Shift Pointers
- Author: Wayne Colonna, Technical Editor
Common diagnostic technique can produce misleading results with VW 01M
One step many technicians take in diagnosing transmissions that have electronically controlled line-rise problems is to unplug the pressure-control solenoid. If the solenoid, pressure-regulator valve and pump are in good working order, line pressure will rise to about 150 to 170 psi at 1,000 rpm in Park. If it rose to only 80-90 psi or didn’t rise at all, this would indicate that a problem existed inside the transmission.
This is a relatively reliable, “general rule-of-thumb” quick diagnostic technique, but you could be deceived if you performed this test with a Volkswagen 01M transmission. Here’s why: If you look at the hydraulics at the pressure regulator valve in Figure 1, you will notice two reducing-pressure circuits. When these circuits are charged, they prevent full line pressure from developing. And as you can see in Figure 2, these circuits are charged in the Park position. When the pressure control solenoid is unplugged in Park, line pressure will rise by only about 30% from the static or idle pressure when it was plugged in. This means about 80-90 psi. And you would be deceived into thinking that you have internal transmission problems when you do not.
To use the diagnostic technique of unplugging the pressure-control solenoid with the 01M transmission, you must place the selector lever into the Neutral or Drive range (with brake applied) to perform a full line-rise test. As you can see in figures 3 and 4, the reducing-pressure circuit with the largest area is exhausted, allowing the pressure-control solenoid to provide full boost.
These partial hydraulic schematics were taken from ATSG’s Audi/Volkswagen 01M Technician Diagnostic Guide, which provides full hydraulic schematics for both early- and late-design valve bodies. Both valve bodies function in the same manner when it comes to having two charged reducing-pressure circuits at the PR valve in Park.
