01M Failure Analysis1
Ross brought me this 01m with a complaint of “no reverse without giving it some gas warm”. Here are the steps I took to diagnose it and the internal failure that made his problem.
Note: order of analysis changed for dramatic effect. 2nd Note: Sometimes, I use some “general” terms that are used in transmission industry, not necessarily VW terms.
First I road tested the car. The trans worked exceptionally, I graphed TCC and it was very good. It had none of the slipping I’ve documented in other places. I saved a graph, but it’s on another computer. I’ll add it later.
01m’s tend to be slow going into gear normally, this is not what Ross had. His car would go into reverse normally cold but once it warmed up, after placed in reverse it would just sit there like in neutral, till you raised the rpm just a bit. Then it would “thunk” into gear and back up as it should. Important piece of info here is that it would NOT do this scenario in forward. This means it’s probably not due to low fluid, but of course you should check this anyway just to be sure.
I put a pressure gauge on to see what it showed. No action shots, but in reverse warm, with problem happening, pressure was 15 psi lower than when cold, without problem happening. Goosing the gas just a bit, would bring pressure up to normal and above, making reverse “clunk” in. In forward there was no significant difference in pressure hot to cold.
This behavior is plain and simply a pressure loss to reverse. It not a problem cold, because the fluid is thicker cold and the thicker fluid would have a harder time, “leaking” off. So the question is, as always, is it fixable or does the trans need to come out.
In reverse the B1 brake and B2 clutch are on. Reverse pressure passes from the valve body, into the B1 piston housing through the case through the “reverse feed tube” in the following picture. Its location is shown in the 2nd picture.
Notice that the “reverse feed tube” has 2 o-rings on it. It has one on the bottom to seal against the B1 piston housing, and one to seal against the case. If either of these is cut or “flat” it could leak the B1 brake pressure. These O-rings are the only hope of repair in this situation, so it’s worth pulling the valve body to check them. Everything else that could leak pressure in reverse is inside, requiring complete trans removal and disassembly to access. This usually means most shops would recommend rebuild or replacement. Unfortunately for Ross, his o-rings were fine.
The next step was to pinpoint if the leak was in the B1 brake or the K2 clutch. You can do this with an aircheck of the B1 brake. I’m showing this step with trans on the bench but just as possible with the trans in the car.
When air checking a clutch you should pull the trigger on the blow gun, then immediately let off the trigger. On a good air check you would hear a “thunk” of the clutches applying then immediately yank the blow nozzle out and you should get a burst of air back out of the hole, indicating good sealing and the piston return springs pushing the piston back, making the air rush back at you. Some leaking of air is normal on some circuits with sealing rings, but this circuit doesn’t have any. Actually on this circuit you can see the clutches apply, there to the right of the blow nozzle under the detent spring.
Ross’s B1 brake I could pull the trigger on the blow nozzle up to about 60%, and just let it blow and it would never apply the clutch. If I took the trigger to above 60% it then the clutch would apply. This is a significant leak! Obviously Ross had a B1 brake leak, causing his “no reverse without giving it some gas” complaint.
Air check of the B1 showed the problem and it’s a bit harder to air check the K2 so I didn’t air check the K2.
What I left out, and why the pictures are on the bench instead on in the car, is because when I pulled the pan to pull the valve body to check the B1 feed tube o-rings, I found this.
You can see the large chunk. Notice how it looks striped? I’ve found this same chunk before in other 01ms. Plus the magnet has more than normal sludge on it, and the bottom of the pan looks like “panning” for silver.
Always remember when pulling your pan, to break open your filter, because the pump makes suction through the filter and it can suck up these pieces and you could pull the pan and think your pan is clean yet chunks are trapped in the filter.
Here’s what inside the filter looked like.
So when I discovered this Ross had a important choice to make. He chose to replace it with a good remanufactured trans with a 5 year, 75,000 mile warranty.
Here’s what I found on the inside. These are the needle bearing from the thrust bearing that goes between the planet and the sun gear. Oddly enough none of these needles made it into the pan.
Pictured is inside the case with most parts removed. The clutch in the pic is the B1 brake. The hole in the case is where the B1 feed tube passes through the case to seal against the B1 piston housing.
Here’s a good thrust bearing out of another trans.
Bearing rides on the sun gear shell and damaged it pretty good, though it can’t be seen in the picture. Excess clearance due to failed bearing allowed sun gear shell to move and contact the B1 piston housing.
And here’s where the “striped chunk” came from.
And I have to say all of this is just a interesting side note. All the metal and broken stuff failing was not related to the b1 pressure leak. Here is its cause.
On the B1 brake piston, the rubber lip seal that is Molded onto the piston had come apart separating from the piston. If it weren’t for the broken up bearing and damaged planet this piston by itself could have fixed this 01m.
Of course “while your in there”…….