1972 Ghia Convert Fuel Gauge

[kdnguyen61]

Dear Members,

I am trying to figure out a problem with my new gas sending unit. I know my tank has about 1/4 tank left by looking inside. After install new sending unit and connect the only wire to the back of the clock, I ran into some weird reading. With only the key on, the needle moves about 1/8 of an inch in reserve area where I think it should be more than that. However when I turned on my light switch, it jumped to full tank. Below is the back side of the clock where the left brown wire is the tank sending wire. The right wire next to it is running from fuse no. 12 and looks like it matches with the wiring diagram. By the way to the right next to no. 31 there are two connections, what are they for as I can't match up to the wiring. Thank you in advance.

[Vladiiiii]

I think 31 is normally a ground

Here are some wiring diagrams by year :

https://www.thesamba.com/vw/archives/info/wiringghia.php

Vlad
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Vlad's 72 Ghia (Once in a Lifetime Restoration topic)
https://www.thesamba.com/vw/forum/viewtopic.php?p=9237746#9237746

[kdnguyen61]

I have looked at that wiring diagram but it seems like my clock is not matching up 100%. The gas sending unit wire is connected to the clock and at right place. There is no grounding wire but the unit body is the grounding. I remembered it works before few years ago. And I checked all wiring from headlight switch not to interfere the fuel gauge in any way.

It is difficult to make sure all those bulbs working correctly after installed (including grounding connection). I can see only some bulbs are working so I may not need to connect a grounding wire to that 31 connection pin. But it won't hurt to connect one any way.

[kdnguyen61]

Is there any member out there has same problem with fuel gauge after install new one and the gauge jumps to full when you turn on headlights?

[Rob Combs]

Good morning,

Not sure if this has been resolved, so perhaps this will help. First lets remember that the fuel gauge and the lamps should have nothing to do with one another. Obvious, right?

Second, lets remember that to peg the gauge, the sender wire needs to have little to no resistance to ground. So, I'm assuming that the power side to the gauge is normal; to peg the gauge means that the sender side of the circuit is finding a ground path outside the fuel sending unit itself.

Third, do you have the right sending unit for your gauge? There are two types that will physically go into the tank but the float-on-arm type works a little differently as a complete circuit than the float-in-cylinder type. We'll get back to that; gauge pegging with lights on obviously shouldn't be happening with either type of sender, but minor gauge inaccuracies can happen with a gauge/sender mismatch.

I'd first recommend jumpering a wire from that empty 2-pin circuit 31 terminal on the back of your clock housing to a good ground and see if your problems go away. Missing and bad grounds give professional techs headaches on the regular, and they can cause some really strange symptoms. If that doesn't solve your problem, try disconnecting EVERYTHING from the clock housing except for the fuel gauge wiring and your grounds and try again. This will eliminate backfeeds. If the gauge then normalizes with the lights on, it tells us that your gauge is finding a path to ground through one of the other circuits connected to the clock. Reconnect them one at a time and retest until the problem reappears - that's most likely the circuit where your problem will be found. Also do not forget that if the gauge is internally shorted to the clock housing you could get some really strange behavior. So let's be certain it is mounted correctly to the clock housing.

Start there and let us know how it goes. I'm going strictly off memory here, but if your problem persists after the above testing I'll try to carve out some time to re-examine the diagrams.

Keep in mind the 71 and 72 had different instrumentation wiring so build dates might come into play if yours is a really early build 72 - I have a really late build 71 and mine has elements of both model years in the instrumentation wiring which makes it a real chore to determine what parts of what diagrams apply. Hopefully that isn't the case with yours...

Good luck.

Rob