The Stovebolt Site Forums The Shop Area The Electrical Bay Fuel Gauge

I see a warning label on the fuel gauge about not touching this (larger) post to hot or the gauge will blow. What is it exactly that blows? A fusible link maybe? And what is the gauge sensitive to? I'm thinking it probably doesn't like a lot of current, but I'm wondering how much is too much, and if you'd get too much from a hand held VOM (on Ohms).

Thanks.

Here's a great site with pictures of the internals of a legacy fuel gauge. That might help you understand the problem. Old fuel gauges use coils with very thin wire to create magnetic fields that move the needle based on current flow that creates magnetic fields.

Too much voltage will create far more current than those wires can handle, and they will burn through, making the gauge useless.

Ohms law says V=I*R (voltage equals current times resistance). So, put another way, I=V/R. If the resistance of a wire is 60 ohms and the voltage is 6 volts, then the current would be .1 amps. Increase the voltage to 12, and the current will double to .2 amps. If the wire is rated at .15 amps, it will quickly burn through.

Having said that, to blow a fuel gauge requires a complete circuit. Touching a single post to "hot" while the gauge isn't grounded or connected to anything won't do a thing. Touching a 12 volt wire to the larger post while the case is grounded might well burn the wires quickly.

And, as you can see if you viewed the pictures on the site I linked to, you can feel perfectly comfortable connecting a modern VOM to your gauge to measure the resistance of the coils.

Thanks for the feedback. What I'd been thinking was that any modern VOM will be current limited and probably to a current safe for pretty much any electromechanical device. After all, you can short the probes together to zero the meter, and not run down the fairly small battery. And I'd seen the site you pointed to, and they were using a VOM to check the gauge they were fixing. So I tried this on my newly re-manufactured gauge I'd just gotten on e-bay, to compare the resistance on my old gauge that I knew was reading properly, to the new one, to see if I'd need to add any extra resistance at the gauge to get an accurate reading. And oddly, the new gauge was reading open, right out of the box. So then I started getting paranoid and thinking maybe a VOM could damage the gauge. An I'd also seen some articles somewhere indicating some gauges had fusible links, and that might be worth checking since mine was reading open, if there were actually any links inside.

I can't think of a legitimate reason for putting fusible links inside a fuel gauge. External fuses do the trick just fine, and a fusible link would require the gauge to be opened up and repaired every time there was an over-current condition.

You new gauge may be reading open because it's transistorized. You won't be able to measure resistance across a diode. On the other hand, it might be faulty.

You can check it easily enough if you have the parts. Connect the gauge to a battery source and to a well grounded sending unit. (Make sure the sending unit has the right resistance for that gauge!) Then move the float mechanism up and down. The gauge should read as you would expect; full when the float is at the top, empty when it's at the bottom.

If you don't have a sending unit, you can probably get a rheostat with the right resistance at Radio Shack or any electronics parts store. Or you can buy a selection of resistors that would simulate various fuel levels from empty to full and connect them one at a time (between ground and the fuel sending unit connector of the gauge.

I agree that fusible links don't make a lot of practical sense; still, I'm sure I saw a thread somewhere indicating that some old gauges had them, and that was typically the first thing to go. But I'm thinking that this was not the case for this era GM vehicles.

I think that in the end, I know what to do. The ONLY thing that was working at all in the old gauge cluster was the fuel gauge, so it was obviously matched to the vehicle voltage, 12 V, and the sender. So I'll simply swap the old gauge into the new cluster. Kind of an odd thing to do, but probably the least work and most likely to work.

Thanks for the help,
Mark