Inoperative temp gauge on a 1986 V6, it stays at the far left position, so first I disconnected the connector...

Checked resistance on the sender terminal opposite of the cutout/notch (for the needle) it measures 3.5 to 4.1 ohms at 68 to 70 degrees F.

- Correct right?

Then put a jumper wire from the green wire (for the red light) to engine ground with the ignition on the red light illuminates.

- Correct right?

Now the other terminal, put a jumper wire from the green/yellow wire (for the needle) to engine ground with the ignition on the needle doesn't move.

- I believe this is NOT correct, the needle should peg right?

[This message has been edited by Skybax (edited 05-04-2021).]

Yes, the needle should peg full right. You can also tell if the gauge is working by turning on the ignition. It should rest on the bottom of the thermometer and move to the 100 mark when ignition is turned on.

You know your bulb circuit is good, so it may be that it's actually your gauge that's gone south.

Ok thanks, I will check the ignition-on needle movement and post back...

Also, I haven't done "wire switch fix" yet to prevent needle from pegging because I want to get it working first, and I don't think it was done before I owned it, which is probably why the gauge isn't working now!

Update: I found answer to my key on / key off question in archives...

quote what the gauge says with the key off - Fieros use an 'air core movement' for the gauges. That is instead of using an electromagnet against a spring, they use an electromagnet against an electromagnet. The gauge won't become inaccurate as it ages because of the natural tendency of springs to weaken over time. But with the key off, there is no electromagnet, and thus the needle may just sit where it was with the key of or might move and then stop. What the needle does with the key off has no meaning with an air core movement.

So back to my temp gauge not working, I assume since the gauge doesn't move with sender grounded and/or key on, next step is to bench test the gauge right?

Found this is archives...

quote Once you carefully remove it (temp gauge) test as follows. A member helped me troubleshoot it using the following instructions. “To check it you will need a 12-13V power supply and a resistor 130 to 140 ohms. Looking at the back of the gauge you have 4 posts. The top center post is ground, the left post is 12+ volts, and the right side post is the reading in ohms from the sensor on the engine block. You place the resistor (130-140 ohms) across the signal post to the ground post. Hook up + an - from the power supply and the gauge should read around the 220 to 230 degrees.” You can buy resistor kits on Amazon/eBay in various values. Then you attach them in series and add the values together to get the resistance you want. Now you can reference the table again and the gauge should indicate the temperature based on the amount of resistance you’ve put inline. Approximately anyways, within a couple of lines.

Is there anything else I can check or I'm forgetting before pulling the gauge? (really don't want to pull that gauge out, lol)

Just wanted to post update on temp gauge fix and results for others that might use search feature in future.

Quick recap... My temp gauge didn't work and the sending unit/red bulb/wiring all checked good. So I bought a used working gauge and had somebody swap the guts and re-calibrate the gauge on the bench. Just for the record this V6 has a 180 thermostat with 210 fan switch that turns fans on around 215 and turns fans off around 205, so it runs 190 in cool valleys, 200 normal cruising, and 210 at stop light in summer or under load (all normal).

On my other V6 Fiero's I had 15-20 years ago, the gauge would be at far left temp symbol with key off, and when you turned key on cold the gauge would go to 100, and when you started the engine it would stay at 100 (if you fixed pegging needle but that's different subject). Once the engine reached full operating temperature (approximately 200 degrees) the needle would run between 25% and 45%. That is also what most people said in the archives.

After reinstalling my newly re-calibrated gauges this week the temp needle reads 25% too far to the right cold with ignition on. I was thinking about taking it back apart and reinstall without clear face, turn key on, remove needle from 25% position, and re-attach needle at 100 position. That's what most people did in the archives and that puts needle very close to what most people consider normal/stock function.

However, after thinking about it for a few days I realized there is an "error" with the factory 220 silk screen number. If you start counting at 100 and you count the lines using 10 degree increments, the halfway point is not 220 but actually 180, and if you continue to count the 10's you will reach the 260 perfectly.

So even though its not supposed to be/work this way, I think its ironic that my "25% cold starting-point error" is actually dead-on-balls-accurate after the 140 mark, if you ignore the printed 220 number. I did a quick Photoshop below to show what I mean, and I'm thinking about making a 180 decal to put over the 220 number instead of messing with the needle, because I'm just happy to have a working temp gauge!

PS: Also added an old-school [COLD---------HOT] to show that the printed numbers are really insignificant when you think about it.

[This message has been edited by Skybax (edited 06-24-2021).]

Even when dash temp gauge works "100%"...
Nearly no-one puts numbers on them even now.
IOW "Old school" gauge isn't old. Visit any junk yard and look at 2000 and later models.

Old setups w/ sender and gauge on own circuit is horrible for accuracy and part of why most had no numbers like Fiero did.
These Gauges will lie a little or a lot just for added resistance in the wire from sender to gauge or for iffy grounds anywhere in the car.
New gauges that are BCM/PCM controlled and use same ECT as ECM/PCM are better because has 2 wires to the sensor but still nearly no-one put numbers on the dash gauge.

When put numbers then every fool w/o a clue tries to put warranty work to dealers that waste time for nothing. Even taken to "3rd party" shops the gauge w/ numbers cause more problems. Many "Trade Rags" hated "Fiero" temp gauges w/ numbers for the same reasons too.

180° or 190° T-stant doesn't matter much or at all because you find most "old school" engines run at 210-220° running on a highway.
New engines can run even hotter because of many reasons and not just turbo/super charge engines w/ "same" 190° T-stat.
Can run hotter when driving city driving, up a big hill, towing or loaded and Worse when over weight meaning at or above GVWR.

Note: Yes, Air Core Gauges only work when key is on.
Temp, Fuel, Tach and Speedo are all Air Core Gauges for Fiero and many others and have No Spring to force them to "zero" when have no power.
These gauges can/will move randomly w/o power for many reasons too. Hit the dash can to that. Have anything w/ magnets near by can to same too. So can't trust Fuel and others gauges when key is off.
Other cars can use Air Core Gauges but have weak springs to "zero" when key is off. Springs here is weaker the magnetic force to run them and doesn't matter. Their Not like most 1 coil meter/gauges using springs.

------------------
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

Just an update to show what I did with the temp gauge for future members if needle falls off. After looking at a dozen Fiero's and finding a photo of my old Fiero from 18 years ago, all the temp gauges point to approximately the bottom of the Temp Symbol with the key off, and they go to 100 when key-on/started cold. Since my gauge was at 100 with key off and 1/4 mark at cold start, that put it into the Red Area after fans came on, so even though I knew everything was ok it was still too unsettling for my subconscious mind, lol. So I took the cluster back apart again, covered the 220 Symbol with a black decal, and re-positioned the needle from the 100 to the bottom of the Temp Symbol. Now it works great in the normal range...

(also did the "temp-light bulb-test bypass" so temp gauge needle doesn't peg on every startup)





PS: These gauges are not linear, first quarter is 20 degree increments, 1/4 to 3/4 is 5 degree increments, and final quarter is 10 degree increments (approximately).

[This message has been edited by Skybax (edited 07-15-2021).]