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Hello, I am having the red message, low voltage conv. features have been disabled error. I have replaced the BCM with another used working one, same result. I checked the inline Aux battery fuse and reading 5 ohms, should this be a dead short like traditional fuses? I took the K57/2 relay out of the car and tested that it was closing when 12v is applied as it should and it is. Alternator is tested and good and I am getting good voltage on my batteries when driving 13.5-13.8. Any other suggestions? Codes are 9091, 9092, and 9071 which is why I tested all the items mentioned. I also have good continuity from the K57 relay to the Aux battery. The error message will come and go as I drive sometimes. It usually takes several miles of driving also to appear after sitting all night. Possibly heat related? I do live in AZ and it is very hot right now 105 - 110. I am leaning toward replacing the relay even though it is testing fine outside the car. Just doesn't't make much sense that it would be anything else. Any suggestions would be greatly appreciated.
 

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start simple, are your batteries still good? These cars can only tolerate an almost perfect battery. even a 1/2 volt drop will make things go wonky. Batteries need to be tested with engine off and do a load draw.
 

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Batteries are good. Anyone else. Come on guys. I know someone has seen this before.
 

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If batteries are fine inclusive of load test before replacing any additional parts consider disassembling front pre-fuse box to check k57/2 relay 150A fuse 70 (211 instructions attached same for 219). Note MB does not offer fuse separately, if aftermarket alternative cannot be found shop part # 21154612417 used i.e.: https://www.ebay.com/sch/i.html?_from=R40&_nkw=2115461241&_sacat=0&_sop=15
 

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Solved! So it turned out the be the voltage regulator. This was actually the first thing I thought of but the batteries were charging fine. I did notice that the charging current was very high (30A) before the regulator change and only around 10A after the change. None of the troubleshooting I did really pointed to the voltage regulator without other symptoms, but I swapped it out as a last ditch effort and it fixed the problem. So, don't overlook the voltage regulator if you see this issue. Actually, I would replace it first as it is the cheapest of all the parts it could be. Anyway, all is good now.
 
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