New member here and would first off like to thank everyone for all the information posted here. I have a 1941 Special Deluxe that’s pretty much original. The gas gauge was displaying empty with a full tank. Here are items I tested: - Tank is grounded - gauge cluster is grounded - removed wire from sending unit and gauge pegged full - grounded wire from sending unit and gauge pegged empty - removed sending unit and testing displayed 0 to 30 ohms through range - extended ground wire from tank to front seat and attached to sending unit then grounded sending unit, gauge responds to moving the float up and down. Problem here is gauge displays aprx 1/4 full thru aprx 7/8 full thru the range. Any ideas? Would that be a gauge problem? Also I believe the sending unit is ok since it test from 0 to 30 ohms but I may need to replace the float since gauge displayed empty when in a full tank. I would appreciate your thoughts! Bob
I agree that the float might need replaced. If it is a cork float they will absorb gasoline and not float at the correct height.
A key point is that these fuel level systems are fairly inaccurate. Think of it this way. The tank is not a perfect rectangle. You have a pivoting arm that is trying to measure a linear depth. And that motion is running a contact over a wound rheostat. There is lots of approximation happening.
Another key point is whether you have an original or an aftermarket sending unit. Sometimes the arm on an aftermarket or universal sending unit will be so long that it cannot travel the full range in the tank.
Thanks for the reply! It does have corks for the float. It appears to be original. It is marked AC and made in USA. I have the unit in hand. Connecting it to the gauge wire that goes to the tank and moving the float by hand is where it moves from aprx. 1/4 full to aprx. 7/8 full.
Ethanol in the gasoline will quickly penetrate the cork and it will no longer float on the surface of the gasoline in the tank. Any corrosion on the coil of the sending unit will cause the resistance to change resulting in even more inaccurate gauge readings.
You posted that the sending unit reads 0 to 30 ohms when you move the float through the full range. You also noted that the gauge reads empty when you ground the gauge wire at the sending unit.
The fact that the lowest reading was 1/4 tank makes me think you had extra resistance in the wire that you used to extend the sending unit wire or you did not have the sending unit connected to a good ground when you tested it where you could also see the gauge.
Run that test again but with a second person watching the gauge while you are at the sending unit near where it mounts to the tank. Use a good 16 gauge wire to ground the sending unit housing to the chassis near the mounting.
One othe test is to check the end to end resistance of the wire from the gauge to the sending unit. You need to disconnect both ends.
FWIW don't automatically blame the floats. This is the sending unit currently in my 38. It's an OEM unit. It's been in there for almost 20 years with the cork floats and it works just fine. I burn mostly E10 fuel.
VCCA Member 43216 Save a life, adopt a senior shelter pet. 1938 HB Business Coupe 1953 210 Sedan
Thanks for the replies! So I removed the tank wire from the gauge and it pegged full. I then attached a separate short wire to the gauge post and the other end to the sending unit post and a ground wire from the battery to the mounting surface of the sending unit. The outcome varies a little here. Sometimes it will post full and other times it stops short. It also posts full more consistently if I tilt the sending unit to the side. I noticed that the float lever has some play where it enters the rheostat housing. Could this worn area be causing my problem?
I expect that the contact that runs against the wound rheostat wire is probably worn and loose in its holder.
I would spray electrical contact cleaner into that area and cycle the arm through the full range quite a few times. That might remove some oxidation and give a more consistent reading. Do this with the unit disconnected from power and let it dry before re-connecting it.
Those corks sure look like that have seen better days. Some people adapt a brass float to the arm.
You need to shorted the float wire. The stroke is way too long for the depth of the tank. The way I did mine was bend it 90deg one direction then 90deg the other. Take into account the depth of the tank (measure it while the unit is out) to dictate the stroke of the wire. This isn't rocket science but it does take some thought. You'll have to tweak it a little up or down but it'll get close. Good luck.
One is getting the gauge to read full range with the sending unit out of the tank. So the float arm is not being restricted by the tank. What is confusing to me is that he noted that the sending unit reads 0 to 30 ohms when he moves it through full range. So that should make the gauge go full range. I am still thinking about that one.
The other is the gauge reading empty when the tank is full. I think that is because the cork float is not floating.
The Empty/Full issue is probably a product of having power on the wrong side of the gauge.
Hook up the sending unit (in your hand) to the gauge wire at the tank and ground the chassis of the unit. Lift up the arm to the full position and look at the gauge. If it's backwards (showing empty) swap the two wires at the gauge. Power to the other terminal of the gauge, etc. See if that fixes it.
