Battery? Alternator?
 

My ‘05 MCS has been the source of ongoing frustration and now it seems to be my electrical system.

Last week I diagnosed a faulty alternator. The battery was load tested and passed. When running the voltage of the battery did not change, which lead me to think it was the alternator. I purchased a rebuilt Bosch unit and installed it myself, taking extra care the ensure that the belt was on correctly during resinstall. I fired the car up and my multimeter showed a healthy voltage.

Yesterday I took the car out and it ran great; no error codes or warning lights.

This morning I started the car and let it run for a few minutes with the heat on (it was a cold morning) and when getting in to leave I was met with the dreaded battery warning light. I pulled out my multimeter and the battery voltage showed 11.2. I then turned the car off and checked again, this time it
showed around 12. I turned the car back on and removed the negative lead and the car turned itself off.

Seems like the alternator, but it was working great 16 hours prior.

I also checked the grounding straps and did a visual inspection of the wiring that I could see. Nothing seemed out of place. I also replaced the #39 fuse in the hope that it might be a blown fuse.

Lastly, I just pulled the following codes from my car:-5E16 CAN connection to instrument cluster

-5E1F vehicle identification number different

-0B safety battery terminal (Car has never been in an accident)

-00 internal fault in control unit:internal voltage (x14)

-03 function switch (relating to sunroof)

I am at a loss. Anyone have any suggestions. I was thinking about just buying a new battery to see if that helps as my current battery is around 4 years old.

Any help would be appreciated!

With the engine running, check the voltage at the alternator output, make sure that connection is tight.
Also check for continuity between the alternator output and the + cable at the battery.
Lastly check for continuity between the alternator body (ground) and chassis ground.

New doesn't mean it's good.

AutoZone should have a machine to test it but it sounds to me the refreshed alternator has died on you. You should be charging in the 14's.

I followed these steps and it seems to indicate that the alternator is to blame. I have returned the rebuilt alternator and will likely just buy a new OEM replacement. Thanks for the suggestion

It seems that the rebuilt alternator is no good. I’ve returned it and will look to buy a new OEM replacement.