So, the 280SE was feeling unwell, and refused to idle when cold. Took three or four tries to eventually get it to start, and it would jump between approximately 200 and 1600 RPM, and misfire when the throttle was touched. After a five-minute warmup, it'd be ready to go.
I eventually took the car to a friendly shop with the Bosch sign above the door, and among its faults (chief of which was the engine being old and worn - who'd believe that when it's only 22 years and 267000 km...) he pronounced the warm-up compensator guilty. Oh, and the fuel pump relay didn't always click in like it should. (Huh? I'd never had a problem with it.)
So I went off, and on the way home, the car died. In rush hour traffic. I didn't take the time to ogle the gorgeous women which are always nearby when I'm in an embarassing situation, being instead busy with jumpering the contacts to bypass the fuel pump relay which the mechanic had obviously placed a curse upon.
I was bloody lucky with the warm-up compensator - these things are hand-crafted in precious metals by Robert Bosch personally, judging by the price for a new one - I have a defunct spare engine and the parts number was right. Changing it was a fascinating job that would have been impossible without removing the oil filter. Good thing I was changing the oil anyway.
But I digress. I also had a spare fuel pump relay. Only problem was that it's smaller, and only has six plugs while the original has nine.
The faulty one has
31 15 87 87s KL V50 TD 30
in that pattern. (If you bridge 87 and 30, the fuel pump will run constantly, even with ignition switched off. That's what got me home, just like the mechanic advised me to, possibly feeling bad about having placed a curse upon it.)
The one I put in has
31 15 8750 TD 30
and thankfully I was stupid enough to try an obviously wrong one, because it works. Rev counter still works, Webasto is unaffected, everything looks normal.
So what is the difference when I'm missing three contacts on the relay? Did I just downgrade my injection system from K-Jetronic w/Lambda (which I think it is, being a 110.989) to just ordinary K-Jetronic with no fancy stuff?