First off, please read this excellent tutorial on troubleshooting the Motronic. Highly recommended! Print it out, run through all the applicable sections, then leave a copy in the glove box for the next enthusiast! Well, that’s what I do anyhow. I can also unreservedly recommend the author’s silicone hose kits (HPSI Motorsports). Great modification for our vac-plagued BMWs, and the black hose looks nearly OEM.
Secondly, 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’ve had this problem repeatedly myself, and heard about it all over the internet. So instead of re-typing this in a thousand blog posts, I’ll put my shortcut notes here for anyone to find with the Google. I’ll pass along a bit of hard-earned wisdom. Some of these notes repeat sections of the Motronic guide up above, please read that first.
- 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 ths plug is decently grounded. A yellow spark isn’t ideal, but OK in most cases.
- 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.
- 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.
My troubleshooting solution is to jump the main & fuel pump relay sockets. You can use a short section 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 your wires. To make the main relay jumper, twist two wires together into a ‘v’ shape, then crimp three spades onto that wire. For the fuel pump jumper, just crimp two spades onto the remaining wire.
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, 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).
Use the main relay jumper to short socket 30 to both 87b. Use the fuel pump relay jumper between 30 and 87.