The Stovebolt Site Forums The Shop Area The Electrical Bay Fuel Tank Sending Unit

Is my understanding correct that the fuel tank sender is supposed to be ~30 ohms when full and ~0 ohms when empty? I've been trying to diagnose a gas gauge that reads empty all the time. I've checked grounds, power at the gauge, etc, just about everything I can think of, and finally I checked resistance at the sender. I made sure the tank is 100% full, and I'm getting 0 ohms resistance. Could be a sunk float or bad sender? Working on my 1954 3100. fuel and electrical systems are all original.

Thanks in advance for the help.

BB

Is there a good ground going to the sending unit? I believe one screw has a brass contact under it for a ground ring termial.

Yes, I have verified that there's a good ground to the tank. I also jumped from the sending unit to the negative post on the battery. Neither corrected the condition.

There is a simple way to verify if the sending unit is the problem or the gauge/wiring. Ground the wire (at the sending unit) that goes to the gauge. No need to disconnect, just use a jumper from a known good ground. That duplicates the full signal from the sending unit. The gauge should read full. If if stays empty, the gauge and/or wiring has an issue.

Note: This test does not eliminate the sending unit issues but with troubleshooting, you have to start somewhere.

It sounds like a failed sender or float. I'd remove it and see. If you want a sender which reads an accurate 0 to 30 ohms and is designed well, there is a seller on eBay (his ID is renbusan) who sells a sender designed and made in Taiwan that is very good and about $29 last I looked.

Stock tank? or under bed

That's sorta what I thought might be going on. I'll pull the sender to see. Thanks for the recommendation on the new sender!

I have a similar problem, in process of putting together a 51 chev 3600. Bought a new wiring harness from LMC truck. Put everything together, gasgauge did not work. I reversed the wires on the back of the gauge, didn't help. The needle just stays near empty, moves slightly when power is applied. Eventually took everything out again. Sending unit reads a nice smooth 0 to 30 ohms when tested, gauge reads 27 ohms. Hooked up gauge, sending unit, and battery with same result. Ordered a new gauge from The Filling Station, does not work when applying power. Checked new gauge, reads 65 ohms, does not seem to be correct, even though it is labeled for 6 volts.
Any ideas?
A rough schematic would be nice, haven't been able to find one.

Schematic? One gauge terminal gets battery voltage. The other terminal gets connected to the sender. If you supply voltage to the wrong gauge terminal you could fry the gauge.
Gauge needs grounded. Sender needs grounded.
Bryan mentioned that his tank was grounded. Just to be perfectly clear, it's NOT the tank that needs grounded, it's the sender flange. One of its screws needs a special wire to the truck frame.
All persons installing new gas gauges and senders need to test it all together on the bench before installing.
I don't think some of the folks here understand how to test. Especially when a new part allegedly "does not work". We would have to see pics of the test or have the exact test procedure explained including how grounding and power was arranged. Vendor Part numbers of parts. Without details, this will be a guessing game and very hard for us to identify the problem. Ground, power, the gauge, the sender, mis wire.

Remove the float and shake it to see if it has gas in it. Some aftermarket brass floats have bad solder which will fail rapidly, leading to empty all the time.
I replace the floats with mid 70s jeep floats. They are plastic and clip right into the aftermarket and original float arms.

Failed sending unit signal is frustrating for sure. I too have a new wiring harness, new gas gauge, and new sending unit on my ‘57 3200. I tried the direct ground suggestion and my needle went to empty. Does that mean my wires are reversed on back of gauge?

My gas gauge was reading empty most of the time but would occasionally jump to correct reading.
Finally discovered seat frame was making intermittent contact with terminal post on sending unit

Did you see my post in general truck talk?
I found that the metal seat frame was touching the wire terminal
the sending unit

At empty, the sending unit in your gas tank should read 0 ohms or as close to zero as possible...it may still show an ohm or two of resistance, but that's ok for now. A reading of zero ohms means no resistance, and that means your sending wire (which goes to the side of your gauge with the red tag) will be sent straight to ground. And this will make your gauge read empty. At full, your sending unit should read 30 ohms or as close to that as possible. That means your sending wire will be connected to ground through 30 ohms of resistance, and that will cause your gauge to read full. So it sounds to me your gauge is doing what it should and now the only thing you need to do is check your sender to see if (at full) it is giving you 30 ohms. Some of the sending units I've seen are made so poorly they don't even come close. I've suggested a very accurate sending unit about a dozen times here. The seller's ID on eBay is renbusan and the 0 to 30 ohm sender is made in Taiwan. For $29 it is excellent and should last a long time.