Motronic 1.0 troubleshooting & jumper kit

One of my favorite troubleshooting techniques for no-start and other flaky running problems in all versions of Bosch digital fuel injection systems is to jump the main & fuel pump relay sockets. It’s pretty easy to jump these contacts with a few short sections of wire, but that’s a jury-rigged solution. Better to make a custom set of jumpers, and leave them in the toolkit or glove box. You’ll need five standard 1/4″ male spade connectors and three short 4″ sections of 14 gauge wire (five 6.3 mm male spades and three 100mm lengths of 1.7mm wire). Strip the ends of these three wires. To make the main relay jumper, twist two wires together into a ‘v’ shape, then crimp male 1/4″ spades onto all three ends. For the fuel pump jumper, just crimp two male 1/4″ spades onto the remaining wire. These jumper wires will temporarily substitute for the relays in your car.

Note: The pumps will power up as soon as they’re jumped, usually with a spark, so be sure pumps are installed properly, fuel lines buttoned up, etc.. So before plugging these jumpers in, take all recommended fuel-related precautions (have a properly rated fire extinguisher handy if at all possible). If you smell raw gasoline and can’t find the source, have the car towed instead. Gasoline is not only smelly, but highly flammable!

Here’s some pics for how they’re used:

As the pictures above demonstrate, you’ll use the main relay jumper to “short” or “jump” socket terminal 30 to both 87 terminals. Use the fuel pump relay jumper between 30 and 87.

A few cars will have the main and fuel pump relays on the dashboard support inside the car. Most other cars, similar to my pictures above, will have them mounted on the fuse box in the engine bay. You can easily verify the proper relay by wire colors. A main relay socket will have five terminal pins, one is always a thick red wire coming directly from the battery. The fuel pump relay is fed from the main relay, it should have a split pair of green wires with a violet stripe running to the fuel pump(s).

As for the rest of the system, I learned a lot from this excellent tutorial on troubleshooting the Motronic. Highly recommended! Print it out, run through all the sections applicable to your car, enjoy the clear & lucid prose, leave it in the map pocket. I make a point to leave a copy with my compliments for the next enthusiast! I can also unreservedly recommend the author’s silicone hose kits (HPSI Motorsports). I’m not sure if they’re still in this business, but I had good luck with them back in 2010 or so. Great modification for our vac-plagued BMWs, and the black hose looks nearly OEM.

Also be sure to get yourself a copy of the proper year and model Electrical Troubleshooting Manual. They were published by BMW, so they’re [nearly] 100% accurate, and can easily be found around the ‘net in PDF format, like at Miller Performance or Wedophones (both highly recommended vendors). The ETM is essential! Don’t leave home without it! It’ll seriously save you hours of hair pulling. I’ll write notes in the margins, or on the flip sides, just so I can recall what I did the next time I’m in there. I’d also recommend leaving a copy of the ETM in the map packet. It’s not like you need it for maps, eh?