[11th Indian]

I found a replacement gas gauge for my 72, the original face-plate was worn and rusty.
The replacement is from another vehicle of undetermined origin. (GTO, GP, 70, 71 72 etc...)

My question is are the gauges from different years that look exactly the same, electrically the same?

Can I just switch the resister on the back of the gauge from my exiting unit, but the movement is otherwise the same?

Thank you!

[Forrest]

69-72 lemans/gto should be the same.. do not change the resistor..best to show a picture of your replacement..it would help..are you using "rally" gauges?

[11th Indian]

Here is an image of the 2 gauges. I do not have the Rally cluster.

My original gauge measured approx 50 ohms, the replacement was closer to 100.
The two had a different resisters across the terminals. Changing the resister on the new gauge to the one from the original made it 50 ohms[IMG][/IMG]

[Forrest]

there is a coil in the gauge ..I believe the resistor is matched to each coil...most tank sending units are set at around 90 ohms....I would try to test the gauge before installing it in the dash

[Peter Serio]

The measurement with the resistor mounted in-place on the 2 threaded copper studs should be right around 44 or 45 ohms.

You can't measure the actual resistor value with it connected onto those 2 threaded terminals. You have to remove it first. The resistor by itself should measure at 90 ohms +- 2%

Is is common for older resistors to change value (from specification) as they age. Most often the ohms increases (more resistance) as those get old. If one goes completely bad (open) it will measure infinity.

In the year(s) of your car & those 2 gauges. Those mid to late 1960s-early-mid and late 1970s Pontiac fuel gauges are all built around a common theme. As long as your sender is working & the gauge has a solid ground at the housing. And your printed circuit is good and the car's wiring is good, you can swap that one out in exchange for the rusted gauge. I would carefully clean the threaded mounting studs using a small brass brush. 2 of those are made out of copper and the other one should be a silver or gray color.

[Forrest]

Good info Pete.. I have replaced those "resistors" in the past..and fixed the problem.. and other times found that the coil was bad..there is two style of resistors, the bare wire wound ones and the sealed color coded ones (I believe tan for gas gauge)..I should say that he should at least check the operation of the new one before installing it.. if the old one was working, hopefully the new one will show the same movement..would be a bummer to install it and dash also to find out there is a problem with the new one

[Peter Serio]

You can bench test any fuel gauge, all you need is 12 volts DC battery power and some jumper wires.

For sure test that before you go to all the work of putting it back into the cluster. Another thing I like to do is replace all of the hardware which are those 6-32 nuts (with flat washers) that hold the studs. Those washers & nuts give a electrical pathway to the copper lines on your PC. After close to 50 years those tiny nuts are usually rusted or corroded. Always put back together with clean hardware.

You will need a way to substitute ohms in place of the sender. I like to test my fuel gauges at least 3 points on the dial. Zero or one ohm = E. 44 or 45 ohms = 1/2 scale and 89, 90 or 91 ohms = Full.

I built a small tester box with a rotary selector switch on top that gives me a choice of different resistance for the stuff I work on all the time.

[Forrest]

Hey indian.. if for some reason you don't get what were saying (we engineers get carried away!) please just pm us or ask for more laymen s terms!..just be sure it works before installing! good luck!

[11th Indian]

Thanks for the info gentlemen!!

The replacement was successful.
I have a large assortment of resistors from past electorates projects, and that was a great idea to to do a bench test.

My updated dash is fully functional.[IMG]http://[/IMG]

[Forrest]

Looks beautiful!! looks like you got your A$$ (engine) covered!>>>glad we could help..