Robgla,

The test switch isolates the 4matic system, but IIRC, it also isolates the rear suspension from the pump. If your system is working properly. I'd leave it engaged. It will tell you when it has a problem and I doubt there's an advantage to running it isolated to running it in service.

Other opinions??

someguyfromMaryland

1991 300TE 4matic 215 k miles
(owned and played on since '96)