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Old 08-15-2011, 12:26 PM   #1 (permalink)
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Join Date: Aug 2011
Location: CT
Car: 1987 535is
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No voltage, 87 535i MCU pin 18

Have a 1987 535is that has the basic motronics engine management control system. Vehicle after injector change and other work will not start. Have checked voltages at main relay and diagnostic connector, both checked out OK. Checked voltages at injectors, voltage was correct. Checked fuel relay, Checked speed and reference sensors, ETC. All seems well. When checking voltages and resistance at motronics connector I found that I have battery voltage at pin 35 of the motronics connector when checked as described in Bentley manual. However do not have battery voltage at pin 18. Both of these pins are fed from the red/blue wire coming from pin 87 of the main relay, through the main harness running along the fuel manifold, and on through the passenger side firewall terminating in the motronics connector. I did jockey the harness around a bit when I was replaced the injectors and at the same time I removed the intake plenum to facilitate replacement of the starter. I did check out the main relay and it was ok. I have not tried to check continuity between the main relay connector and pin 18 of the motronics connector yet. Plan to do that shortly. I also plan to cut the wire ties holding the main harness to the fuel manifold and exercise the harness a bit to see if voltage returns (maybe a corroded or broken wire). I would appreciate any advice and ideas to further trouble shoot this problem and determine, if necessary, how I get power to pin 18. I.E. do I try to pull a new wire through the harness, do I tap into the wire feeding pin 35 inside the car, do I run a new wire and marry it to the outside of the harness. Again, any comments, advice, or help is appreciated.
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