I have a 1991 G wagon, the previous owner has put the om606 engine on it. The issue i have is that since i bought it the differential lock wont lock in. If i press the button the yellow light comes on but the red light doesn't. I have changed the diaphragm in the transfer case but that didn't help. Does my g wagon have a auxiliary vacuum pump or does it take the vacuum from the engine? If it does take the vacuum from the engine then what does the switch do, what does it switch on?

Drive at slow speed when you press the button sometimes it takes a while to lock in.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

I've tried everything, it won't lock.

are you even getting vacuum to the diaphragm at the t-case? the most common "failure" i see for these systems is the vacuum line literally just falls off the switchover valve from a rubber vac line drying out. they'll be by the brake booster

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Allen Kroliczek
Oak Grove Autosport | Oak Grove Autosport
01 G500, 82 300TD, quite a few more.....