intermittent voltage drop while on (not battery, ground, alternator)

[BlueM3Wagon]

Long time no see! The e91m3 wagon has been operating beautifully for the last three years, had to replace some wear items like TAs and battery but have put about 17k miles on it since swap. I do have one issue I can’t seem to solve. I’ve take it to three independent shops with no fix and any BMW dealership as well as EAG refuse to look at the car because it’s swapped.

I’m getting intermittent low charge to the battery while it’s on. Certain times I’m getting a clean 14.5v to the battery from the alternator, other times it stopped to 9v. I’ve replaced the alternator three times, ground cables and battery. The last visit they were able to test signal wire from alternator and determined no grounding issues and it was operating as it should, however still have voltage drop. They also replaced the IBS in good measure. They’ve reached the limits of their expertise and testing. They recommend I look into DME, but when I called ECU Pros, they said 99% of the time, DME in the M3 Will throw a IBS fault code. I’m throwing no faults. They said they’ve seen some voltage issues caused from coolant or oil level sensors as well but those also usually come with faults. The only faults I have is from running catless rear O2s.

If anyone had an idea, resource or person I can reach out to, I would be insanely grateful!

PFA!!

[Redd]

Check the main cable behind the fuse box inside your glove box. Many are due for replacement due to age. If you see discoloration on one end of the cable then replace it.

[BlueM3Wagon]

Originally Posted by Redd

Check the main cable behind the fuse box inside your glove box. Many are due for replacement due to age. If you see discoloration on one end of the cable then replace it.

From what I was reading this is more of an issue where cars won’t start at all, correct? Car and electronics function fine until the battery is depleted from undercharging.

[Redd]

Are u running standard wet/AGM batt or Lithium?

[BlueM3Wagon]

Originally Posted by Redd

Are u running standard wet/AGM batt or Lithium?

Standard: Duralast Gold 900 cca, battery was registered and system reset.

[Redd]

I had some charging issues when I moved to a Li batt. Car would charge the batt, use it til it went flat, and then only charge it again after it was flat. It was weird. Tried everything, nothing worked. Went back to wet batt and everything is OK now.

But that doesn't sound like what you're experiencing. Sorry man, wish I could be of more help.

[BlueM3Wagon]

Here more context from the Indy.

“When the car doesn't charge, the signal from the dme is commanding it not to. This would indicate that the dme is seeing an input that is causing it to stop charging but here inlies the problem. All inputs to the DME for charging protocol check out fine. Also all wiring from said inputs and within the charging circuit check out fine. This is why we believe there is something internal to the dme that is at fault. It appears nothing is failings externally on the car and nothing it telling the dme to stop charging. it internally is just changing its mind to "charge" or "not charge". Most likely a circuit board issue.”

Think I might just need to see if the DME needs repair or replacement. Hate throwing money at things but I’m at a complete loss.

[Redd]

Before u spend bigger money, try swapping in another brand of battery?

[amrazM]

probe the harness for the oil level sensor, if its shorting out somewhere it will cause this charging issue. you need to check for continuity.

[BlueM3Wagon]

Originally Posted by amrazM

probe the harness for the oil level sensor, if its shorting out somewhere it will cause this charging issue. you need to check for continuity.

Thanks for this insight. Exactly what are you probing and what continuity? From the sensor to the harness? Since they checked signal wire square wave and it eas not disrupted while the charging malfunction was happening, makes them assume any other short in a circuit would disrupt the square wave pattern.

[sh0ckc0re]

The Oil Level Sensor, Alternator and the IBS are all on the BSD Bus that the DME is the controller of. If you aren't getting BSD faults I would definitely check behind the JBox for the Batt Cable there. I have also seen the B+ Connections in the Battery Box rot out on the e9x from the inside and the outside. Wouldn't be a terrible idea to get some eyes on those Connections and make sure there is no corrosion. FYI, my shop had an e92 get towed in recently for a similar issue which the Engine Ground was completely burned in half and similarly a G01 that had water in the floorboards which was turning the square wave into a sine wave and the DME couldn't interpret anything on the Bus Line.

Are you getting good voltage anywhere in the car while you are getting this message (i.e. Alternator is @ ~14.5V and the Battery is @ 9V indicating a wiring issue)?

Is the Alternator putting out a good Voltage (Like @ the Alternator Connection)?

OE or OEM Alternator (Valeo or Bosch)?

Are you able to scope (not volt meter) the BSD Line (12V Bus System)?

I know it's been about 2 weeks, but did you ever get any resolution on this issue?

[Assimilator1]

Curious, so sub'd.

[mapster37]

Could be a software issue in the DME. Try reprogramming it and go from there