2000 P32 Fuel Gauge

[TrekkerSmith]

I have a 2000 P32 chassis with the GM 6.5TD engine. The fuel gauge is pegged to the past full side. Occasionally it seems to work, but not accurately. This has been the case for at least two years, and 15,000+ miles. Fuel pump operates just fine. Is the wire that feeds the instrument panel gauge a hot wire, or the ground wire, and with intermittent working, where should I look and what should I do to fix it?
Thanks,
Gary

[dieselclacker]

Sounds like a bad fuel tank sending unit. If you can get to it, disconnect the wire to the sending unit, the fuel gauge should go all the way up past the full to the top peg. Grounding this same wire should cause the gauge to go the opposite way, to all the way past empty to the bottom peg. Could also be a poor connection in this same sending unit wire circuit. Check it at both ends.

Dieselclacker

[TrekkerSmith]

I'm 2,000+ miles from home and can't get to a point of dropping the tank to pull the pump/sending unit combo for a while yet, but if grounding the wire from the sender to the gauge would cause it to peg to full, that is what I am seeing most of the time. Could be the wire is grounding out somewhere along that line most of the time, or even all the time. Any idea what wire that is out of the ones coming from the tank? I would have to short it out somewhere in it's run along the frame rails.

Gary

[dieselclacker]

Gary, a short to ground would cause the gauge to go to empty and beyond. An open circuit would cause the gauge to go beyond full. Not sure if your dash gauge plugs into a printed circuit, but a poor connection there could cause your problem. Can't help you with the color of the wire to the sender. Maybe someone reading this has a wiring diagram. You need to make sure this sender circuit is good all the way to the tank. Checking it this way will also eliminate the gauge as a problem. The sending unit is a variable resistance path to ground for the gauge. Taking away resistance in this path causes the circuit to be out of balance, and the gauge will move due to this imbalance.

Dieselclacker

[TrekkerSmith]

Had it backwards. Will check gauges behind dash this weekend and see if I can find a loose connection, and maybe identify wire color and size.
Thanks for quick comeback.
Gary

[KC1BUD]

Quote:

Originally Posted by TrekkerSmith I have a 2000 P32 chassis with the GM 6.5TD engine. The fuel gauge is pegged to the past full side. Occasionally it seems to work, but not accurately. This has been the case for at least two years, and 15,000+ miles. Fuel pump operates just fine. Is the wire that feeds the instrument panel gauge a hot wire, or the ground wire, and with intermittent working, where should I look and what should I do to fix it? Thanks, Gary

I am not sure I can help, although I had the same problem with a Chevy Pickup many years ago. I pulled the gauge from the dash to find it had two copper clips on the back that slipped over studs behind the gauge. Thinking that it wasn't making good contact I first tried to make clips grab the studs tighter, this seemed to help it for a while, then it was as bad and unreliable as before. My final solution was to solder wires on the clips and bolt them to the studs, problem solved.

Even though your chassis is newer, I would pull the gauge first to see if the gauge contacts are making a good connection, then trace it back from there.

[TrekkerSmith]

Today's observations: Started off this morning with tank over 3/4 full, and gauge showing pegged full side. Drove around 100 miles (tank should have been about 2/3 full at that time) and gauge started moving. Filled up anther hundred miles down the road and gauge showed 3/4 full. Tank took 1/2 it's capacity to fill. Gauge pegged to full side upon fillup. Drove another 130 miles and gauge hasn't moved yet. I know it should be around the 3/4 mark. I fill based on miles driven, and am always conservative in my calculations, so not concerned about ever running out of fuel.

Since gauge actually does (eventually) move downward as the tank becomes more empty, wonder if that changes anyone's opinion on where to look to fix it. I will have time this weekend to pull the instrument panel and check the wiring at that end.
Gary

[DriVer]

When my tank is full, it'll takes a bit more than a hundred miles before the needle starts moving off the "F" peg.

[TrekkerSmith]

Thanks, DriVer. The first 80,000 miles or so, my gauge reflected a reasonably accurate display of my tank level. When absolutely full, the gauge was just barely past the full mark. Now when full (and until it finally starts moving) the gauge is horizontal, about 1/2 inch from the full mark. I just want it to more accurately reflect the level of the tank.

Gary

[TandW]

I get 300 miles until the needle is on the full mark. From there to 1/4 I get 400 more. When I don't tow anything. Good luck!!

[TrekkerSmith]

Thanks for that comment. I spoke with a long time auto tech who suggests (because the gauge does eventually start moving) that the problem lies within the fuel tank, and it likely needs a new sending unit. This can be replaced without replacing the fuel pump, but is mounted on the pump unit.
Gary

[TrekPR]

After going to the trouble to drop the tank to replace the sending unit I would be real tempted to replace the fuel pump and anything else that could go wrong and be ready for the next 80,000.

[dieselclacker]

Quote:

Originally Posted by TandW I get 300 miles until the needle is on the full mark. From there to 1/4 I get 400 more. When I don't tow anything. Good luck!!

Chief, if my math is right, thats 700 miles on 3/4 tank of gas!! What size tank does that Allegro have in it?

Dieselclacker

[TrekkerSmith]

OK. Spoke with a fellow who used to train GM mechanics for bunches of years. He says the problem is almost assuredly the sending unit in the tank. The wire to the gauge carries no voltage, but provides a variable grounding signal based on a float operated wiper and wire coil. The wiper changes position on the coil as the fuel level changes, changing the ohms in the wire running to the gauge. Says sometimes the wiper arm simply needs to be slightly bent to bring it into contact with the coil. If it's making contact, but not providing the proper ohm output, then the coil in the sender needs to be replaced. Tank has to be dropped (obviously) to get to it. Will have to tackle this job this fall when my trip is over.

Thanks for all the responses.
Gary