Unfortunately you probably have the early design valve body that had numerous problems. One of those problems was a conductor plate (the internal wiring harness) that had unshielded leads. Accumulation of normal wear debris can give electronic glitches to these. The plate also holds the temp sensor and speed sensors.

While the plate is less than $200 the valve body is available from MB for an outstandingly low price (less than $400 last one I used). The valve body includes the plate and all the solenoids so it makes a hugely better deal, except that for the early models one will also have to replace the controller.

Again, experience is a valuable commodity in the one you pay to fix this.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician