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Bosch ignition problem 700 1989

posted by  someone claiming to be jmarsnc on  Fri Feb 15 08:09 CST 2008 [RELATED]

My 89 745 turbo automatic has become unreliable at 240k miles because of what I believe is an ignition problem. It will stop suddenly and refuse to start, indeed the tachometer will not even bounce while the starter motor is turning over: Naturally it will not start in this state. Over the past 6 months I have found that I can unplug and then reconnect the power stage (near the battery) and it will often restart. Thinking the problem was the power stage, I replaced it with a non Bosch unit that FCP had for sale. This solved the problem and I had a trouble free 2000 or so miles. A few months ago the problem returned, thinking it was the not Bosch power stage, I replaced it with a new Bosch unit ($120.00) I found at a local supplier because FCP no longer had any available (Bosch or non Bosch). The new Bosch started acting up in the same manner within a month. I could get it to restart by unconnecting and reconnecting the power stage. eBay provided a used Bosch power stage which I mounted on the fender above the old one. Switching the stages provided restarting for a while but that didn't last long. At this point I could not make a trip to the dump (about 10 miles round trip) without an incident. Figuring that the power stage connected the ignition ECU to the coil and controlled the ground connection to the coil, I purchased a used ECU on eBay. At first I though I had it solved, the engine felt stronger and ran fine for a few days but now I am back to stage one. Today it quit before I got to the dump, I dis&reconnected and got to the dump only to have it happen again on the way home. This last time I had to swap out the ECU back to the original before I got home on the newer Bosch power stage as reconnecting or switching the power stage seemed to have no effect. The power stages both have plenty of thermal grease and heat sinks and don't seem to be even warm much less hot when they fail. Fuel consumption is up and the engine feels weak and surges.

So I am back to where I started. I am certain this is an ignition problem located in the ground circuit to the coil (which I also replaced with new one) but all my attempted fixes have not seemed to have any effect. Should I buy another new Bosch power stage? Get one of the ECU's rebuilt?Forget about the 745 and enjoy the new C30 that is reliable? Any input will be welcomed.
JMars






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