I got in my sedan the other day and as soon as I turned the key the fuel gauge pegged to full even though the tank was not full. I checked all connections and they all seem good. I also disconnected the sending unit one wire at a time and both at the same time. The gauge pegs to full an all of those conditions. Any ideas? This kind of stuff isn't really my thing.
You should get fantastic mileage then.. ... On a serious note, sounds like a bad fuel float or sender? Maybe the float is stuck? Or the ground wire somewhere is shorted to bare metal somewhere along the frame, or? The gauge has power to it, and it finds it's ground through the sender unit. So, if the wire to the sender has insulation rubbed off somewhere against a bulkhead, frame, or where ever, it will get it's ground right there in lue of through the sending unit, resulting in the gauge pegging out full as soon as it gets power.
If you unplugged the sender and still have a full reading it could be a short in the gauge. Which gauge and sender do you have (ohm rating)?
Before I shot down a gauge though, I would physically check the wire, because a short anywhere from the gauge to the sender would do the same thing with the sender unplugged. Odds are, a wire has the insulation chaffed off along a metal edge somewhere.
I haven't pulled the tank to check the sender yet. It could be bad but I doubt it's stuck. I'd think if it was stuck it would show the fuel level that was in it (less than 1/2 tank) when the problem started. Also, it doesn't show full as if the tank was full - it "pegs" on the full side. I'll try running a "jumper wire" in place of the hot wire to the tank as a test for a ground in the existing wire.
I don't know what ohm rating the gauge and sender are but I know they're matched. They were purchased as a set and have worked perfectly for years.
sounds like a dead to ground short then. Remove the wire from the gauge (the one that goes to the sender). If the gauge doesn't peg, then it is a direct to ground short somewhere in that wire to the sender unit. If it pegs when you disconnect the sender unit, it is a short in that wire, and like I mentioned, that wire going through bulkheads, along the frame, etc are places it would chaff against to rub through the insulation. You would have to either physically inspect that wire all the way back for the bad insulation, or just replace that wire.
that said, if you have a meter, disconnect both ends (sender end, and the gauge end.) Ohm from the wire to ground, if you get infinity (pegs the ohm meter) you have a direct short in that wire somewhere. With both disconnected on each end, that wire should read open (no meter needle movement)
Is FULL or EMPTY the normal off position for a VDO? I'm suspecting you lack an actual ground, that the sender is grounding to the tank as expected but that the tank itself is electrically isolated from frame OR that sender is isolated from tank. Try a jumper from one of the sender mounting screws to bare metal on chassis and see if the thing moves.
All the reading is essentially a resistance in relation to ground. The gauge gets it's ground through the sender but reads the resistance to ground via the variable resister in the sensor. Sounds like his problem happened suddenly, and was working normally, so he is getting a ground with NO resistance if it is pegged over to full which indicated a direct short to ground in lue of getting it's resisted ground through the sender. The sender complete's the circuit usually through it's metal to metal contact. Unless he has a plastic gas tank, he shouldn't need a hard wired ground from the sender.
All GM metal tank cars came with a sender ground, because the tank is sort of hung flexibly to prevent torsion from body flex.
That's true, but if he lost that ground, the gauge would always read empty (not working at all) because it would have no ground at all. With it reading pegged at full is indicating it has a ground, too good of a ground and not going through the sender's resistance, indicating a direct short to ground at some point before the sender.
If you simply disconnect the sending unit, and the gauge moves to empty. That would indicate the problem is with the sender unit, either a stuck float, or a straight shorted resister in the unit. If it stays pegged over to full, the problem is it is shorted to ground either along the wire itself bare and touching ground, or the gauge is shorted internally to the metal to metal contact ground it would have. If you also disconnect the wire from the gauge (the one that goes to the sender unit) and it still shows pegged full, the problem is the gauge is shorted. If it goes to empty then, it is a short in your wire. inspect the wire and find where it has broken or chaffed insulation and touching the frame, or metal ground somewhere along the way. Repair that, or just replace that wire completely and you should be golden.
So, backwards from Ford...here's the VDO page, all on the general idea of replacing the sender link with a jumper and watching results: http://www.discoveryowners.com/VDO.asp
Thanks guys. I'll try your various tests tomorrow and see if I can get it sorted out. Sounds like a ground problem, I just need to determine where.
Don't assume the gauge is grounded on its own due to the mounting. It needs its own separate ground to the (-) input on the gauge itself, since this may be floating.
Ok guys, I disconnected the wire from the sending unit from the back of the gauge and it's still pegged to full so I guess a new gauge is in order. Right?
Make sure that the bracket that holds the gauge in is not touching the sending unit connection, make sure you have the power to the gauge to correct location. If those look good I would say the gauge is bad.
I don't know if this will help you or not, but I just bought some new VDO gauges and when I put them in the dash, before I installed the sending unit into the fuel tank, I put power to them to see if I had it wired correct. Without a load on the gauge, the fuel gauge pegged to full when power was applied and not connected to a sending unit. When I connected the new sending unit up, the gauge went to empty (which it was) when power was applied. So maybe you ought to check the sending unit before you buy a new gauge.
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