Troubleshooting the gas tank gauge.

Decided to come up with a way to test the gas gauge, beyond the shop manuals simple but flawed way.

If your gas gauge doesn't work at all or reads low, the shop manual instructs to disconnect the feed wire at the back of the trunk wall, behind the filler door.   This will cause the gauge to spike to past full. Then ground the connector, and the gauge will spike to empty.

On my own car, this proved to be a false indicator of a good gauge, My gauge read about 1/4 tank low, at all times.  Swapping the sending unit didn't work.  I swapped a gauge, and it worked, and has worked for a while now.

This is how I devise a test for the gauge.

The sending unit rheostat reads at 1.5 to 30 ohms.  I picked up several different types of resistors from radio Shack for some testing.  Here is what I found;

30 ohms,  resistor reads full on a gauge.

15 ohms reads 1/2

10 ohms read 1/3 tank.

I have several gauges, so I have backed up the readings. 

I then tested the known bad gauge. (I dont throw away anything)  The gauge read low, like it had in the car. remember that it passed the shop manual tests.

The packs that I picked up were 10 ohm and 15 ohm 1/2 watt.   I soldered the 2 15 ohm resistors together to create the 30 ohm.


So, with that said, get your self a 5 pack  15 ohm  1/2 watt  resistors from Radio Shack.    Since your gas tank is already out,  put the resistors in line at the feed line, where it hooks up to the gas tank sending unit.  Move your way up, until you have found the problem. I believe I twill be a bad wire, or a bad gauge.  If the gauge even reads  1/8 low, with the resistors, the gauge is bad, or a bad wire.

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Comment by Russ Austin on May 25, 2015 at 1:30am

Here is my test panel.  I put 3 bolts in the panel, that are insulated from the panel. Al I do is ground the panel, and touch the feed wire to one of the bolts.

The test panel also tests the headlight switches that I refurbish.

Comment by Russ Austin on May 25, 2015 at 1:25am

The pictures make it look complicated, but all you need is 1.50 for the resistors, and just hold the resistor against a ground, and the feed wire.

Comment by Russ Austin on May 24, 2015 at 11:27pm

Yes Jason, the brown feed wire to the gas tank sending unit  is either disconnected from the part, or at the connector behind the license plate door.

You would turn the ignition switch on to test the system.  I have a test gauge I use to test the sending units that I repair, so I have to power it with 12V

I'll get some pics of a cleaned up work site later.

Comment by Jason Edge on May 24, 2015 at 9:51pm

Russ, Looks like a good approach to test. Just a couple of questions and maybe we can post to our help page. I am assuming you are unplugging the connector on the fuel gauge and testing with the resistors directly connected to where the brown wire would have connected to the gauge (outside if I recall), and you have a 12 volt source where the pink and black wire connected. A couple pictures would be great. I use to work at Radio Shack part time and should have some of those 15 ohm resistors. You can just twist the wire leads together and disconnect as needed.

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