Woodsie3

My 2001 225hp TT's gas gauge has been stuck on full for about a year. I tried all the tricks with pulling fuses or resetting electronics and such to no avail, so I pulled the sending units and read the Ohms off of them with a multimeter at full and empty. (And yes, I scrubbed the senders' metal with a pencil eraser as well, but it didn't change anything.)

Passenger's side
Empty: 11.5 Ohms
Full: 136 to 147 Ohms

Driver's Side
Empty: 27 Ohms
Full: 126 Ohms

IIRC "Full" should be in that 147 range, but all of the other numbers aren't correct for what they should be. The car's gauge cluster was rebuilt a few years ago, and currently my mileage is 158,000.

When this all started, my rebuilt cluster had been working well for quite a while. Then I'd notice sometimes the needle would be down on empty, or it would be on full (both incorrect), or even sometimes it would be in between and would GAIN fuel level over the course of the trip. For quite some time, though, it's been stuck on full whenever I start the car.

I recently got the VAGCOM cable and the Lite version of the software, and it allowed me to see some more numbers:

FUEL LEVEL: 66.0 L
FUEL SENDER RESISTANCE: 290 ohms

By my calculations I've used roughly 3.8 gallons of fuel since my last fill-up. If the tank were full, that would equate to roughly 60.6 L, not 66.0.

Any thoughts? Do I have a bad cluster or a bad sending unit? How can I tell which one is bad?

Thanks!

Mister Bally

With the sending unit wire(s) disconnected and ignition "on", dies the gauge change at all? If not, I'd say the sending unit is fine. Troubleshoot per the guided fault finding steps in the service manual.

Woodsie3

Thanks for your advice!

I unclipped the wiring harness from the outer cover of the sending unit, and when I turned the ignition on, the gauge moved from its resting position to just below empty.

Since I don't have the manual you're referencing, what are my next steps? Does this confim the unit is bad? Then how do I localize which needs replaced?

Thanks again!

Woodsie3

Is this a Haynes manual? Chilton? I'd love to know the next step, or even just what manual to order since I did step one.

Thanks!

Mister Bally

The official service manual (for North America) from Bentley Publishers.
It is available as a CD-rom or download from their website.

Woodsie3

I was able to get a copy of the Bentley manual (book form), but I see nothing in it about "Guided Fault Finding," even in the instruments section.

What are the steps? Or where are they?

etischer

What voltage do you read at the level sensor? I had a similar issue with my temperature gauge being stuck at full (hot). Normally there is 5 volts at the coolant temp sensor, but the fault with my dashpod resulted in the reference voltage being only 2 volts instead of 5. I was able to repair a bad solder joint on the dashpod to fix it. Might be a long shot, but then again, it seems your level sending units are working properly.

Woodsie3

Thanks for the advice! This may all seem pretty haphazard, but I appreciate you being willing to help me through it:

I rechecked the resistance on each of the senders, and I got 147.8 to 11.7 ohms on the passenger side (full to empty, respectively) and 136 to 26.6 ohms on the driver's side. So one one, the top number is spot-on, on the other it's the bottom/empty number.

Where exactly should I check the voltage? On the four-pin harness I unclip from the top of the sending unit? Or on the cluster somewhere?

I pulled up the wiring diagram and see which pins/fuses the senders correspond to, and I see the fuel pump one goes through fuse 28 to "Connector -1- (87) in the instrument panel wiring harness" (labeled A99) and "Ground connector -3- in the wiring harness interior" (labeled 277) (so that would account for its positive and negative).

I also see "Sender for Fuel Gauge" which has wires that lead to "Ground connector (sensor ground) -1- in instrument panel wiring harness" (labeled 269). I see another one from the sender terminate in a box that has "142" in it, but I'm not entirely sure where that goes to trace the positive. If I look in the "Standard Equipment" section, though, I can find the sender and the fuel pump again, and it looks like the same diagram.

In this one, the positive heads up toward the gauges and the low fuel light, and it says "T32/5" just below it. So that looks like it would be a 32-pin harness, terminal 5. Then from there it looks like another wire comes out (T32/3) and heads to A27, which is listed as "Wiring connection (vehicle speed signal), in instrument panel wiring harness." Huh?

So am I checking terminal 5 of a 32-pin connector for the voltage? And what voltage should I see?

Thanks again! Sorry for the lack of understanding on this!

Woodsie3

I should mention, that T32 is confirmed in the book to be the blue connector on the back of the cluster.

Woodsie3

Anything? I'm kind of in a holding pattern waiting to try to fix this myself. Thanks!