I have a 86 fiero gt and the coolant gauge reads nothing most of the time, sometimes after hitting a bump in the road or going down a gravel road it will flicker on for a short stint then go back to not working.
While the car is running if I push the ignition key forward slightly the gauge comes on and works as intended. I've already replaced the coolant temp sensor and check the gauge cluster for a loose or broken connector/connection between the pin and gauge. My dad said it was probably the ignition switch module( the box the rod from the key switch itself turns into) and before I go into replacing the ignition switch module does anyone else have a clue as to what it could be?

Welcome, gauge could be dying or connection on sending unit loose, this was just posted yesterday...

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 jumper wire from green/yellow wire (for the gauge needle) to engine ground to see if the needle pegs. (if not gauge is dead)

[This message has been edited by Skybax (edited 09-15-2021).]

Sure you replace right sender/sensor? Fiero V6 have 3...

If gauge "works" when starting the car...
Likely isn't anything in the S-column.
OE wiring makes the gauge to slam the "top" of unit.

Notice the coolant Light Does Not Turn On at same time? This is because of a "Defect" from GM that wired both wrong.

See https://www.fiero.nl/forum/Forum2/HTML/145468.html

At the Sender plug, ground light wire to block/frame, light should turn on. Should not need to remove the plug to jump back of terminal.

Sender for gauge is Resistive to block. 1365Ω = 100°F/38°C 55Ω = 260°F/127°C
Test at sender needs to pull the plug a use above resistor or any value in between to see if gauge reads. Gauge wire - resistor - engine block
If nothing the pull C500 next to battery and test at C2 on body side.

Get resistors from old dead radios etc. Many newer things have surface mount parts and not useful here.
Careful remove w/ a lighter and small pliers.
Should easy find 1000Ω (Brown Black Red) and smaller google color codes.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

quote Originally posted by Skybax: Welcome, gauge could be dying or connection on sending unit loose, this was just posted yesterday... 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 jumper wire from green/yellow wire (for the gauge needle) to engine ground to see if the needle pegs. (if not gauge is dead)

Sorry for using the wrong terms, I replaced the coolant temperature sender. And I'll be sure to try these tests as soon as I get out of work. Thank you for your advice

quote Originally posted by Tyome Jackson: I have a 86 fiero gt and the coolant gauge reads nothing most of the time, sometimes after hitting a bump in the road or going down a gravel road it will flicker on for a short stint then go back to not working. While the car is running if I push the ignition key forward slightly the gauge comes on and works as intended. I've already replaced the coolant temp sensor and check the gauge cluster for a loose or broken connector/connection between the pin and gauge. My dad said it was probably the ignition switch module( the box the rod from the key switch itself turns into) and before I go into replacing the ignition switch module does anyone else have a clue as to what it could be?

Mine did the exact same thing. What I did was change the coolant gauge sensor, switch the wires around at the sensor and disconnected the battery for 20 minutes. After everything hooked back up, the gauge worked perfectly and still does. It might be worth a try.