My 91 4Matic just had the 4Matic dash light come on. The car has 110K miles. I checked the hydraulic reservoir and OK. Put the control lever in test mode as per the manual. Light is still on as per manual states. Is there any other obvious things to check out? My wiring diagram doesn't help much other than show all the moduals in the system and I don't want to start replacing stuff without knowing what I'm doing. What normally fails on the 4Matic system? Thanks for all input.

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Handyjay