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 August 14, 2015 at 9:54pm

Devised a new tester for the fuel gauge. You can sit in your seat and turn the knob while you watch your gas gauge move. The test set has calibrated marking to test your gauge.

Plug the long wire into the connector for the sending unit. Its located behind the license plate.

Connect the short wire to a tested good ground.  Turn the ignition switch on, and test the gauge.

I made 2 of these. One for myself, and another to loan out to members.

Comment by Mike Foose on June 24, 2015 at 8:41am

I use a cordless drill with a clutch so I can get them all evenly torqued - I can tighten them a lot more by hand.  These aren't head bolts and I don't torque em like they are - they are NOT over tightened by any means - but they ARE even - (except for the one that HAS to be loose to get a OHM reading).  I replaced the OEM screws with 10-28 X 3/8 zinc coated.  Was using Stainless and thought that might be adding to it so I went back to steel.  Also using Toothed washers under the ground's ring terminals

I am using a fiber gasket material - Napa VT 102? (don't have the roll handy now) - and no adhesive

Comment by Russ Austin on June 24, 2015 at 7:53am

Mike, you mentioned running the bolts down with a screw gun.  The original hardware was a screw, so you may be over tightening  the fasteners.  I have only used a screw driver to retighten the screws, with no problems.

Comment by Mike Foose on June 22, 2015 at 10:28pm

Thanks for that Mark - It kinda makes sense, that I'm flexing it but, the flange is not bent and neither is the mounting area.  In other words, everything looks normal / undamaged.

Why would it always be the last screw?  Doesn't matter which one is last to get tightened - that's the one that sends it over the edge.  AND, why, if I let that last screw a little loose so my meter reads the float arm correctly, would it make the gauge read wrong.  Remember I've tested every part of this outside of the tank and it all works as planned.

The area "where the wire passes through" - is it easy to find?  I've not taken it apart to look for that -YET.  But I am pretty good at taking the tank down and making gaskets.  Got any pics of what you're referring to?

Thanks again for the reply

Comment by Mark S Anderson on June 22, 2015 at 6:50pm

sounds to me like when you're tightening the sender flange, you're flexing the unit a little and it's grounding out where the wire passes through.  </mytwocents>

Comment by Mike Foose on June 22, 2015 at 2:20pm

Let me complicate this even further.... Russ is aware of my issues and has been VERY helpful but I think I've stumped even HIM.

-Had a gauge that would only read 1/2 when tank was full, 1/4 at 1/2 and E at E.

Thought the sender was bad so I got one from Russ (Mine didn't even have a pick-up screen - his did)

-Went through ALL KINDS of troubleshooting and determined that the gauge was bad

- Got a good gauge from him and it all tested fine.  I could operate the sender's float arm outside of the tank and the gauge would read correctly.  The brown wire running from the gauge pod to the back of the car is not broke.  The system worked when I had it in my hand and I THOUGHT the problem was solved. 

- Here's the fun part

When I installed the sender back into the tank my OHMS dropped to zero - even though I could operate the arm up & down through the fill hole (tank is empty).  I even added a second ground under another of the 5 screws that hold it onto the tank.

If I remove - or even back one of the 5 screws out - even 1/8 turn it will read fine.  If I tighten it the thing drops to 0.0 ohms - even with the float arm at the top of the tank - WTF!  I could reliably reproduce this and it did the same thing EVERY time I tightened or loosened one screw - usually the one closest to the fill tube

- SO... I put lock tite on it and tightened it to the point where the meter WOULD get 30 OHMS at full - tested THAT many times and it seemed solid.  Put my tank back in (for the 4th time!) and added fuel

Now, My gauge reads 1/2 at Full, 1/4 at 1/2 etc - just like before.  This is CRAZY and I'm reaching out for some more help.

All my grounds are tested solid and clean - I think I'm gonna drill a hole in the flange of the tank and add a ground wire from the tank itself directly to a battery ground

I have no idea what to do - THIS MAKES NO SENSE - got any ideas?

Thanks again Russ for all the help!!

Comment by Russ Austin on June 3, 2015 at 11:03pm

Doing some field research. Hope to have a better process on testing the sending unit, wiring and gas gauge.

Comment by Russ Austin on May 28, 2015 at 8:00am

Just to clarify a bit of confusion.  The test harness actually simulates the gas tank sending unit at a FULL tank of gas. Plug it in, in place of the sending unit, and you now have a signal of a full tank of gas to your gauge.  

If your gauge isn't reading at the full mark, then test the brown feed wire on the back of the gauge for 3.36 V  If the voltage is correct, then the gauge is bad.

Comment by Russ Austin on May 26, 2015 at 9:46pm

If anyone needs this test harness, I can sent it to you.

Comment by Jason Edge on May 25, 2015 at 6:01pm

Russ, that is a 15 ohm resister that actually read 14.75 ohm with my meter so it was slightly less than the advertised 15 ohm rating, and read slightly less than 1/2 full. I have always preferred a gauge to read a tad low rather than high anyway when it goes down toward empty.  For my 2nd test, I used 2 different resistors in series that read 29.98 ohms and the gauge read full (see pictures below), so this is not a bad gauge.  The 2nd set of pictures you posted helped as I really didn't have a clue with the first pictures with the test panels.  You should be able to test wherever you can tap into the brown feed wire side with the resistance and have power to the gauge. My car is in the shop and all I have to test with are gauges off the shelf.


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