boost switchover valve
 

I finally teed in my boost gauge in my '87 190DT. I put it in past the switchover valve. I saw a whopping 0 lbs of boost then. Well, I unplugged the switchover valve and everything was good. Got a smooth 0-15lbs of boost. So my question is, why is the switchover valve constantly energized when the key is on? I tried to search for it but could not find anything.

My experience is with an 87 OM603, which I suspect is similar to the OM602 you have.

There is a pressure switch in the manifold. It looks like a big nut with an electrical wire on the back side.

If that switch fails closed then the dump valve will be energized.

It may be plugged with goo but I would be afraid of damaging the diaphragm in there trying to clean it out. Since its function is rather critical I would replace it.

My understanding is that if you unplug this, it should bring the boost back if it were the issue. However, unplugging it makes no difference.