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Alternator Wiring - PMU - how to shut down

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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Hi folks - hope to get help here. Placed similar topic in another forum but maybe wrong area.

I do have an issue with my alternator wiring - also I thin an understanding problem, since I have bno clue how this could ever work.

back information:

BMW e28 with M50 turbo engine

Ecumaster PMU16 power management

Maxxecu engine management

PMU16 gets it start impuls by Klemme 15 (ignition switch) and powers then ECU, Coils etc...

Klemme 15 (ignition switch) is also connected via charging bulb with D+ on the alternator (2 Pin alternator).

So what happens is pretty clear: when I switch the ignition off with the key, the engine keeps running, since the alternator is "feeding" the line now, so the PMU16 gets still power on its 12v switch (even the key is turned off).

How should this ever work?

I tried to repplace charging bulb with a 3w resistor and a LED working as a diode in row -> now engine shuts down with key, but alternator will not charge (even LED goes off I only see 12,2V)

i know i could program weird stuff and shut off coils with an extra button, but I want to understand how in the serial production version this could have everworked si nce there is no diode between KL15 and D+.

Another idea: could i use a power output of the PMU16 with a 3W resistor connected to the D+ terminal of the alternator to get the alternator to charge? any risk I could hurt the PMU by doing this?

I do attach a wiring scheme of the actual install

Thanks for helping me out - stay fit and healthy

Attached Files

How did you get on ?

Hi Ross - I solved it by running an output of my PMU to the alternator with a 3W 39 hm resistor in between...

But still dont know how in the original setup from the factory this could have ever worked without a diode...

Hi Philipp,

It is possible that from the factory, they use a main relay that cuts the power without affecting the alternator load?

Best regards,


The "logic on" signal to the PUM16 must be tolerant to a larger voltage range to denote "ON state" to keep the PMU16 on while cranking when battery voltage can drop to 9.0v or lower.

Wiring like this worked in the 80's because the DME, fuel injectors, and ignition coils received power from relays that needed more current to turn on than that particular input of the PMU16.

When you turn the key off, the alternator is still sending positive voltage to the battery warning light and, even though you've wired the PMU after the light, there is still enough voltage to keep the PMU16 on. That's why adding that resistor solved your issue. You finally created enough of a voltage drop to the PMU16 to be less than "logic on."

If you're still willing to rewire the E28, you can take the logic on signal from a relay that's only on with ignition in start or run.

You can also wire in a diode between S213 and pin 22 of C1 at the instrument cluster to keep the positive voltage from returning to the PMU when the engine is off. Just make sure the diode can handle the wattage.


"Wiring like this worked in the 80's because the DME, fuel injectors, and ignition coils received power from relays that needed more current to turn on than that particular input of the PMU16."

That is the answer I was looking for and nobody until now could explain. thanks a lot.

I got it working, since the reason was quickly found - I tried both -> diode and PMU and kept it with PMU for now...

Thanks a lot

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