I guess the "engine speed" input came in coincidentally with that blue vacuum servo thingie on the L/H firewall. The one that mostly produces an improved simulation of
manifold vacuum for the transmission.
Here's where the plot sickens, as my aged mother used to say. My car is a bit of a bastard child. At some point in time, somebody installed an engine from an '81 300TD wagon in it. This is a turbocharged engine - the wagon got the turbo one year before the sedan. The under-hood environment is a pseudo-random mix of '81 stuff and '85 stuff. For example, the existing condenser was appropriate for a 240D! Took some research and a trip to the junkyard to get the right condenser....
No, the tach doesn't work. And it looks like they installed the '81 transmission along with that '81 engine, because I have been unable to find an engine speed sensor on the transmission. Best I can tell, this
sensor should be up on the L/H side of the bellhousing. Nothing. Another thing needed to make the blue servo work, is an injection pump with a rack position sensor, which mine does not have. In fact, my injection pump does not even have a proper ALDA - it has the earlier, round "altitude compensator". And even if I got the blue
vacuum control unit working, I don't know if it would do a good job with the '81 transmission.
When I bought the car, it did teeth-jarring
shifts. It turned out somebody had blocked off the vacuum to the tranny. I researched the system, understood the problem ( transmissions need manifold vacuum, diesel cars don't have it ), and hooked up the Vacuum Control Valve ( VCV ) on top of the injector pump per the early '80's system. This gave good shifts at low power levels, but the simulation still breaks down when you stomp on it. Close enough for government work....
Another thing that's missing is the "engine cold lock" sensor. The connector dangles at the end of its cord.
I don't even know if the wires that I connected to the compressor clutch were the right ones. The brown ground wire, that's
a slam dunk. The other wire was green/blue/black, and I can't find that in any of the schematics.
Assuming that it's not reasonable to retrofit the sensor to the transmission, I may try to adapt a harmonic balancer sensor to do the job.
But I see no reason to have engine RPMs affect the action of the compressor. Hopefully, I can downgrade this system to work properly as an '83 or so. With 12V from the pushbutton box through the aux fan and compressor relays, and input from the ETR switch. O if only I had a proper schematic!