I am new at posting to this forum, however, I have been reading it for quite a while. Thanks to the folks who have answered, unaware, many of many questions.

I have engine wiring/vacuum questions for my 84 300SD. I have searched alldata but they come up short in many areas.

One of the POs bypassed the switchover valve for the boost control. Where exactly should it be mounted and do you know who would carry a replacement?

Also, can anyone tell me where the engine control unit is mounted and what is its function? According to the diagram in alldata, it is wired to a temp sensor, rpm sensor, switchover valve, vacuum converter and throttle position sensor. Much of this wiring is missing and I will probably have to search the salvage yards.

This is my first Mercedes and it has been a great car. I have put nearly 40K, with over 320k on the odometer, over the past year in business and personal use!

Thanks

I don't know about the SD, but the '85 300D has the EGR computer (Big brown box) behind the passenger kick panel.

I have a spare one if you are interested in it.

On my 81SD the switchover (also called the crossover) is mounted on the left side firewall, above the brake master. Best way to locate is to trace the rubber line from the banjo bolt on the very rear of the intake. It should be in a tight area right between the intake and firewall.
Follow that line to the switchover. It should have an electrical plug into it, and then another small plastic line running out and to the ALDA.
That line carries boost pressure to the ALDA and in turn the ALDA dumps more fuel into the engine when running a boost. If you note weak accleeration and little or no black smoke on take off, you will likely find that line, or the banjo bolt, or even the valve is plugged up with goop. Clear it and find a lot more power.
The electical wires running to that valve serve to relieve an overboost pressure to the ALDA. To check it run 12v power to it and it should open and relieve the pressure on the incoming line.

The engine computer only controls EGR operation. You may as well not worry about it.

However, the overboost protection circuit is important.