Motronic 1.0 troubleshooting & jumper kit

One of my favorite troubleshooting techniques for no-start and other flaky running problems in all versions digital fuel injection systems is to jump the main & fuel pump relay sockets. Bosch makes it easy to jump these contacts with a few short sections of wire, but that’s such a jury-rigged solution. I make a custom set of jumpers for each car. You’ll need five standard male spade connectors and three short 4″ (100mm) sections of 14 gauge (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 each end. For the fuel pump jumper, just crimp two male 1/4″ spades onto the remaining wire.

Use the main relay jumper to short socket 30 to both 87b. Use the fuel pump relay jumper between 30 and 87.

Before plugging these jumpers in, take all recommended fuel-related precautions (have a properly rated fire extinguisher handy if at all possible). Gasoline is not only smelly, but highly flammable! The pumps will power up as soon as they’re jumped, usually with a spark, so be sure the wire harnesses are secure, pumps are installed properly, fuel lines buttoned up, etc..

A few cars will have the main and fuel pump relays on the dashboard support inside the car. Most others will have them mounted to the relay 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 ’10 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 anymore, eh?

As for my personal troubleshooting technique, I’ll put my shortcut notes here for anyone to find with the Google. My method isn’t nearly as comprehensive as the Motronic guide up above, so please read that first. My method:

  1. I like to test for spark first. Pull a plug wire, plug in a spare spark plug, lay it on the engine and crank her over. You should see a pretty nice bluish spark if this plug is decently grounded. A yellow spark isn’t ideal, but OK in most cases.
  2. Then I’ll test the crank angle sensors and the coolant temp sensor. Older Motronic 1.0 cars (mid-80s mainly) use two senders in the bell housing. One senses the flywheel position with a single pin, the other one senses the flywheel starter teeth to estimate RPM. The position sensor (CAS) and coolant sensor (CLT) signals are required for the motor to start! You can swap the speed sensor with the CAS sensor in an emergency. They’re the same part, just be sure to match it with the proper plug on the manifold support. Please use a pick or small screwdriver to remove those PITA Bosch locking wires. Pulling the plugs out directly is possible but will eventually break the mating plug, allowing humidity and water into the connector. Later plugs have spring-loaded locks that are much improved.
  3. Then I like to test for power to the appropriate ECU (DME) terminals. If that’s all OK but you’re still not getting power to the fuel pump, keep reading. Here’s the cause: your DME can’t sense the running engine, so Bosch shuts down the pump as a safety precaution. To use the old programmer’s phrase: “This is a feature, not a bug.” Alternatively, the relay could be working but unable to provide enough amperage through the load terminals. This commonly happens when incorrect parts are specified and non-resistor relays end up in those sockets.
  4. …to be continued…