The Stovebolt Site Forums The Shop Area The Electrical Bay Gas guage ISSUES!

Ok, I know this is a old and boring topic, but my gas guage has a mind of its own. First it wouldn't work at all. The truck was converted to twelve volts before I purchased it. I was playing around last night and noticed the voltage reducer was on the negative side, I switched it over to the positive, or "hot".The gas gauge now works, at the "full' reading when the ignition switch is turned to the "on" position. I checked all the wires, all "seem" ok, all connections look good. Is this a grounding issue with the sending unit? What is the proper way to ground the sending unit? Right now it is grounded by a small wire right off of the top and the wire is screwed into the back frame area of the cab. Tonight I can run my ohmmeter on the sending unit. I also ran my test light to the sending unit but it did not register any juice. I also ran the test light underneath the truck and again no juice. PLEASE HELP ME!! Any advice/tips would be greatfull. It has been raining here for 3 days now and this gag gauge deal has been fun but it is also frustrating......

Phillip:

1) You need an inline voltage drop to handle the 12 volt blast going to your 6 volt sender and guage.

2) Check your wiring against the factory GMC wiring diagram.

3) Your truck was converted from 6 volt positive ground to 12 volt negative ground or 12 volt positive ground? This will also possibly affect the guage.

4) Cut a nice fresh curved branch off a forsythia bush and keep it behind the seat to stick the tank! (I'm kidding I'm kidding!)

5) Using that factory wiring diagram and a volt/ohm meter, you can trace your guage wiring to make sure you have the wires where they are supposed to be.

Phillip,

Check the ground at the gauge (check continuity from the gauge housing to ground) and at the sender unit (sender unit "cover" to ground).

Tim

Is there supposed to be a "extra" ground on the guage that runs to the sending unit to ground with the other ground wire that is already there with the sending unit????.......

Wire harness diagram shows a "Fuel Jumper" wire running from the gauge to the sending unit....

Phillip,

Sorry about the lack of clarity in my earlier message.

The wire you are describing is the sender lead that runs from the sender post on the gauge to the sender post on the tank sender unit. Both those posts are isolated/insulated from ground.

One diagnostic for fuel gauges is: if the gauge spikes to full when the key is on, then the gauge is most likely good and something is not grounded properly. There are two parts that have to be grounded: the body of the whole gauge assembly (assuming that the gauge itself is properly grounded to the body of the gauge assembly); and, the non-post part of the sender unit on the gas tank.

A good test is to run one wire from the battery ground to the back of the gauge assembly unit and a second wire from the battery ground to the metal fuel line that goes into the gas tank. If the spiked needle does not come down, you have to do other tests to determine/isolate the problem (at that point, I usually take out the gauge unit and the sender unit and bench test them.

These testing procedures can be found in this forum by searching for "gas gauge" and "fuel gauge".

I hope that someone else who can explain this better (or who has an easier or better testing technique) will also post a reply here.

Good luck,
Tim

Tim, I think I understand now. So, run a extra "test" ground from the ground on the sender unit to the ground on the frame or gas line, and then also run a "extra" ground from the gas gauge to the ground on the battery. Then see if the gauge changes, then I can remove one of the grounds, and that should tell me which side is not ground properly, either the gas guage or the sender unit. Is this what you mean? I sure hope so. Sometimes trying to read things like this will confuse me more, but this is a big help to me.

What do you think about my test light showing no activity on the sender unit? That makes me wonder if there is some "break" in the line somewhere I can't see, like on the lower firewall where the harness comes out at.

I have read the past posts, but I still would like all the help I can get!!

Unless you have some kind of custom gauge, your fuel gauge is just a ma meter. What that means is it senses current flow. The fuel sending unit is a variable resistance that is at it's smallest value when the tank is full, and has it's largest resistance when the tank is empty. Because your unit reads full all the time, either the inline dropping resistor is not correct or the tank sending unit or wiring is shorting to ground. This is not cause by a lack of a good ground connection.

Thanks for the correction truckernix,

I left out disconnecting the sender lead to test the fuel gauge,(in that case, "full" indicates a good gauge. I think). Do a quick test of your sender lead by running a good wire from the sender to the gauge. If the gauge still reads "Full", it is probably time to check the sender unit.

Here is a link to an earlier thread on this topic. It should add to the confusion.

Tim

http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=3;t=000606#00 0003

I listened to these guys, mostly the venerable tc. Mine was intemittent and jerky. I took apart the sender and I could not see why that rusy spring looking resistor worked in any position. I bit the bullet and bought a new sender from a reputable stovebolt parts catalog. The dam mounting holes didnt line up. I know they are supposed to be arranged so you can only install it one way, but it was no way. I wanted to finish, so I just drilled my own holes. Worked tho. - Scott

Heres some more confusing information. If a gauge is good, it will go to full when you ground the sender wire. Unplug the wire, and gauge should drop to empty. If it doesn't go to empty with wire unhooked from the sensor, unhook it from the gauge, it should drop to empty. If not, your gauge most likely is bad. If it does drop with wire off gauge, your wire to tank is grounded out.
Another thing, the sender could be stuck on the full side, though pretty hard to do. With the gauge reading full, something is grounded out or gauge is bad other wise it would read empty. Since the sender and wires going to it are all grounding wires, your test light will not show anything unless you reverse the leads and hook to the positive side of battery. Even then its so little you might not see it anyway. Joe