Mystery Tachometer

^Learned something new today. Didn’t know the resistor affected gauge sweep. Thought it just kept the gauge from bouncing

Grit dog said:

^Learned something new today. Didn’t know the resistor affected gauge sweep. Thought it just kept the gauge from bouncing

Click to expand...

Yeah, I thought it was only to keep the gauge from swinging back and forth with fuel slosh.

But I know I've swapped around fuel gauges in multiple GM cars and trucks and they read correctly. Pickups, Burbs and K5's all interchange, regardless of tank size. I even had an '83 S10 fuel gauge in my '79 crew cab for many years.

Grit dog said:

^Learned something new today. Didn’t know the resistor affected gauge sweep. Thought it just kept the gauge from bouncing

Click to expand...

bucket said:

Yeah, I thought it was only to keep the gauge from swinging back and forth with fuel slosh.

Click to expand...

Andy you are not wrong, the resistor on the back of the fuel gauge is part of the design so the gauge does not bounce. I had to do some research to see why the old mechanic told me my GM C60 fuel gauge would read correctly in my C10 with the resistor from my original gauge.

The resister serves two purposes.

If you were to look at the internal layout of a gauge you'd find two coils wound opposingly and one coil directly across the said resistor. One coil is used to pull down the pointer towards empty as the voltage across it increases (this coil has the resistor directly in parallel with it). The tank sending unit and the wirewound/ceramic resistor form a voltage divider that varies the voltage across each depending on the position of the wiper which is connected in the float arm in the tank. The voltage across the resistor changing helps "buffer" the movement rather then trying to drive a coil directly. The other basic operating coil opposes the change wanting to pull the needle to full.

So, this being stated, a longer variable resister in the tank might need a slightly different ceramic resistor value.

Older fuel gauges had a pulsing voltage regulator.
More info:

That's why the simple GM design is great. You can't guarantee system voltage, but you can give constant set values of the resistors in which the voltage drop ratios will still give accurate readings. In other words, it doesn't matter if the input voltage is 9 volts or 15, the resistance values are constants and you will have a greater voltage across the resistor on the back of the gauge when the tank is empty resulting in more current in the opposing coil to pull the needle to empty. No need for complex and expensive voltage regulators like other car makers.

I finally got some time today to re-install the gauge cluster and wouldn't you know...problems.
The tach didn't work, the temp gauge pegged out when I turned the lights on, and all the gauge needles would bounce with the blinkers. After a couple hours of electrical diagnosis I found that the cluster had a bad ground and apparently the printed circuit board does not supply enough amperage to run the tachometer. A new cluster ground and a separate key on 12V to the tach later and everything is working perfectly. I upgraded all my backlights to led bulbs since the plastic was all burnt/melting at the bulb locations. I even found all the wiring for the heater controls and factory radio (which sadly doesn't work, but it does light up). Thanks to everyone that helped here!

Excuse the engine in the video...it was stone cold and I still haven't set up my choke correctly. Plus I have some more carb tuning to do. Yes, the fuel gauge is correct...it's running on fumes lol.




I also ordered a new gauge bezel from ebay for $12!! I have no idea how it can be sold for that cheap, I was expecting to get an empty box but it came today and it rivals the OE bezel in quality:


Pick one up if you need one. They have them with AC too for like $40. The box said it came from Holley.