I recently made repairs to my 280sel 4.5 engine wiring harness. The connections to the sensors and injectors were not very good.
There have been threads about this topic with some out of the box ideas like converting to L-Jet connectors etc. but I wanted to share this as it didn't cost much and you can do it with the harness in place.
Here is a pic of one of the existing injector connectors with remnants of the old boot. This is typical of what unrestored d-jet connectors look like. Poor connections to injectors and sensors will lead to all sorts of running problems.
Now you can find d-jet connectors at places like Rock Auto and other and they will look like this below - where the terminals are already inserted and wires attached. This set up forces you to solder or crimp the wires onto the existing harness wires. I don't like this approach as it looks clumsy and leaves more chance for bad connections
Connectors, terminals and boots may be available through MB (I checked pricing but not availability) but I did find a site called Finjector.com that sells all sorts of fuel injection supplies. These guys are based in Finland and were very good to deal with. Prices for the connectors, terminals and boots were quite a bit less than MB. They also sell wire sheathing which I got as well.
I only replaced the 8 injector connectors, cold start valve connector, coolant and air temp sensors - 11 in all. Finjector also sells similar set ups for the throttle switch and MPR
After cutting off the old connector and cutting (make that cracking) off the old wire sheathing, put on the new sheathing and boot first. Then crimp on the new terminals with a simple crimping tool
After that, insert the new terminals into the new connector
Connect it up and slip the boot over - job done
I also made some repairs to the ECU end like putting on a new ECU cover boot (my recommendation is not to do this task- too much trouble and harness terminals to the ECU are now NLA)
Car does run really well now.