D-Jetronic chapter 10: Engine wiring harness

After 40 or more years a wiring harness falls apart. First the rubber sleeves tear and let water get to connectors. Then connectors corrode as well as mass contacts and you loose proper contact. Additionally cable isolation falls apart and you will have unwanted and partially vibration dependent short cuts. That's the point in time when you are in urgent need of a new engine wiring harness. But, oh dear, your car manufacturer does no longer supply a new one. And in the market you often only get rubbish, see the experience that Markus (foxbravo) made in I want my money back. Why is business with wiring harnesses so complicated? Because it must vary not only by engine type but also by car model and year of make.

Some examples: MB's 350 engine M116 was installed in series R/C107, W108, W109, W112 and W116 engines. Depending on car model ECU and MAP sensor are located at different places of your car body. Either in front right or behind glove box. Additionally you have specialities depending on year of make like thermo time switches with screw connection or with a push-on connector. Trigger contacts can be connected via cable or a 5-pin connector etc. Other car manufacturerslike Volkswagen started with absolute pressure switch, then used full load transition in MAP sensor and finally a full load contact in throttle switch. In the end you have to de-install your existing engine wiring harness, put it on a large work bench or carton where you mark all distributions and exits and connectors. And then of course build a new wiring harness to be exactly like the old one. There are only very few cases where car manufacturers still deliver new wiring harnesses. And you can ask yourself how long they have been in stock and how well rubber sleeves and cable isolations have been conserved.

I recoomend you to do this job in winter by yourself. There are no reliable suppliers. Old wiring harnesses that are used or have resided in stock for ages don't pay off and let's be honest, doing a new wiring harness is just a matter of effort if you have a some technical abilities. This work consumes a lot of time and you would have to pay a lot to a professional. That is why a price of up to 1000 USD for a new wiring harness is not surprising. With the tips by foxbravo, matze280 and me, you will be able to source the material for less than half of this price and you can later be proud of your achievement!

What do you need for making your own wiring harness?

10.1 Analyzing the old wiring harness

Your job is easiest if you anyhow take the car apart for a repaint. Of course you can also replace the existing engine wiring harness without taking everything apart. But your engine won't run as long as you do so. Strip your wiring harness from all sensors and mark every single connector with a label describing what was connected. It is necessary, the air temperature fits on an injector and vice versa! I already had such failures. Now you distribute your old wiring harness on the floor of a big room or a large work bench. Your beloved one will love to see you do so in salon. Now mark the wiring harness including all junctions and connectors on a carton. For every connector, note down the following, Mathias did even take a photo of each connector:

1. What for is the connector
2. Pinout of connector, cable colours and dimensions (0,75 mm² is standard, I would use 1 mm²)
3. Type of connector (like blade receptacle 2,8mm or ring connector 4 mm)
4. Where does each cable lead to (Use multimeter to measure if necessary)
5. Diameter and length of each PVC isolating tube
6. Draw shape of connector on carton, so that you remember exact placement and length.

After analyzing and noting down everything in you PC in a table, compare it to the cable plan that you find in Bosch's workshop manual for your engine. If they match, you can now find out those points in wiring harness where in cable is split up in two or more continuing. After that you can measure the length of each cable and thus create a shopping list of cables with colours and diameters, connectors, contacts and rubber sleeves. Markus (foxbravo) has provided his plan for his MB 350 SL Please login for downloading and Mathias (matze280) for his MB W108 280SE 3.6 Please login for downloading via an Excel spreadsheet.

10.2 Sourcing material

First you can of course ask your car manufacturer whether he still supplies components. But do ask for the price. MB charges 400€ for rubber sleeves for an 8-cylinder. "Quite shocking!"

I do attach links to alternative suppliers here. Unfortunately repro-parts does not publish prices, but they used to charge 2 to 4 € per rubber sleeve.

I do not earn anything on these recommendations and I do not trade these goods. I only received samples form repro-parts.de and I recommend them as car manufacturers and Bosch like to milk us on these parts.

Regarding the crimping tool: It is important to have one that properly locks the unisolated blade receptacle on the cables. Such a hobby tool is by no means suitable. It is important that bothpins are bent in a way that they lock well on wires and isolation. One variant of a good tool is Knipex PreciForce 92 57 34 for unisolated blade receptacles with a well determined pressure.

10.3 Let's get started

Now you have all information in your computer to build a new wiring harness. You bought all material and now let's get started. Biggest difficulty now is connecting the old 25 pin connector towards ECU to your new wiring harness. As the contacts in that connector are not available any more, neither from car manufacturers, nor from Bosch or form AMP/Tyco, you have to reuse your old contacts. Edit: I happen to have found a limited no. of NOS contacts. I recommend to extract them piece by piece from the old connector. Use a needle and push away the locking pin from the side through the front. Now you can extract the cable including contact to the back. Brave fine mechanics now open old crimpings and attach new cable via soldering plus a short heat shrink tube. Alternatively you leave a short piiece of wire and solder the new wires to the old ones. There is no better solution as you will not find these contacts as new ones. But I cannot create any hopes for you as I have contacted every possible supplier. Who plans a longer time for a new wiring harness should consider to buy a used 25 pin connector. But be careful: depending on car and motor type the number of contacts vary.

Now you start from ECU connector and move from distribution to distribution. Hereby you fix PVC isolating tubes via a dhesive sockets. The advantage of these is that you can easily open them again if you did a mistake. Do not forget bushings! When you did all wires, verify correct connections with a multimeter. If correct, use a good crimping tool for receptable blades. It is a good idea to use writable heat shrink tubes with labels according to the wiring plan in Bosch's workshop manual. Brother offers such solutions. I intend to use black heat shrink tubes with white writing for my own wiring harness.

In the end you attach the rubber sleeves and insert the receptable blades into connector housings. Be sure to check well that they properly lock in the housings. I once searched for ages, to find an unlocked blade in a connector of a new 1000€ wiring harness. I would see contact when using multimeter, but during driving contact would often fail. Don't ask how long it took me to find this fault.

Finally there is the big moment, when you install the new wiring harness. First lay it parallel to the old one on the engine and verify that all connectors are at right location. Now remove the old wiring harness and install the new one. And if your engine runs smoothly after first start, feel like a king!

This article was created with the help of Markus foxbravo and Mathias matze280. Many thanks to them for proof reading, photos and tables and the approval to use them here.

Wishing you a succesful replacement of your wiring harness!

Your Dr-DJet (Volker)

Your Dr-DJet (Volker)