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Fuel Sending Unit Repair/Replacement (long)...


Old 11-18-2002, 04:36 AM
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Default Fuel Sending Unit Repair/Replacement (long)...

I posted a note a few weeks ago (<A HREF="https://forums.audiworld.com/a4/msgs/1385527.phtml">original note</a>) about problems with the fuel gauge on my 98.5 1.8TQMS and I thought I would follow up now that I've solved the problem. The problem was intermittent in that after filling up the tank, the gauge would function properly until I burned off approximately half a tank. After that point, it would basically read zero, with occasional spikes to a normal level. As the tank got lower, the gauge wouldn't respond at all and stayed at zero.

After getting tired of the bouncing gauge, I decided to investigate further and see if I could get at the sending unit. However, the procedure outlined in the Bentley required two special tools, with a rather hefty price (over $200 IIRC). I didn't want to spend that much considering the part was only about $50 so I figured I would see how far I could get without them. Turned out that they weren't necessary at all.

The access point for the sending unit is located on the floor of the trunk,behind the rear seat (right side), under a round panel which is secured by three screws. Removing the panel reveals two fuel lines and an electrical connector which come out through a plate attached to the top of the fuel tank by a ring or union nut, in Audi speak (sort of like the top of a mason jar). After marking (one is inbound and the other outbound) and removing the fuel lines (secured by single use crimp clamps) and the electrical connector, I was able to loosen the union nut by tapping on the outer ridges with a ratchet extension and hammer (being careful not to destroy it in the process). With the union nut off, you can pull up the top plate far enough to clearly see the fuel sender on the side of the fuel pump housing.

Now at this point, the Bentley says to remove the fuel pump but now that I could see the part, it didn't seem that this was necessary. I found that I could reach in and release the catch on the side of the sending unit with my fingers (you can hear it click), at which point the sending unit was easily removed. Inspection of the sender revealed the source of my problem. I found that the thin wire that connects between the fuel sender body and the float arm had broken in half. With a full tank (arm up), the two pieces of wire braid made contact with each other and the gauge worked normally. However, as the tank level lowered and the float arm descended, eventually the two pieces of wire would separate and the gauge would go to zero. I replaced the wire (soldered on a new piece), reinstalled the gauge, closed everything back up again and all is well with the world once more.

Some additional observations for those that might attempt this: Be careful when you pull the fuel lines from the top of the plate. While the inbound hose did not leak very much gas, the outbound hose did (despite my pulling the power to the fuel pump with the car running to try to relieve any pressure). Also, be sure to run the tank down as low as possible before you attempt this so you don't have to stick you hand into a pool of gasoline. I went to the station right after completing the job and put nearly 15 gallons of gas in my car (the most I've ever put in) but I didn't have to wade through gas either when I pulled the sender.

Finally, the Bentley appears to be in error in the range of resistance values it lists for the sending unit. While I found exactly 40 ohms with the arm up (full tank), I only got down to about 200 ohms (197-198 actually) as I rotated the arm down (tank empty) before the unit reads as an open circuit (infinite resistance). This seems to be perfectly normal as after installing the sender with my tank nearly dry, I did get a reading on my fuel gauge (albeit a very low one). This would seem to indicate that the range of motion of the float arm may be limited somewhat by the tank installation.

Anyway, I hope someone finds this information useful (sorry for not taking any pictures).

- Charlie
98.5 1.8TQMS (40,200 miles)
99.5 2.8 Tip Avant (37,300 miles)
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Old 11-18-2002, 04:48 AM
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Default Funny you mention this...

I did a write up with pics and stuck it on my website
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Old 11-18-2002, 07:30 AM
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hehe j/k...nice name
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Old 11-18-2002, 08:34 AM
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Default Wish I had seen it before I attempted this ...

would have eased my apprehension quite a bit.

Thanks for the pointing out the link.

- Charlie
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