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 2:21pm

Mark, the voltage reading after the resistors is 3.36 VDC

Comment by Mark S Anderson on May 25, 2015 at 1:30pm

now with the car running and the resistor in place, what does a voltmeter read on the "upstream" side of the resistor?  this is essential information, since we could replicate your test. if the alternator voltage is normal and voltage reading before the resistor matches yours within a fine range, the circuit is good all the way so we can eliminate the wiring as a problem.

Comment by Russ Austin on May 25, 2015 at 1:22pm

Jason, you probably have a good gauge that you tested. It just reads low doe to low amperage.

Comment by Russ Austin on May 25, 2015 at 1:21pm

Mark, your absolutely correct.  I changed the gauge, and noticed that the gauge wasn't reading exactly where it should.  I started the car, and the gauge reads perfectly with the test harness in place.

What was I doing different?  I was bench testing the gauges with a 12V 40 amp power source.

The gauge I puled out bench tested bad, as it reads excessively high.  My problem was it would read way past the FULL mark , and not move for a while, then drop to the half way point, and move fast to empty.

With all that said, you need to perform the test with the car running.   

Comment by Mark S Anderson on May 25, 2015 at 1:01pm

Russ, I love the approach you're taking.   But there needs to be a voltmeter in the process.  if your battery is discharged, that could make a big difference on the gauge reading.  and you'll use it for voltage drop tests now that your resistor is in place to make finding the wiring issue you have a breeze.

Comment by Russ Austin on May 25, 2015 at 12:06pm

Made a on car test harness. Interesting enough, my car failed the test with the 30 ohm resistors. The gauge should read full, and the gauge read between 3/4 to full. So I have a problem too.

I'll be taking the dash pad off too troubleshoot.   Pictures explain it, but will narrate.  The purple wire is connected where the gas tank feed wire goes.  The other end of the purple wire goes to the gas tank ground bolt. Make sure you have a good ground.

Comment by Russ Austin on May 25, 2015 at 11:52am

Jeff, since I just recently found out how to troubleshoot the gauge, I couldn't tell you how often the gauge is the culprit.

Comment by Russ Austin on May 25, 2015 at 11:09am

Jason, there is no need to take th gauges out. I noted iit right below my last picture.

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.

To clarify. I would use alligator clips to hold a good connection, and a strand of wire would be needed.  Use some connectors to get a good connection at the feed wire.

   If your tank is reading low, disconnect the brown feed wire behind the gas tank filler door.  Connect one end of the resistor to the brown feed wire that is on the trunk back wall. Do not connect it to the wire you disconnected, as that runs to the gas tank sending unit.

Connect the other end of the resistor to a good ground. Turn the ignition on, and see what the gauge does.

BTW Jason, with the 15 amp resistor, the gauge should be dead center on 1/2 tank.  Looks like you have a bad gauge. What does that gauge do with a 30 amp resistor?

Comment by Jason Edge on May 25, 2015 at 8:06am

Hey Russ, What I was after was a simple description or illustration showing connections on the back and how we can bench test our own. I pulled a couple of mine and used the Radio Shack 15 ohm resistors (part # 271-003), and took the following pictures showing the hookup on the back using the one 15 ohm resistor which should peg it about 1/2 tank.

Note to bench test with fuel/temp gauge removed, connect 12 volt + to connector on the right where the pink and black wire would have connected (I use a small backup battery as seen in pictures but you can use any 12 volt DC source such as your car battery), and connect ground to body of the fuel/temp assembly and jumper the 15 ohm resistor over to the left connector where the brown feed wire from tank would have connected. I use an extra alligator clip to hold to connector. Connect the two resistors together in series for the 30 ohm resistance and test in same manner.  MAKE SURE CONNECTIONS ARE TIGHT.

Comment by Jeff Kinzler on May 25, 2015 at 3:28am
Russ-
Thanks for posting a most helpful article.

I am tempted to ship my car out to you to troubleshoot the whole electrical system.

How often is the dash gauge the culprit? Mine reads about 1/2 when the tank is full and quickly shows 1/4 to empty after 5-7 gallons are used.

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