I have an 89 S4, 141,000 miles. Well maintained. Occasionally it will not crank. I have replaced the battery, the starter, the electrical portion of the ignition switch (checked the drive cam on the ignition lock, OK), cleaned all the connections at the battery and starter and measured the current drop from the battery to the starter (.09V). This last time, I had been driving the car daily, I brought it home parked in the garage. The next morning Nada. For a month, nada. I had to wait for my mechanic to return from vacation. Today, the flatbed came and voila' fired right up (no jump start, just turned the key and vrooooom).

Anyone had similar experience and find the cure? Please help. I love my shark and want him to be a daily driver.

 

Did you check the connections on the engine ground strap? It's located under the car on the passenger side, and connects the engine to the chasis. Check that the cable is in good condition, and that the conections at each end are clean and tight.

 

Go to Home Depot or similar store and buy eight feet of lamp cord (18 gage two-conductor fine-stranded electrical wire)and two alligator clips. Get a 12 volt tail light bulb.
Solder an alligator to each conductor on one end. Solder one conductor to the bulb center contact and one conductor to the bulb base on the other end. Touch one alligator to the jump start terminal and the other to ground - the bulb should light.
Open the central power panel access, and find the oxygen sensor wires at the base of the panel that lead to the rubber grommet in the floor next to the tunnel. Pull/push the grommet out of the hole, and feed the end of your tester with the alligators thru under the car.
Get under the car (SAFELY!!) and attach one alligator to the starter solenoid terminal that has the small yellow wire. Make sure that it won't touch the large electrical terminal. Some electrical tape on the alligator might be a good idea. Attach the other alligator to a good ground nearby. Make sure that none of the wires, including the ones going from the grommet to the oxygen sensor, will get on the exhaust as you drive the car.
Run the wire end with the bulb out to where you can see it, and close the power panel access.
Every time that you hit the starter, the light should glow brightly.
If you hit the starter and the light glows brightly, but the starter does not function, the problem is in the starter or the power to the starter.
If you hit the starter and the light does not glow, you know that the problem is in the yellow wire or its feed.
Let us know what happens, and we will go from there.

 

Wally, If it is the yellow wire or its feed, then what? Are those problems capable of being this sporadic?

BTW, we checked the ground cable from the engine to the chassis.