How to check a fuel gauge?

[HanSoloWS6]

I just got my '82 Diesel home after replacing the water pump and the fuel gauge is either bad, or the sending unit is shot. I drove it a good distance, but the needle is about 2 extra notches past full still and I can't notice a change. Or it's really good on fuel lol.

Is there an easy way to check the sending unit without dropping the tank, and the fuel gauge?

[C-10volvo]

take the dash apart and clean the terminals. this is a common problem with 73-87's. to test the gauge itself i would think you could take a low voltage battery and see what it does (AAA or AA).

[boxrodz]

The fuel guage acts like an amp meter, so it senses the amount (and direction) of current based on the voltage applied across a specific resistance. So, once you gain access to the fuel gauge terminals, you can check your gauge using the power from a 12 volt source (i.e. battery set-up or the truck battery). Connect the ground terminal of the gauge to ground. Connect the positive terminal of the gauge to positive side of the battery. Then, connect a resistor (90 ohms or less) between the 'sensor' terminal of the gauge and the ground. If you use a 45 ohm resistor, you should see a '1/2 tank' reading. Other resistor values should give you a proportional reading on the gauge. You could also stick a 100 ohm potentiometer ('variable resistor') and vary the resistance to check the different readings of the gauge. But, usually with meters it either works at all values or not at all.

The sending unit acts like a potentiometer in which the resistance varies as the level of the fuel changes. If you can gain access to the sensor wire coming from the sending unit, you could use an OHM meter to check the resistance of the sending float (between the sensor wire and ground). You should get a reading between 0 ohms (empty) and 90 ohms (full). Other than that, I think that you have to drop the tank to get to the sending unit to test it further

From your description, it sounds as if you may have a bad (open) connection between the gauge 'sensor' terminal and the sending unit. When the ignition is swiched on and power is applied to the gauge, if a bad 'sensor' connection exists, then the gauge would read past full at the 3 o'clock position. You could check the connection between the connector on the gauge's 'sensor' terminal and the connector at the sending unit using a long wire lead and your meter (ohms). Connect one end of the long wire to the gauge 'sensor' connector and the other end to one lead of the meter. Connect the other lead of the meter to the sending unit 'sensor' connector. In the ohms setting, the meter should read 0 ohms for a good connection. If it reads infinite, then you have a bad connection. You can use the same process to pinpoint the location of the bad connection.

As C-10volvo said be sure to check you connections well.

Good luck.

[C-10volvo]

you took the words out of my mouth!!! hahaha

[us111]

hello,,also do a site search for fuel gauge,,you will find 10 other people at least with same question and answer's,,including me,lol (still aint fixed mine)

[HanSoloWS6]

I got a chance to put some more miles on the truck today, and it does work, it's just way off it seems. I now have a "full" tank according to the gauge lol. I'll do the checks when I get a chance, thanks for the info.

Another question is about the tank switch. I have the dual tanks, but I haven't been under the truck yet to check things out to see why I can't switch to the right tank.

It's an old truck and I'm sure I'll have a ton of little bugs to work out lol. A lot of things were pretty well rigged up instead of replacing the part.

[Tigger]

The fuel gauge was dancing in my Chevy when I first got it home also. Like an idiot I threw money at a new sending unit and installed it in the tank. Nothing changed but my wallet thickness. Yes, go to the sending unit last. Once the good weather comes I'm using your advice and cleaning the contacts of the gauge. Thank-You for the experienced advice .

Rob

[Slonaker]

My gauge moves very slowly at first, then picks up speed. It will go from 3/8 to 1/8 of a tank in about 10-15 minutes of driving. The closer it gets to empty the faster it moves. Weird, but you get used to it.

Slonaker