Deceived
Common diagnostic technique can
produce misleading results with VW 01M

By Wayne Colonna
Technical Editor
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
Shift P fig 1.tif
Shift P Fig 2.tif
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Shift Pointers
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 on page 36, 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 (page 38) and 4 (page 40), 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.  TD