fried ECU! Cause? Or random?

[tfritz]

My m30 swapped e30 randomly died on me last fall while driving. Just lost power, RPM's slowed, didn't sputter, just quit running. After trying to figure it out I ended up replacing the CPS, still wouldn't run. Tested the main relay, fuel pump relay, looked for broken wires and found nothing wrong but the car still wouldn't start. I randomly decided to pull the ecu, upon removal i noticed that the ecu case was coming apart (case has been open several times due to Miller WAR chip). I opened the case and found a burnt spot on the circuit board. Would the be a cause for this? Or just random old part failing? I just don't want to install another ecu and instantly fry it due to some other issue. Before the car dying last fall it ran nearly trouble free for 5 years with the swap.

Thanks!

[bmwman91]

Take a pic of the ECU PCB and post it up (decent in-focus shots). It's hard to diagnose without seeing what failed. My bet is on a fried coil driver IC since that is by far the highest-power dissipating component.

[tfritz]

Burnt area

[tfritz]

Hopefully these pics are a little better

[e30davie]

Damn you really let the smoke out on that one.

What's on the other side of the burnt patch?

[tfritz]

Burnt spot is at the lower right of that yellow thing

[bmwman91]

OK, I dug out an old Motronic 1.3 board (Bosch 0 261 200 173). It is the same board, but populated with components a little differently than the one in your pics. It looks like one of two components burned up on the bottom of yours: R615 or R305.
- R615 is 89 Ohm which connects pin 10 (O2 sensor ground) to the rest of the grounds.
- R305 is a 0 Ohm jumper which connects pin 30 to ground, and pin 30 is not listed as being used in any M20/M30 wiring diagrams that I have. The other endpoint is pin 5 of A250/1, which looks like an unpopulated location for a daughterboard for some other vehicle application.

There are a few possibilities that I can think of:
1) (not likely) There was an internal short in the O2 sensor which caused Vbatt to appear on pin 10. Even at max system voltage (14V), this would only lead to 2.2W of power dissipation in R615, and since the O2 sensor/exhaust is connected to the engine (which is grounded) a short would just rip through there and blow a fuse.
2) (possible) The engine ground harness is broken in such a way that all of the current which normally flows through the ground strap was shunted through the O2 sensor ground. If the ground was being forced through the 89 Ohm resistor, the alternator voltage could have climbed significantly which would have put a ton of current through that little resistor. Check the continuity between the ends of the ground strap while wiggling it since there could be an intermittent break.
3) (not likely) Something is connected to pin 30?
4) (possible) Repeated opening & closing of the case caused a piece of metal debris to float around under the PCB, which ended up shorting something in that area.

Can you take a pic of the whole board, top & bottom, so I can compare to mine? Also, can you use an old toothbrush and some rubbing alcohol on the cooked area to clean it up a little so we can see what, exactly, let the magic smoke out?

[tfritz]

Bmwman91, thanks!
So some additional info...
The board was loose inside the case (possible arced off the case?)
While I was testing main relay and fuel pump relay (before I found the burnt ECU) the ignition coil was getting hot! Hot as in I could not hold my hand on it.
I'm not sure if the hot coil was a result of the burnt board or what..

[tfritz]

As clean as I could get it

[bmwman91]

OK, so it seems like R615 is what burned up, if was in fact a component that got cooked. It is hard to tell from the photos, but is the copper trace that runs to R615 (the middle of the 3 that you can see bare copper on) completely obliterated / gone?

Here is my guess:
If the PCB was loose inside the case (plastic clip thingies were removed, board was just sliding around on the bottom metal part) then I think that's the answer here. See the black spot on R615's soldered end, on the left side? I would guess that it's evidence of arcing from making contact with the case, which is grounded. The ground on the PCB is on the right side of that 89 Ohm resistor, and they put it in there for a reason, so it was effectively bypassed when the left side contacted the case. Is there any sign of arcing or burn marks on the case near there?

As for why the car isn't running, I would check the fuses. I would imagine that one of the ones related to the fuel injection system blew. You may also want to check the operation of the related relays as well (main, fuel pump, O2 heater) since they may have been stressed during this as well.

[tfritz]

Yes that copper trace is completely broke. The plastic spacer pins are still in the board, and I didn't see an arc spot on the lid

[bmwman91]

I am a little stumped then. Check the engine ground strap, and I am not sure how to test the O2 sensor.

[tfritz]

I'll check all grounds and probably just replace them. The O2 sensor is most likely the culprit if that's the circuit, as the wires are cut (exhaust falling off issue). Those wires have been cut for several years but I suppose something could have rubbed through. With the WAR chip I THINK I have the O2 "turned off" but I could be wrong