BMW M52 engine.

Black mot88 connector lock is slightly different from the natural color M50 (either type can be used with a bit of filing).

The mot88 pinout is VERY different. So do NOT plug into the other type.

See [M50-vanos-vs-s50b32.xls] columns.


We received a report of some relay prelling (when ignition off) that we cannot reproduce.

What should be hooked to the mot88 (which pin) to simulate the "ignition off" state ?

  • The flyback=mot88/87 (not pin54 in the xls beware, that is cam sensor +12V!) in the car also comes from the main relay but could possibly be moved to a constant 12V (pin26) instead to solve this?
  • good find: apparently the mot88/54 = mot88/49 (12V connection wire) inside the ECU is simply not needed, if the mot88/54 is supplied from the harness (weird, why is it connected back to the ECU at all ?)
  • Can a failed diode in the relay cause this?
  • possible, but more likely the mot88/54 cam12V seems to be unnecessary from the ECU if the harness feeds 12V there (via main relay) anyway

OEM wiring information here

Page 108 shows that flyback 12V mot88/87 = mot88/54 cam12V in the harness so how that same connection can cause any problem in the ECU ?

The bold, or last 2 are suspect to cause prelling, especially if switched 12V (ECU main supply) is connected to cam12V in the harness

New Tests

No prelling at all. ECU does not power up.

If ECU forced ON with mot88/49 = 12V than ECU turns on, with minor 0.4sec delay the main relay turns on and no prelling: everything seem perfect.

Maybe the BMW has some very low resistance relay


Dave suggests:

The problem with the PNP relay prelling is caused by the


p259 diode >| mot88/87=EC36/23

not connected to EC36/25 (#15) inside the ECU.

Somehow mot88/54 is (suspected) involved in creating the path...

But how can we be sure ?

Actually, perhaps the main relay itself connects the #15 to injector supply, EC36/23, and possibly causes inability to turn off main relay ? (reproducing failed because of some relay differences ?)