The Stovebolt Site Forums The Shop Area The Electrical Bay Fuel guage sending unit ?

I took out my gas tank today as it needs to boiled and cleaned. Lots of tar like stuff from old gas sitting in there as well as some rust. The sending unit float is pretty much deteriorated to nothingness and so far, the gears are not turning. I have the unit in my ultrasonic cleaner and hopefully it will free up. And hopefully, the sending unit still works, the electrical part anyway. To test it, will I just read the ohms as the arm goes up and down to see if there is a change?

Yes, not sure what your truck is but most of the sending units are 30 ohms.

Do yourself a favor , deep 6 tank and sending unit , buy a new ones ....

Gazz,
If you have an ohmmeter, you can take the sending unit out of the
tank, and measure the resistance from the center terminal to the
housing (disconnect the wire from the sending unit first!)

It should read 0 ohms when the float is all the way down, and 30 ohms
when the float is all the way up, and the resistance should change
proportionally as you move the float up and down. The actual readings
full and empty may be off a bit, but they should be within a few ohms
of spec.

Check the float to see if it has a leak and is full of gas, if so that will
make it sink and read low all the time.

Check to see that the wire to the gauge is not shorted anywhere; it
will read low all the time if this is the case.

Check to see that the sender is properly grounded to the truck's body,
it will read full or past full if there is much resistance.

Check to make sure the sender and power wires on the gauge are not
switched.

Check to make sure the gauge housing in the dash is properly grounded.

Harold

Great info here Harold. I'm having trouble with mine. I replaced the sender, but the fuel gauge only drops about 1/8 of a tank below full and goes no further. The ground seems to make no difference. Isn't the unit grounded via the tank and tank straps anyway? When I disconnect the fuel gauge wire, the reading goes all the way to the right and out of sight. When I reconnect it, the needle comes down but never past 1/8 of a full tank even if it's empty. I could sure use some help on this one. Both the old sender and the new do this. Thanks!

Wish I knew your name, it is nice to know who your are talking to (guess I'm from the old school).

Try this site. You may get additional info.
Sending unit repair
Harold

I am having a similar issue as o2bnkc. I have a rebuilt sending unit, and a new Jim Carter sending unit. both test out just fine (30 ohms full, 0 empty) but my gas gauge never gets higher then slightly above half. I have tested them both with a ground cable and without, it seems to make no difference. I know the float is not getting hung up, but I can't figure out what the problem is! BTW they do register empty accurately.
Curt

Curt,
Did you read the info I posted above?
This is part of what it says:
If the gauge still reads incorrectly, the wire from the tank unit to the gauge may be faulty. Check by using a jumper wire to ground the tank unit terminal on the gauge (the one not connected to the ignition). If the gauge now reads empty or below, the problem is in the wire leading to the tank (or its connection to the gauge). If grounding the tank terminal still doesn't cause the gauge reading to drop to empty or below, the gauge is faulty.

When the gauge reads lower than it should, check at the gauge by disconnecting the wiring from the tank unit. Turn the ignition on and if the needle reads full or above, the gauge unit is probably good and the tank unit or the wiring leading to it are probably the problem. Reconnect the tank unit wire to the gauge before proceeding.

It takes patience just try the procedure a step at a time.
God luck,
Harold

Harold, I did read the post. I installed a new wiring harness recently so the wiring is good. I am getting the same ohm reading at the sending unit, and the gauge. Today I adjusted the float level slightly and was checking the ohm's, when I noticed that they dropped significantly when I hooked up the gauge wire. This happened if I had a ground wire, or relied on the sending unit bolts to ground. Without the gauge wire connected to the sending unit I was at 28 ohms, once I connected/touched the gauge wire it dropped to 10ohms.
I am thinking the gauge housing is not correctly grounded, or the gauge its self has a problem/short? What do you think?
Curt

With the sending unit out of the tank. Not to tough just pull the seat back off and you are there. Put one lead from the Ohm meter to the center connector and one lead to the mount ring. With the lever all the way up and with lever arm moving down. Should read 30 Ohms to 0 Ohms. If it does then check the float and see if it floats. Mine was a submarine. So all I ever saw was "empty". The other one that I fixed was a "low floater" It was always about 1/4 tank off of full. About half full of gas. Sorta a slow leaker. The last one I worked on had a bad gauge. Sender was good, wire was good, and the ground was good. The wire from the ignition switch was good to. Last thing left? The ground on the instrument cluster. It was good so all that was left was the gauge. Took about a whole darn week end to figure that one out. Stick with it you will get it. And, Thanks!, to so many on this site who let me know what to check.

FYI I have had problems with the universal sending units from MACs. I used one in a tank I made for a '52 3100 and when it went bad I assumed when I welded near it I burned it up. I took the bed off replaced the sender with another from Macs and it lasted a couple of months when it too went bad. I pulled the bed off last week and replaced it with a different suppliers unit.
To test I operate them with gage out of dash and sender in hand, i repeat each test as the pieces are assembled, the final piece being the gage returning into the dash. I also run a ground jumper to a screw of the sending units' flange as if the tank were plastic.