VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 1931 Fuel Gauge Question

went back out to a friends house to finish of some work i started a month or so back on his horn. we got it all wired up and work. then he asked if i could look at his Fuel Gauge, as it did not work, he has to us a stick to check the fuel level.

I had my meter, and followed the guide on a previous post


i know i validated and checked how the one on the 1930 works, i know it is a 30 Ohm.

so on the 1931 we turned off ignition, pulled the line from the fuel sender off the back of the gauge and then turned on the ignition. gauge did not moved, stayed at empty. tried a few times, removed wire, put back on, validated we had no voltage if ignition was off, 6 v with ignition on. the gauge did not move.

went to the back of the car and used my meter on ohms to check to see if the fuel sender was functioning. nothing, read 0 ignition on, ignition off. looks like it was replaced somewhere along the line as it has a neoprene style black float and and modern black rubber coated wire coming out. validated it had a good ground to the base of the unit. i opened up the tank fill and moved the float up and down a few times, it was floating in the gas at nominal level. no matter what meter did not move.

did not drop the tank or anything, but went back up front and if i jiggled the gauge stems on the back and messed with it some on occasion the gauge would jump to full and stay there, then randomly drop back to empty. validated wires wire not an issue by removing them and running a jumper wire.

based on this it seems that the sending unit is shot and needs to be replaced, and that the gauge is not functioning properly. i know that i can get a replacement sending unit 30 ohm from TFS or Gary no problems. problem is can the gauge be rebuilt or do we need to search for a replacement ?!?

So this is for sure a 0-30 ohm gauge? Just like a later Chevy? If so.....

The disconnected sender measures in ohms, from it's post where the gauge wire connects, to the flange where it mounts to the tank. On ohms your meter should read infinity when not connected. If digital, maybe "OL" or something. Short the leads together. The meter should read almost zero.

On an analog meter, zero the meter with the zero adjust knob. On a digital meter you will be measuring the resistance of the leads, and that is what you will see. If your digital meter has a "delta" function, push it while the leads are shorted together, and the meter should go to zero. If not, memorize or write down the number you see.

Now test the sending unit. It should be very close to zero with the float down, and 30 ohms or a little more with the float up. If you had a digital meter with no "delta" function, subtract the number you saw with the leads shorted from both your readings.

Now about the gauge. With the key on, and the wire disconnected from the sender, the gauge should have pegged past full. That is like having the wires apart on your ohmmeter. It is basically infinity, and the gauge only needs 30 ohms to get to full. If you short the sender wire to ground, that is zero ohms (or close) and the gauge should go empty.

If it does not behave like this, you still have a problem (as you suspected) other than a sending unit. Either the gauge is missing power or ground (it needs both, so hanging out of the dash it wont work), or the gauge is bad, or the gauge wire is shorted to ground somewhere between the gauge and the sender.

When you get it figured out, be sure to add a ground wire to the sending unit, whether it needs it or not. Put it from a sender mounting screw to the frame.

Exactly what Bloo said. I find more often than you think it’s a bad gauge and not a bad sender. Inside the gauge there’s a fine single copper wire to the post and many times I’ve found it broken. I resolder the wire and the gauge works. Just remember, full ground reads empty if the gauge is working. No ground or no wire rom sender attached, gauge should pin over full. I’ve probably repaired a dozen gauges to a couple senders even though you’d expect the opposite.

...sender...

disconnected the sender wire going to the dash. i connected my ohm meter up to the unit, one to ground (screw on base of unit) the other to the threaded stem (wire out) my meter read zero NOT O.L. cleaned up contacts, swapped around wires, etc... always read zero. so there was continuity in the unit. used a stick and moved the float up and down, nothing. did this a few times, same results.

did the exact same test with same meter on my 1930 and it reads around 8 ohms about half a tank of gas, using a stick and moving the float up and down the reading changes accordingly.

...gauge...

i removed it and cleaned up the contacts, reinstalled, verified there was a good solid ground (continuity & ohm check). also verified there was +6v to the ignition stud on the gauge when key was on. when i remove the fuel sending wire, nothing happens ignition on or off. but if i moved hte sender wire around on the stud the gauge would sometimes jump to full, sometimes half, etc.. nothing consistent. but reading ohms on that wire to ground i get zero on the meter not O.L.

did the exact same test on my my 1930, when i remove the sender wire from the gauge, and turn on ignition the gauge pegs past full, turn off ignition, goes back to below empty. consistently. I connect the sender wire to gauge and it reads around half a tank with ignition on, below empty with ignition off. consistently.

did a continuity check on the wires for hte unit & gauge. all check out fine, even jiggled them around to check for a short in the wire, nothing.

Yep, sounds like they are both bad, just like you thought. The sender is literally shorted. You might be able to clear the short somehow, but over the internet it's so hard the guess what to do next. As for the gauge, it sure sounds like you probably have a broken wire inside like Chistech spoke of. Maybe it can be fixed.