster and switch wires. Luckily I didn't have to do that. So if your temp gauge isn't working, try that first.

Using correct terminology to not get confused, on the V6 the coolant temp sensor is for the computer and located in front of the engine. The coolant temperature sender for the dash gauge/light is located behind/below the ignition coil. Most older cars have a gauge or a light, but the Fiero has both, so the 2-pin connector is just that, one for the gauge and one for the light. The factory connector often degrades over time and you can buy a replacement connector, Delco #PT110

To check and see if everything is working correctly...

1. Check resistance on sender terminal opposite of the cutout/notch (for the gauge needle) it should measure 3.5 to 4.5 ohms at 68 to 72 degrees F.

2. Then put a jumper wire from the green wire (for the red dash light) to engine ground with the ignition on and the red light should illuminate.

3. Now the other terminal, put a jumper wire from the green/yellow wire (for the gauge needle) to engine ground to see if the needle pegs.

That is how everything should work.

PS: The switching of wires behind the instrument cluster is a completely different issue, that is a mod to correct a factory flaw in the original GM wiring engineering, that flaw being the voltage being routed incorrectly which causes the temp gauge to peg/slam every time you start the car, and eventually it damages the gauge. You don't have to switch wires behind dash to correct this factory flaw and prevent gauge from pegging/slamming on startup, that is a more complicated and permanent mod. There is an easier and safer way, all you have to do is disconnect/remove the double light-green wire from the ignition switch located on top of steering column and tape up the end so it doesn't touch anything when left hanging under the steering column (I actually taped a note on mine so future owners who find it know why I did it). This bypasses the red temp light bulb test feature on startup and the gauge will no longer peg/slam on startup and the temp light will still work, it just won't light up anymore briefly when you start the car.

[This message has been edited by Skybax (edited 09-14-2021).]

"Temp gauge fix" to fix gauge slamming when starting the car is Either...

You switch wires at the Sender AND in the Dash so Light and Gauge works "right."
(Or at C500 and in the Dash.)


Switching wires at C500 is likely "safer" because Sender Plug body likes to break. Plus Engine wires stays the same colors for people working on them later. Switch car side of C500 and if engine wire is replaced for any reason, the "fix" keeps working.

OR

You Cut/Delete Gauge Wire to the Ignition Switch on the S-column.
Optional you splice the I-switch end to the Coolant Light. That often needs added wire from I-switch end to reach the dash plug.

Delete is drop the S-column to pull the plug for the I-switch to release the gauge terminal. This is nondestructive.
Optional can add new end if have extra "pigtail" from other plugs on other GM cars to "fix" the light.

What you did to fix "dead" gauge isn't same and have little to no way to know what you actually did.

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(Jurassic Park)

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