Gas Gauge

[Lowell]

My son picked up a 72 Jeep Commando and we are having some fun making improvements to it.  The gas gauge doesn't work.  Can anyone tell me if there is a way to see if the problem is in the gauge or in the sending unit, short of dropping the fuel tank to exam the sending unit?

 

[Shayne]

Good project,  I dont recall exactly on that unit, but That should be a gauge and not a light.  By pulling the instrument panel open you should with an OHM meter, check the wiring and the gauge while the panel is exposed.  Thus, should it be there, problem could be solved quickly. Good luck

 

[John From Detroit]

There are three possible problem areas.  Gage, Sender and wires.  There are two kinds of gages but for this you can consider them to be the same.

You are going to be surprised when I tell you the most likely cause too.

To test the guage you need a small resistor, a 12 volt light bulb will do, and a Multi meter  If you can find the guage terminals use the "Sensor" side (one side should be something like six volts, the other should too if the problem is not the guage, if you hook a test light you should see the gage move)

To test the wires.. You will need to access the sender unit on top of the tank.. Same test, measure voltage, hook up test light, you should see the gage move with the light.  NOTE: on some rigs there is an inspection panel and you don't need to drop the tank.

Next comes the tank to ground connection.. Yes tank to ground (Frame)  Using the multi meter measure voltage from the tank, or if the tank is plastic from the ground connection on the sender, to the frame of the coach, ANY reading above zero is unacceptable, it should be zero.. If you get a reading hook up the test light,  (Same connections) if gage now appears to work you have a ground fault.. .This is my prime suspect,  A ground fault.  (Ground faults are the problem as often as not)

Finally there is the sender unit itself,  With fuel in the tank (half a tank is nice) using the multimeter on OHMS (Resistance) scale, unhook all wires from the sender and take a reading,  ZERO is bad, INFINITY is  bad, anything else is good.

NOTE: Another suspect is the connections you just unhooked,,,, If everything tests good, hook them back up and see if it now works. (It is not a miricle, You simply cleaned them by rubbing them against each other)

 

[Lowell]

Thanks for  the comments!

I'll follow your directions and see what happens. The Jeep isn't here right now but should be back in a few days.  I've printed your comments and will see what I can learn.  I was able to find a shop manual for it on a CD from an on-line source and it provides the resistance that should be read at zero, 1/4, 1/5, 3/4 full.  So that should be of help. 

My son wants to do as much of the upgrades as he can himself.  We're looking for improving reliability, not excessive off-road capability.  It's a hoot to drive down the street because of the attention it draws. Any and all suggestions are appreciated.

 

[John From Detroit]

One nice thing about jeeps.. It is very easy to drop that gas tank, so long as it's near empty