[This message has been edited by theogre (edited 06-22-2020).]

The correct use of an Ohmmeter includes touching the probes together prior to use, to see if any parasitic resistance exists in the loop. If you don't have a REL button you can do the subtraction manually. But typically I see 0.1 Ω (on Fluke, BK Precision, and whatever else I use), so this is not a problem for taking measurements in the tens of Ohms. If you have more than this, you have a problem with your setup. For lower-resistance measurements, use a 4-wire setup.

Here my resistor is installed:


I'm not expecting perfection, but I am expecting better than what it was before! Before, the needle was either touching or in the red zone during normal driving. I don't know what went wrong, but anything is possible on a 35-year-old car. Maybe it's the aftermarket sender I used? Anyway, I will test this over the next week, comparing with the ECU's temperature reading.

To a first approximation, the fuel and temp gauge accuracy does not depend on the system voltage. For a single gauge, the needle orientation is determined by two coils, so as the system voltage varies, the currents in both coils remain proportional to each other. I guess you could add a regulator if you wanted to stabilise things more, but I don't think the complexity is warranted. I did not observe any difference in fuel gauge reading with engine running/stopped.

As for a quality tachometer restoration, you should replace both R and C. I agree that C should be changed for a film capacitor, but don't forget to change R as well.

[This message has been edited by pmbrunelle (edited 06-23-2020).]

Here is what I used for my temperature gauge solution:
Resistor: Digi-Key ALSR3F-47-ND (I wrapped the leads around the studs and cut off the excess lead length)
#6 nylon washer: Digi-Key RPC6806-ND
Standard zinc-plated steel hex nuts, #6-32 (these were from my parts bin)

Take note that the fuel gauge resistor has some different requirements:
1. The plastic insulation around the base of the studs rises above the level of the metal can. The stock ceramic resistor includes counterbores to clear the insulation, so it can sit flat. A PCB should have large holes to clear the insulation, or should include counterbores/countersinks as appropriate.
2. When the fuel tank is empty, the fuel gauge resistor will see the full system voltage, whereas the temp gauge resistor sees a fraction of system voltage. So watch out for power dissipation.

For a PCB-based solution, I would not use a through-hole resistor like I used. Aside from being a non-standard value (for Fiero gauges), the solder joints on the bottom side would touch the metal can. Not good. So I would look at a single-layer PCB with surface-mount resistors. Possibly two in series/parallel to allow for better fine-tuning, and more power dissipation.

To keep things simple, I think you could build boards with a "GM default" resistance value, whatever that may be.

Then, people who want to fine-tune their gauge could do something like I did:

Replace the stock resistor with a multi-turn pot:


With the engine running, adjust the pot such that the gauge's reading agrees with reality:


For temp, use ECU data.
For fuel, fill the tank to the brim.

Depending on the resistance that is found to be ideal, power dissipation could increase/decrease vs. the GM default resistance values, so I suggest putting the biggest SMT footprints that will physically fit, so as to give the most flexibility with regards to power dissipation. For folks who want to solder their own fine-tuned resistor values, it may be easier to hand them unpopulated PCBs.

[This message has been edited by pmbrunelle (edited 06-23-2020).]

[This message has been edited by theogre (edited 06-24-2020).]