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supersuk
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PostPosted: Mon Jun 04, 2018 12:17 pm    Post subject: Help With Fuel Gauge Reply with quote

Ever since I completed the fastback, i've been battling the fuel gauge. No matter what, it always reads full. Here is a list of things I tested:

1. I tested the fuel sender ,which is new VDO (brazil) replacement, and that reads about 80 ohms empty and about 5 ohms full. I'll take that as acceptable.

2. With the ignition on, I removed the wire from the sender and the gauge went to empty. Touched the wire to ground and the gauge goes to full.

3. My tank is currently about 3/4 full so I took a resistance reading with the sender in the tank at the sender and it reads about 35 ohms. Then I read the resistance at the wire that plugs into the gauge back piece and it reads the same 35 ohms.

4. I tested the gauge by connecting the wire from the fuel sender to appropriate pin on the fuel gauge and applying 12v power to the other pin from the fuel gauge and it went straight to full.

This leads me to believe there is a short somewhere in the gauge...Anyone know how else I should test the fuel gauge? Seems like the sensor isn't the problem.
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KTPhil Premium Member
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PostPosted: Mon Jun 04, 2018 12:37 pm    Post subject: Re: Help With Fuel Gauge Reply with quote

I would disconnect the gauge wire both at the gauge and at the sender, and check if that wire has any voltage on it, or if it is grounded. It should be totally isolated from power and ground. If not, trace the wire for a fault or replace it.

Also, bear in mind the gauges ground using those springy wings that physically hold it in place at 3 and 9 o'clock. Usually this results in a jumpy needle, not a full reading, though. Worth a check just in case, adding a grounding jumper to the case temporarily. I think some later models also have a ground wire to improve things.
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supersuk
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PostPosted: Mon Jun 04, 2018 12:45 pm    Post subject: Re: Help With Fuel Gauge Reply with quote

Hi Phil, I did test the wire for continuity and it was good with 0 ohms resistance. I'll check for voltage tonight. I did however try to ground the gauge and that didn't change anything.
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KTPhil Premium Member
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PostPosted: Mon Jun 04, 2018 12:52 pm    Post subject: Re: Help With Fuel Gauge Reply with quote

supersuk wrote:
Hi Phil, I did test the wire for continuity and it was good with 0 ohms resistance. I'll check for voltage tonight. I did however try to ground the gauge and that didn't change anything.


If it is grounding from frayed insulation, it might cause your symptoms, so check for ground isolation, too.
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supersuk
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PostPosted: Mon Jun 04, 2018 1:37 pm    Post subject: Re: Help With Fuel Gauge Reply with quote

Will do. I doubt it is frayed insulation because this is a brand new wiring kit, but will check to see if the wire is being grounded. Wouldn't the resistance read 0 if it is the insulation being bad? I'm consistantly getting about 35 ohms with the gas at 3/4 tank.
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PostPosted: Mon Jun 04, 2018 2:11 pm    Post subject: Re: Help With Fuel Gauge Reply with quote

It wasn't clear to me just what you are measuring, so I recommended the "clean" measurement to rule out the wire 100%.

After that, the gauge may be your next suspect. If ours is good cosmetically, you can buy a good but "cosmetically challenged" used one on the Samba pretty cheap since you are just taking out the guts.
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Bobnotch
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PostPosted: Tue Jun 05, 2018 9:00 am    Post subject: Re: Help With Fuel Gauge Reply with quote

supersuk wrote:
Ever since I completed the fastback, i've been battling the fuel gauge. No matter what, it always reads full. Here is a list of things I tested:

1. I tested the fuel sender ,which is new VDO (brazil) replacement, and that reads about 80 ohms empty and about 5 ohms full. I'll take that as acceptable.

2. With the ignition on, I removed the wire from the sender and the gauge went to empty. Touched the wire to ground and the gauge goes to full.

3. My tank is currently about 3/4 full so I took a resistance reading with the sender in the tank at the sender and it reads about 35 ohms. Then I read the resistance at the wire that plugs into the gauge back piece and it reads the same 35 ohms.

4. I tested the gauge by connecting the wire from the fuel sender to appropriate pin on the fuel gauge and applying 12v power to the other pin from the fuel gauge and it went straight to full.

This leads me to believe there is a short somewhere in the gauge...Anyone know how else I should test the fuel gauge? Seems like the sensor isn't the problem.


It sounds like you tested it correctly. I believe your sending unit ohms values are low though. Should be 90 and 0. I don't know if your values being low are causing the problem or not though. Your gauge and it's wiring tested out correctly, disconnected = empty, and grounded = full. This means the wiring to it is good too.

