R129 SL600 Main Engine Harness Rebuild

My primary reference for this job was from: MB SL600 7,2 Väth

As a DIY project this is and 8/10, and not really expensive. Maybe $50 in wire and $60 is parts (that you’ll break during disassembly.) I did buy some good tools to make the job easier including a premium soldering iron, shop lights, and multimeters. It took about 30 hours. The next time you could probably get it down to 20. A replacement is $3,000 so rebuilding it yourself is a reasonable option. What tips the scale in favor of DIY is that you must also rebuild the (2) MAF & (2) Throttle Body cable sets too and either no shop or dealer will do this or you’re replacing un-obtainable throttle bodies. In short, DIY is the only option. And if you’re DIY’ing those harnesses, may as well do the main harness too.

Here’s the harness removed from the car:

Watch the removal video here:

MB SL600 7,2 Väth has a great schematic that was referenced constantly during the rebuild. I updated it a bit for a broader audience and modified some notes that were confusing. Feel free to email me changes you feel are necessary:

M120 Main Engine Harness Schematics

After considering the tasks at hand for quite a while I settled on the following process:

Chop off the termination block.
Measure and label each wire from where we chopped the wire to the connector, then add four inches for the travel from the cut to the termination conductor.
Don’t measure the ground wires from the connectors to the shared ground wire. We’ll do that later.
Note the shared ground wires 16 & 17 are heavier gauge.
Cut the wires at the termination block about an inch high off the top of the block.
Solder each wire to the termination block.
Cover with shrink wrap and heat.
Replace the top of the termination block.
Cover wires with some sort of braided sheath to the firewall grommet.
Replace firewall grommet.
Cover wires from firewall grommet to where the first set of fuel injector leads branch off.
Place the new harness over the old, sort the wires to their destinations, and zip tie in place using small ties.
Measure shared ground wires from connector to shared ground wires. Note that in the original harness some travel down the harness and double back for easy connection to the shared ground. Zip tie in place.
Swap all the fuel injector connectors and wiring covers and boots. Be sure to swap the covers and boots before soldering on the conductors.
Label the fuel injector cylinder on the new wire. This really helps keeping things straight especially at 5,6,11,12 where they all branch from the same point.
Fuel injectors are now done.
Carefully cut open the round connectors. (See Below)
De-solder conductors.
Swap wire covers.
Solder conductors to new wires.
Epoxy round connectors together.
Break open rectangular connectors and de-solder conductors.
Swap wire covers.
Replace rectangular connectors with new from the dealer. They’re about $2.00 each.
Carefully swap the thermometer connector.
Verify conductivity of each wire from conductor to conductor and to grounds.
Reinstall onto the black harness tray from the car.
Reinstall into the car.
Fire it up!

Round Connectors
I’ve seen a few techniques on opening these and the truth is they’re built in such a way as to foil attempts to cleanly open. You see, they’re connected on both these surfaces:

So the only way to open is to cut along both sides, then brake around the round edge. When cutting the sides, note that you’re cutting both the outer cover AND a little interior part that holds the wires in place (the dremmel welded them together but the video clearly shows the two pieces):

Both my breaks were rough, but a little epoxy and careful matching afterwards I was able to get them back together. You’ll probably get epoxy inside the outer channel (that the car’s connector slides into) that I dremmeled out with the tiny mill like attachment. It’s a hard, messy job that I spent most of my time with.

The top of the connector will have one conductor marked with its pin number. That pin matches the pin number on the car’s connector.