Not to be quick to judge or anything, but when there’s a dead gunshot victim on the floor, and the person’s spouse has just found out about an illicit affair, and has a history of jealousy and violence, and is still holding the gun which is still smoking and the ballistics match, and the ink isn’t even dry yet on the large life insurance policy on the deceased …. then, yes, this person is the most likely suspect.

That’s what I’m trying to find as to our 2000 Audi A6 4.2 project cars slipping in first gear. There are many components, but which one is the most likely suspect to be causing this symptom?

To recap either of our Audi project cars’ transmission issues:

• In reverse gear, the car is quick off the line and shows no sign of juddering or problems.
• In first gear, the transmission slips, as if it’s mostly in neutral plus a little bit of inching forward, and then with a bang and a jolt, it engages and goes into limp-home mode. Or perhaps it goes into limp-home mode and that causes the jolt. When this happens, the error code in the transmission control module shows a ratio mismatch, as in it analyzes the transmission output speed as compared to the input speed, and if there’s a mismatch, as in: it’s slipping, that triggers a fault code to be stored, and limp-home mode to be engaged. Interesting. So maybe first gear doesn’t eventually engage at all. Maybe the jolt is due to limp-home mode (i.e., 4th gear) engaging.
• Neutral works just fine.
• So now when I study the truth tables for these three gears, I see that all three of them work by having the pressure regulation valves 1, 2, 3 and 5 (not 4) energized. So, something else must differentiate them, and meanwhile I can rule out all five of the pressure regulation valves and their solenoids as being suspect.
• That leaves the three solenoid valves.
• For reverse, solenoid 2 (not 1, not 3) is energized.
• For neutral, solenoids 1 and 3 (not 2) are energized.
• That’s puzzling to me. That covers all three of these solenoids in some way, and doesn’t enable me to suspect any of them. So on to door number three.
• For 1st gear, solenoid 1 (not 2 or 3) is energized. So the difference between 1st gear and neutral is solenoid number 3 being energized or not. For neutral it’s supposed to be energized. For 1st gear, not. So, what if instead of being “not” it’s “kinda sorta?” Then first gear would behave somewhat like first gear, meaning the car would move forward, but first gear would also act a lot like neutral, which means: slippage.  So channeling the spirit of Sherlock Holmes, I conclude that a kinda-firing-when-it-shouldn’t solenoid number 3 could cause this, or a valve that’s sticking and behaving as if it’s kinda sorta closed when it should be fully closed.
• Mechanically, the big difference as to 1st gear is that the freewheel gear comes into play, so the transmission probably energizes the freewheel when the valves for 1st gear are in the correct on/off combination.

I feel good about this. It is all starting to make sense to me now.