Hi. I have been working on, (and learning about), my 91 740 Turbo. I had a similar problem with fuse #11 not powering up. Here is what I found. First, according to All-Data, it takes a minimum of 6 volts from the engine speed sensor to the FI ECU to power fuse #11. When I started, I had only 5.8 volts. I cut the wire from the FI ECU at each connector it passed through, and spliced/soldered the wires together. I also removed the speed sensor itself & filed a LITTLE off the bottom of the sensor to get the magnet a touch closer to the flywheel. The result of the above was the engine speed signal jumped to 6.5-7.5 volts, and the fuse 11 powers up now just as it should.
GOOD LUCK!
Ken

If you were missing spark and corrected that by replacing the crank sensor, it is likely that this was your original cause of not running.

The only times you should find power at fuse 11 is; 3 seconds after turning key to II after being off for > 15 seconds, during cranking, while engine is running. Exactly what times did you check for power there?

Recheck. If you find it immediately after turning key to II but not during cranking, then you are missing RPM input. If you never get it.. suspect relay first (unless it is new, not used). Next step would be to check for ground potential (voltage drops from +12 to near zero) at both 86/1 (red) for AMM and supressor relay power, and 86/2 (yellow/black) for fuel pump power. If they both drop below 1 volt during cranking - you are back to the relay itself.

The "new" relay is still suspect. There is nothing between it and fuse 11, so if the relay works, fuse 11 will get voltage.

Another possibility is that the ECU is failing to complete the relay coil ground (86/2 yel/blk wire) when it gets timing pulses from the Ignition CU. If you can probe 86/2 while cranking and it stays at +12V, the ECU isn't grounding it.

Or the timing pulses aren't making it from ICU to ECU (on opposite sides of the car). There has been one case of that here in the recent past, due to wire chafing on the sharp edge of a bracket. That was the yel/gray wire from ICU 17 to ECU 1, with 2 intermediate connectors. Same for Bosch or Rex/Regina (which is yours?).


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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.