Every once in a while you run across a scenario that has you scratching your head. This was one of those units.
Our warranty dept. received a call from one of our retail locations. They had a 2002 Volkswagen Jetta with the O9A transaxle. The unit had been out approximately 300 miles and came back because of a no reverse complaint. All the external checks that could potentially inhibit reverse were investigated, everything was working properly. It was determined the unit would have to come out for further evaluation.
We received the unit back at the plant, and ran the unit on the dyno. Sure enough, when reverse was selected you could feel a partial engagement then a neutral. The drive engagement was fine. After air checking the unit I started with the valve body, taking it apart. Everything looked good, all the valves were free, the casting surfaces, separator plate and gaskets all looked good. No problem found.
I had all the solenoids retested and they checked out fine.
Ok, it’s time to go into the unit, checking all components on the way out. Nothing was stripped or broken and the unit was assembled correctly. The unit was reassembled and it was run on the dyno, still no reverse engagement and no low/reverse clutch pressure. Something is inhibiting reverse, somehow, somewhere.
Forget about looking at an oil schematic, Volkswagen does not publish them. We pulled the unit off of the dyno removed and retested all the solenoids again, they retested good. I replaced the full set of solenoids, as a starting point and put the unit back on the dyno. The unit had a good reverse now. The issue was resolved, now its time to figure out why.
We increased the control pressure to 75psi and it would work properly. Now its time to duplicate the actual operating pressure of the solenoid in the vehicle. To do this we had to do some creative engineering. Our machine shop fabricated a “test” solenoid with a pressure port. Figure 2.We installed the test solenoid in the unit. Figure 3. The pan was then installed. Figure 4.Then the vehicle was started and the actual solenoid operating pressure was approximately 66psi. Figure 5. It was amazing that only a 9psi difference of control pressure would influence whether the solenoid worked properly or failed (stuck). Now that we determined when the solenoid would fail we had to find out what was going on inside the solenoid.
The solenoid was disassembled and inspected (Figure 6) there was nothing obvious with the pintle or solenoid body, after studying the pieces to see how the solenoid operated mechanically I noticed the small plastic snout of the pintle would seal in the corresponding pocket in the plastic snout of the solenoid to shut the flow of oil off. What was happening was the tip of the pintle would wear enough (not much) and was sticking in the sealing pocket of the solenoid snout. Figures 7 and 8.