280SE injection weirdness

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 V
50 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 87
50 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?
--
Roy G. Ovrebo

You don't know audio horror until you've listened to all 3000 railway
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
One of the functions of a fuel pump relay is NOT to pump if the engine stalls. Seems odd but after a collision in which the engine is snuffed one doesn't want fuel spraying into the wreckage. Be sure your pump relay stops the fuel pump when the engine stalls.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
[The fuel pump relay]

Ok, I got hold of a fuel pump relay with nine plugs. Most noticeable difference is a bit higher idle. Around 900 rpm, as opposed to ca. 750 with the six-plugged one.
I suspect my theory above holds true.
Roy G. Ovrebo
--
You don't know audio horror until you've listened to all 3000 railway
announcements used at Gatwick Airport to check for errors. -- Random
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.