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Erratic fuel gauge

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Ever since I have owned my 84 924 Lux (5 years) the fuel gauge has always been very erratic. Sometimes it will go from half full to empty and other times it will flick about at high speed. The engine doesn't even need to be running for this to happen, (but of course the ignition needs to be on). I have cleaned every single earth I could find, I have tried another gauge and it has a new sender unit in the tank.Being fairly new to the forum I have been reading some of the older threads, on the one entitled "little red light" 924Nutter stated that there was an earth supply from the alternator regulator through the fuel gauge. Having tried everything else does anyone think that it could be the regulator that's affecting the fuel gauge? If so how easy is it to change, how much does it cost and where can I get one from?
Just for more information I have changed all the main earth leads and also renewed the loom from the starter motor/alternator. I hope somebody can help because it drives me mad not knowing how much petrol is in the tank![:(]
 
What I actually said was'
"This could be heralding the beginning of the end of the regulator in the alternator. According to the wiring diagram, there is the possibility of an earth return via the fuel gauge, if the battery can back-feed through the regulator to the lamp and then to earth. Checking the ign switch is easier in terms of accesibilty, but from my experience with prices, the regulator is half the price of a new ignition switch."

This would also need a failure of the diode pack in the regulator allowing reversed current flow. Now that I have my new reading glasses, it is apparent that no such possibilty of an earth return exists.

When you say new do you mean new or different, because my gauge was erratic, and I replaced the sender with a secondhand one sourced from ebay. The gauge has been fine up until now.
 
The replacement gauge was secondhand but the sender unit was a brand new one from the Porscheshop in Halesowen (£65). I was just clutching at straws really because I've tried everything else I can think of. There was one strange event that happened a couple of years ago. I broke down on the motorway (electrical connection on the distributor at fault), the RAC man gave me a solid tow just to the next junction, along the hardshoulder. I had my hazard warning lights flashing and everytime they flashed the fuel gauge needle flicked in perfect harmony. Does this maybe suggest were the fault lies?
BTW you're lucky I've got new reading glasses and now I can't see a thing!
Paul.
 
Clearly there is cross connection there then. Try to reproduce the symtoms again with the hazzards, and I will scruitinise the wiring diagrammes for possibilites, but not before I've had a pound of fillet off the barbie and sunk a few low carb Michelob ultras.
 
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