Repetitive ECU break down

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OMiddelhede
Posts: 1
Joined: Tue Nov 22, 2022 8:43 am
Location: Denmark

Repetitive ECU break down

Post by OMiddelhede »

Gentlemen. Your kind advice is much appreciated. My ECU keeps breaking down every time I accelerate from app 100 to 130ish (70 to 90 mph). Not if it is a soft acceleration, but when I do a pedal to metal move. It has happened five times since I bought my 1983 635 from a Dutch premium dealer in 2021. Judged from the fact that there was an extra CPU in the boot it probably happened before my ownership, too Two transistors in the ECU simply melt, among them the main power transistor.

My mechanic and I are running out of ideas. So far we changed all injectors, both crankshaft position sensors, opened and checked the whole wiring loom, and obviously every time disaster struck a car electronic specialist mended and repaired the two ECUs we have. My mechanic inserted an extra circuit fuse surrounding the ECU, but the fuses went on each occasion along with the ECU. We also hired a specialist consultant who monitored and measured all data values while driving, but not during breakdowns. Everything was within the normal.

What else can have gone wrong, gentlemen?

The car is a 1983 635csi, first registered in Italy, then with years in the south of France, and now with me in Denmark. It is fully refurbished and in a good nick, drives really well - anyway. 
bakemono
Posts: 62
Joined: Mon Oct 28, 2013 7:23 am
Location: NY, USA
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Re: Repetitive ECU break down

Post by bakemono »

This sounds like an unusual problem. Any update? I am wondering if they already checked that the system voltage is not excessive and the waveform does not have excessive ripple.

Since you mentioned crankshaft position sensors, I gather that the ignition is also controlled by the ECU on this model (ie. it has Motronic rather than L-jet). So perhaps the ignition coil impedance and ignition dwell should also be checked as potential causes for overloading the ECU.
'82 528e - M50B25 + GS6-37BZ
'83 633CSi - Megasquirt and a 3.46LSD
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