What you could do is take the small nut off the bottom of the sending unit (after you remove it from the tank), add a jumper wire (for ground), and add the gauge wire, and cycle the float up and down to see if your gauge is working correctly (or if it's still stuck on full). You might have an issue with the sending unit being internally grounded or just not "free floating" causing the incorrect reading. If you can get another sending unit to try, you might get a correct reading to verify everything is kosher on the car side of things.
What happened to the old sender? Jim Adney does rebuild sending units, even with broken wires inside the tube.
I hope this helps.
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supersuk
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PostPosted: Tue Jun 05, 2018 9:52 am    Post subject: Re: Help With Fuel Gauge Reply with quote

Ok, I retested the wire and made sure that it was not shorted. The wire was removed from both the tank sender and the bulb holder, connected one end of the multimeter to a good known ground and the other onto the wire. No continuity. Then I retested the resistance of the wire, which gave me 0 ohms. Next was to see what resistance value the sender gave me with it in the tank at about 3/4 full and it was 35 ohms again. Then I connected the wire to the sender and bulb holder and read the resistance again at the bulb holder, which was 35 ohms.

When I removed the sender from the tank, the float did move freely. I did the test that was called out in the bently that said to read the resistance as you slowly remove the float from the tank and the resistance went higher to 80 ohms when it was completely out of the gas. I also tested the sender with it out of the tank and me turning it upside down so the float moves and the gauge reading doesn't change, but the resistance does. I also tested the resistance of the resistor (or whatever it is) in the fuel gauge and it was around 70 ohms. Can anyone confirm what the value should be?

Bob - I'll remove the sender again tonight to do what you suggested.

I did however purchase another fuel gauge/bulb holder from the classifieds last night to see if that helps. Maybe there's something wrong with the wiring in my bulbholder or gauge. I'm just so frustrated with the fuel sender!
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PostPosted: Tue Jun 05, 2018 9:54 am    Post subject: Re: Help With Fuel Gauge Reply with quote

Just double check that the gauge itself is grounded by adding a ground jumper.

But other than that, it sounds like it's the gauge itself. Like I said, I think the gas gauge portion is removable, so you can cannibalize the old and new to make a clean, functional gauge.
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supersuk
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PostPosted: Tue Jun 05, 2018 9:57 am    Post subject: Re: Help With Fuel Gauge Reply with quote

KTPhil - I made sure that the body of the gauge was grounded too. Put a jumper wire from the gauge body to the known good ground and nothing changed. I read the resistance from the ground pin on the bulb holder to ground and it was continuous with a 0 ohm reading. Then I read the pin on the gauge to the body and that was also continuous with a 0 ohm reading. Lastly I connected the bulb holder to the gauge and read the resistance from the gauge body to the known ground and I got a continuous reading with about 1 ohm resistance.
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PostPosted: Tue Jun 05, 2018 11:12 am    Post subject: Re: Help With Fuel Gauge Reply with quote

Good, so it's gotta be the actual gauge.
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PostPosted: Wed Jun 06, 2018 7:32 am    Post subject: Re: Help With Fuel Gauge Reply with quote

supersuk wrote:

I also tested the resistance of the resistor (or whatever it is) in the fuel gauge and it was around 70 ohms. Can anyone confirm what the value should be?


There shouldn't be a resistor there or a "vibrator" like a beetle uses. That might be the real cause of your issue.
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PostPosted: Fri Jun 08, 2018 11:04 am    Post subject: Re: Help With Fuel Gauge Reply with quote

The replacement fuel gauge came in last night. Changed everything out and now I have a working fuel gauge! Thanks everyone for your help!
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gt1953
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PostPosted: Fri Jun 08, 2018 12:11 pm    Post subject: Re: Help With Fuel Gauge Reply with quote

I the process of going through my new to me 72 squareback, the fuel tank out as well as the sending unit. I did not get any ohms at all. Disassembled the sending unit cleaned the wires, the only way I was to get resistance was at the internal connection of the sending unit.
So what I did was to add a ground wire from the internal connection thru a small hole in the top of the sending unit. The hole is sealed up. Still have to test it with gas in the tank.
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PostPosted: Sat Jun 09, 2018 8:10 am    Post subject: Re: Help With Fuel Gauge Reply with quote

supersuk wrote:
The replacement fuel gauge came in last night. Changed everything out and now I have a working fuel gauge! Thanks everyone for your help!


Did the new gauge look like the old one you swapped out?
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71 Notch ...aka Krunchy; build pics here;
http://www.thesamba.com/vw/forum/viewtopic.php?t=249390 -been busy working
64 T-34 Ghia...aka Wolfie, under construction... http://www.thesamba.com/vw/forum/viewtopic.php?t=412120
Tram wrote:
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Tram wrote:
People keep confusing "restored" and "restroyed".
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supersuk
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PostPosted: Tue Jun 12, 2018 11:53 am    Post subject: Re: Help With Fuel Gauge Reply with quote

Bob - Yes the fuel gauge was exactly the same. I did notice that the print of the numbers were a little fuzzy looking compared to the gauge that it replaced. I could change it, but it isn't enough to bother me.
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