Fuel Gauge Reading in Reverse?

[vwwestyman]

I started a thread on this 25 months ago but it quickly devolved into a live update/trip report thread which ended with the engine in my Bus dying and me never figuring out the issue. (And eventually forgetting the issue until today!)

Anyway, I actually got the Bus running again today, well over 2 years later! When I got it going, I noticed that the fuel gauge was reading quite low-a little under R. So I got to the fuel station and topped the tank off. I was surprised that it was reading even lower! Then remembered that I was having the issue before and dug up the old thread.

In short, for no reason that I could figure out and without (that I recall) doing any sort of wiring stuff, my fuel gauge suddenly started reading backwards.

When the tank is full, it reads all the way to the left. As I drove on that ill-fated trip, the needle would rise as fuel was consumed. And go right back to reading all the way to the left when the tank was nearing empty!

So, now that the engine is running again, it might be nice to know where I'm at with the fuel levels.

I'd love to hear any ideas on what might be going on or what I could check.
_________________
Dave Cook

President, Wild Westerner Club

1978 Champagne Edition Westy, repowered to '97 Jetta TDI
1973 Wild Westerner
My Thing

[vwwestyman]

Here's a current pic of the back side of the fuel gauge.

Anyone see anything crazy here?

_________________
Dave Cook

President, Wild Westerner Club

1978 Champagne Edition Westy, repowered to '97 Jetta TDI
1973 Wild Westerner
My Thing

[timvw7476]

If you've got the gauge/vibrator/wiring untouched since you can remember it is probably some nonsense going on with the float & sender in tank. Hope you already have the handy access hole already cut......most buses do.

Yes you can pull the tank without pulling engine, but most induction stuff must be removed to get the slide-out space, & the two final screws for the cover plate are usually well-hidden & upside down to boot. : l

[busdaddy]

I'd rig up another sender outside the tank and see what the guage says about it, or you could simulate a sender with a cheap potentiometer from Amazon, 0 ohms empty, ~70 ohms full. The sender wire is just inboard of the RR shock absorber coming out the bottom of the tank compartment.
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[vwwestyman]

The problem is definitely the sender.

Had to drain the tank anyway (fuel leak under the Bus was making everything smell of diesel!) so I decided I might as well pop the cover and investigate further.

The sender is reading in reverse! Float is attached and floating. The resistance changes throughout the motion. It is just somehow backwards. First test was to connect the signal wire directly to ground from back. Gauge went to Full.

Next, I ran a jumper wire from the signal wire at the back all the way up front, and hooked up a ground wire up front, and attached the sender up front so I could easily observe the gauge. It consistently read in reverse of what it should! Float up: Reading drops. Float down, reading raises to Full.

Finally, multimeter proof (had to get creative trying to hold everything together):

For reference, this is how it should read. Almost exactly the opposite of what I'm suddenly getting.

I guess I simply just need to get a new sending unit. But am absolutely befuddled as to what could have caused this to happen (and would love to hear any theories!)

I installed this sender a few years ago, and it worked great for a long time. Then somehow switched it's readings.

And this shouldn't be surprising, but since the gauge is simply reading resistance between two points, reversing how the wires were hooked up didn't change anything.
_________________
Dave Cook

President, Wild Westerner Club

1978 Champagne Edition Westy, repowered to '97 Jetta TDI
1973 Wild Westerner
My Thing

[vwwestyman]

Are there any sources of good sending units that don't need futzed around with to work? I had to do a little bending of the arms to make this one work.
_________________
Dave Cook

President, Wild Westerner Club

1978 Champagne Edition Westy, repowered to '97 Jetta TDI
1973 Wild Westerner
My Thing

[vwwestyman]

Don't know if it helps much in figuring out what could be making the sender seemingly reverse itself, but here's a pic of its insides.

I'll probably just have to put things together for now to prep for this camping trip and deal with it later. But if you spot anything or have any ideas on WTF is happening, I'm all ears!

_________________
Dave Cook

President, Wild Westerner Club

1978 Champagne Edition Westy, repowered to '97 Jetta TDI
1973 Wild Westerner
My Thing

[mikedjames]

I think the sender is a unique badly made example with the arm assembled 180 degrees out. You could fix it by reversing the 90 degree bend just under the pivot..
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[busdaddy]

I was just going to suggest the same thing, bend the arm over to the other side.
_________________
Rust NEVER sleeps and stock never goes out of style.

Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
Stop dead photo links! Post your photos to The Samba Gallery!

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[vwwestyman]

I guess I can try that.

But what on earth would cause it to reverse??

It worked for several years with no problem, and I'm very sure I didn't do anything to it before that ill-fated trip.

So weird.
_________________
Dave Cook

President, Wild Westerner Club

1978 Champagne Edition Westy, repowered to '97 Jetta TDI
1973 Wild Westerner
My Thing

[Abscate]

Has the Bus been to Australia lately ?
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[vwwestyman]

[SGKent]

I wonder if somehow the tank has voltage on it. Meaning that it is not at a true chassis ground. The wire to the gauge is assuming that the float knows the difference between ground and resistance. But if voltage gets into that circuit then the tank is no longer at ground. The same would be true if somehow voltage was leaking into the wire going to the gauge.

I think that the 73 and 78 systems are different and I don't know which bus you are working on.

This is my 1977 but I have the electronic regulator and not the bimetal kind like you have.

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[vwwestyman]

I figured it out!

I don't know how it happened, but apparently the float arm somehow rotated 180 degrees on the pivot that the wiper arm is attached to.

It was mikedjames and busdaddy's comments about the arm being bent incorrectly that caused me to investigate this further. As stated before, this sender did work correctly for several years; it wasn't as if I had just purchased the sender and the gauge wasn't working after.

So I knew it wasn't exactly the arm being bent wrong, but started looking for other options. I took the cover plate off and it still initially looked ok, but I finally noticed that the arm seemed loose in the pivot, and I was able to fairly easily swivel it 180 degrees.

And with that information, I decided to use a little solder to "glue" the pivot to the arm in the correct orientation.

And now the gauge seems to read correctly!

Here's a pic of the pivot soldered to the arm.

_________________
Dave Cook

President, Wild Westerner Club

1978 Champagne Edition Westy, repowered to '97 Jetta TDI
1973 Wild Westerner
My Thing

[Clatter]

Damn! Nice work!!


Way to sleuth it out my man..

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[vwwestyman]

[timvw7476]

well this takes all the fun out of my 'glitch in the Matrix' guess. : (
Damn problem solvers finding the true issue. Nice workx2