Bellybutton Logic

While driving up to my shop one bright Monday morning, I noticed a silver Audi A4 sitting in the service drive. “Someone must have dropped it off over the weekend,” I thought to myself. After performing my morning ritual of turning the lights on, firing-up the computers, the coffee maker and the air compressor, I retrieved the “Early Bird” envelope from the drop box. When I tore it open, there was only a key and a short note inside that read: The fuel gauge is wrong…my son’s bellybutton is more accurate! Call me…John.

Of course this really peaked my interest. I had never heard that analogy before – comparing an automobile’s fuel gauge to a kid’s bellybutton. I called the number John had written on the envelope and a woman answered. I identified myself and asked who she was.

“I’m John’s wife, Penny,” she answered. “The Audi is my car.” After I related the details of the note, John’s wife started to chuckle. “He’s talking about our son Bryce’s bellybutton,” she laughed. “When he’s full, his bellybutton sticks out and when he’s hungry, it goes back in. As for the Audi’s problem, the fuel gauge never reads full. John thought there might be something wrong with the sender, whatever that is. Anyway, John said the tank was full when he dropped it off at the shop. I hope you guys can fix it.”

“It could be the sender or it might be something else,” I advised. “As soon as we diagnose the problem, I’ll get back to you.” After I hung up the phone, entered all of the pertinent customer information into the computer and generated a repair order, I looked for a TSB that might shed some light on the problem.

I found one that addressed the fuel gauge problem perfectly, printed it and attached it to the repair order. Here is the procedure recommended by the TSB:

The fuel gauge on a 1996 Audi A4 with a vehicle identification number (VIN) prior to 8DTA004690 could fail to read “Full” after filling the gas tank completely. The cause may be a significant voltage drop at the common ground for the fuel gauge, fuel pump and tail lights.

Repair Procedure

1. Obtain the radio code and note the customer’s preset radio station settings.

2. Disconnect the battery ground (GND) strap.

3. Remove the rear seat bottom.

4. Locate the 0.5 mm ground wire from the fuel sender (It comes from a 4 wire harness near the right rear seating position).

NOTE: There are two brown ground wires in the harness (1.5 mm and 0.5 mm), isolate the 0.5 mm wire.

5. Cut the 0.5 mm brown wire from the sender and attach a new wire (approx. 12 ft long). Solder the joint and install a heat shrink sleeve on the connection.

6. Remove the driver’s side sill molding.

7. Carefully route the new 0.5 mm brown wire along the existing harness to the driver’s side of vehicle, forward along the carpet edge to the ground lug under the left side of the dash.

8. Solder an 8 mm eyelet connector to the end of the new ground wire and attach it to the ground lug on the A-post.

9. Reinstall the driver’s side sill molding.

10. Reinstall the rear seat.

11. Reconnect the battery ground strap (GND).

12. Reprogram the customer’s preset radio stations.

NOTE: You will need to reprogram the power windows as follows:

1. Lower all the windows.

2. From outside the vehicle, raise all the windows using the key in the door lock cylinder.

3. After the windows are raised, turn the key in the lock position once, wait 3 seconds and turn to the lock position again.

4. Check that all the power window features are operational.

I called Penny and got her approval to do the work. Within an hour, the technician had successfully solved the fuel gauge problem using the information outlined in the TSB.

That afternoon, John, Penny and Bryce dropped by the shop to pick up the Audi. “I guess vehicles these days can be fairly complicated,” John remarked. “I never would have guessed it was a wiring problem.” Smiling, I replied, “If only cars could just be as simple as Bryce’s bellybutton.” John and Penny laughed. “You know, we just had lunch,” said John. “Hey Bryce, show Rich your bellybutton.” Proudly Bryce raised the front of his T-shirt. Sure enough, his gauge read “Full.”

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Written by ALLDATA Technical Editor, Rich Diegle. Rich is an Advanced Engine Performance Certified, ASE Master Technician with an AA Degree in automotive technology and 22 years of dealership and independent shop experience.