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Wiring problem with fuel gauge - really strange
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NorthernStar
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PostPosted: Sun Mar 09, 2008 6:05 pm    Post subject: Wiring problem with fuel gauge - really strange Reply with quote

Hi all. I have a really strange thing with my fuel gauge. Ever since I bought my '84 W.B. a couple of years ago the fuel gauge didn't work properly. Last summer I replaced the sending unit (which was really old), but the gauge, while better, still would not give accurate readings. Following the archives and my Bentley, I decided to check the voltage stabilizer, and when I did I found that instead of having 3 connections, there are only 2 ON THE CIRCUIT FOIL. The picture in the Bentley shows it to be a 3 pin rectifier that should have 10 volts between pin 1 and pin 2, and it shows pin 3 also in a socket connection. The wiring diagram shows all 3 pins connected in the circuit. My circuit foil only has 2 connectors - one for pin 1 and one for pin 3. Pin 2 connects to nothing on the foil. The water temp. gauge works OK, and all connections on the multipin connector seem to follow the wiring diagram. The foil all looks in good shape and original - not like somebody haywired a fix. What do you think - did some models only use a 2-pin stabilizer? Has somebody replaced the original with the wrong one? The voltage across pin 1 to 3 was less than 3 volts, so I bent the center pin 2 across into pin 3's socket and when I connected it up the fuel gauge started to move and the overheat led started blinking as it should. The only problem was something started to smoke behind the cluster so I quickly shut off the switch. It shows 10 volts between pin 1 and 2, so do you think I should try it again and see if anything starts to burn? Anybody else ever encounter something like this?
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mightyart
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PostPosted: Sun Mar 09, 2008 6:35 pm    Post subject: Reply with quote

My 81 only has 2 connections on the voltage stablizer, the center one is not connected at all.
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Captain Pike
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PostPosted: Mon Mar 10, 2008 5:18 am    Post subject: Reply with quote

Me thinks you have an Air box foil on a wasser. Sad
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OilNBolts
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PostPosted: Mon Mar 10, 2008 6:13 am    Post subject: Reply with quote

Tuning for smoke is never a good idea in electronics.

The regulator requires three connections- input, ground, and output. The Bently manual, page 97.20, current track 38, shows exactly that: a pin going to power, and a pin going to the fuel gauge. The third connection is shown going to ground, but may happen through the mounting tab. The problem is, this is shown as the air-cooled application. As Bill W. noted, "Me thinks you have an Air box foil on a wasser".

On the later models, the mounting tab is attached to a heat sink plate- hence the need for thermo-silicone compound to improve the component's life expectancy- but isn't electrically connected to anything. I have to wonder if yours provides the ground connection.

The pinout of the later three-pin regulator (stabilizer) is output, ground, input as viewed from the front, or the side with component markings. The center pin is connected, or electrically common to, to the mounting tab- I just confirmed this on a workbench with an ohmmeter.

I'll bet this is true. Do your voltage test using the mounting tab as the negative connection, and compare the two pins. The right pin should show something approximating the battery voltage, and the left pin should show a regulated 10V, +/- .5 volts.

If your replacement is a three pin device, do not connect the center pin to anything!! And, the two pin device will not function with the mounting screw removed.
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NorthernStar
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PostPosted: Mon Mar 10, 2008 7:53 am    Post subject: Thanks guys Reply with quote

Thanks for the help guys. Probably what happened is that somebody replaced the cluster and used one from the air-cooled model. Particular thanks to you OilNBolts. I'll do those tests as soon as I can. This forum is always helpful.
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OilNBolts
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PostPosted: Mon Mar 10, 2008 6:14 pm    Post subject: Reply with quote

A pleasure, hope it helps. I think you may be right about the substitution. It is probably worth your while to do a pin-by-pin comparison of the two gauge pods to see if any other surprises lurk undiscovered.

For what it's worth, I recently did a repair to one of my fuel gauges, this one on the '88 GL. The instrument, when functioning correctly, measured 50 ohms.

Roger R.
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bucko
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PostPosted: Mon Mar 10, 2008 6:30 pm    Post subject: Reply with quote

My 84 cluster also has only the 2 tabs used (center is missing). I too figured the the mystery "leg" is missing because it is a ground, and that the mounting tab that the screw goes through to mount this gizmo grounds it.

My cluster is from an 86 or later unit (has the tach and oil buzzer circuitry).
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NorthernStar
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PostPosted: Mon Mar 10, 2008 6:52 pm    Post subject: Reply with quote

My gauge measures 60 ohms across the terminals, which could be close enough. I've found the part number for the stabilizer is 171-919-803 and you can get them from AutohausArizona for $16.26. The dealer had the part number for the gauge itself as 255-910-31 and shows it as obsolete. Nobody seems to have them under that number, which is strange because it is so easily removed from the cluster you'd think they'd be worth stocking. I'm going to take my cluster to a local vanagon specialist in the next couple of days and see what he says. I'll post anything new that I find.
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OilNBolts
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PostPosted: Mon Mar 10, 2008 6:59 pm    Post subject: Reply with quote

If you have a pick-a-part style junkyard in your area, you can find the regulator in any number of VWs of that age, as well as Audis and something else, maybe it was Volvo. The same trip is also a great opportunity to get a lifetime supply of instrument panel light bulbs.
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