2E7C - BSD data bus communications fault

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veer90

Lieutenant
Nov 16, 2016
929
West Nyack, NY
Quick writeup on my misadventures dealing with this stupid fault code

A few months ago I got randomly code 2E7C BSD comms fault. Cleared it, went away. It would come back and eventually got worse from intermittent to present all the time. It can be a royal PITA to diagnose especially if it's intermittent at first like my car.

Symptoms (in the order I started noticing them):
  • random 2E9F (oil condition sensor) fault code
  • checking oil level results in no reading, with INACTIVE displayed
  • rough idling, noticeable if you have upgraded motor mounts
  • idle hunt under certain conditions. most noticeable if you clutch in and drop RPMs to idle when moving.
  • oil temperature gauge suddenly reads impossibly high (250+ in the dead of winter with easy driving)
  • faster than normal warm up
  • water pump runs unnessarily. ~3 seconds when locking / unlocking car, full tilt when ignition on / engine off
At this point you're probably thinking the car is possessed and a part out is in order.

Explanation: the BSD or bit serial data bus system links several components together on the N54: alternator, IBS, water pump, oil condition sensor, and DME. Failure of one BSD component takes down communication over the data bus and causes other BSD components to misbehave. Proper diagnosis is only possible when the fault code becomes persistent. At which point the DME fault code memory is checked after unplugging each component, in sequence, with ignition on / engine off. Sequence and explanation courtesy of a BMW tech I spoke with who helped walk me through the process:

1. Ignition on
2. Unplug BSD component in question (I started with the alternator)
3. Clear codes
4. Wait 15 seconds and re-read codes

If code 2E7C persists, then the part unplugged was not faulty. Plug it back in and repeat steps 1-4 for all other BSD components. If all other BSD components are good, then the DME is bad (rare but it happens). According to the tech, most of the time it's a faulty water pump. Another possible culprit is damaged or corroded wiring.

Me being me, I didn't listen and replaced basically everything else first. New oil level sensor... didn't fix it. New alternator... didn't fix it. No IBS (weird how some cars have it and some don't) so didn't need to replace that. Finally replaced water pump this past weekend and the code so far is gone for good. My oil temperatures are back to normal, oil level monitoring works again, and it no longer idles funny.

I don't know enough about water pumps to say for sure what happened, but when I removed it there was coolant pooled in the electrical connector. My guess is the pump housing developed a micro crack and the pump controller said see ya. Very disappointing with just under 40k on an OE continental branded pump.
 

fmorelli

Colonel
Staff member
Aug 11, 2017
2,458
Virginia
In summary, score another integrated electric water pump sucks.

Thanks for the write-up. Very thorough and clear to understand. I sure hope I never have to remember that I read this ...

Filippo
 
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SJ_1989

Sergeant
Aug 7, 2018
268
Illinoisssss
This is why I decided to go away from making a custom harness for my drag build that interfaces with the OEM DME....damn CANbus system. It can be done and has been done, but it's expensive. I'd rather put that money toward a standalone and be done with it.
 

veer90

Lieutenant
Nov 16, 2016
929
West Nyack, NY
This is why I decided to go away from making a custom harness for my drag build that interfaces with the OEM DME....damn CANbus system. It can be done and has been done, but it's expensive. I'd rather put that money toward a standalone and be done with it.
I don't think CANbus has anything to do with this. as far as I know BSD (bit serial data) protocol is bmw specific whereas CANbus is a general protocol for different controllers on a car to communicate with one another.
 

BrazyN54

Private
Jun 1, 2018
27
Same thing happened to me 2E7C the BSD line would go down and I wouldn't be able to check oil level.
Long story short it was the waterpump,
It stayed on even after the car was shut off for a while, ran very loud and had a lot of current draw put excess load on the alternator.
 
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dfpascarella

New Member
Sep 11, 2019
2
I want to verify ISB BSD two wire connection in trunk. It may have been repaired. Can someone verify the wire color positions? Does grey go to grey and the two striped wires together?one of the wires pulled out with pin. May have happened to both so I want to be sure.
 

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