Rear power distribution box meltdown

relative4 · 10-16-2020, 05:07 PM

'07 E90 335xi auto. Anybody seen this? Ideas what would cause it?

I have no idea how long it's been like this. The car runs fine, I was in there for unrelated reasons.

smass · 10-16-2020, 05:45 PM

Is the nut under there loose? Flip up the melted cover and tighten it up. If loose it could get hot.

relative4 · 10-16-2020, 06:33 PM

Just checked the nut, it was snug. The pattern of melting in the long skinny section makes me think a huge amount of current has been passing, or at least passed at one point, through the fusible link to the main cable, but I have no idea how to go about verifying that or identifying a cause.

Mustafa.e92 · 10-16-2020, 08:23 PM

That doesn't look good. Use a volt meter see if the alternator is over charging

relative4 · 10-18-2020, 12:06 PM

The voltage is about 14.3 at idle. When the engine is running, the temperature of that terminal is just above ambient, and also there is no heat damage on the compartment cover, so it's not an ongoing problem.
I suspect this happened when I was doing some engine bay work with the B+ terminal uncapped and shorted it to ground with a wrench. Nice loud pretty spark, and louder profanity. Seems like a lot of heat to dissipate from a 1/10 second discharge, but it's all I got.
Ordered a used distribution box from eBay and will see if problem repeats. If not, will install a new box from FCP.

10-16-2020, 05:07 PM	#1
relative4 Major 612 Rep 1,272 Posts Drives: '08 335xi E90 MT stage 1+ Join Date: Feb 2019 Location: Denver, CO iTrader: (0)	Rear power distribution box meltdown '07 E90 335xi auto. Anybody seen this? Ideas what would cause it? I have no idea how long it's been like this. The car runs fine, I was in there for unrelated reasons. Attached Images Last edited by relative4; 10-17-2020 at 11:41 AM..
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10-16-2020, 05:45 PM	#2
smass Lieutenant Colonel 401 Rep 1,579 Posts Drives: 06 325i - 10 335i Join Date: Sep 2010 Location: Scottsdale, AZ iTrader: (1) Garage List 2010 E90 335i [0.00] 2016 F15 35i [0.00] 2003 E53 3.0 [0.00] 2006 E90 325i [0.00]	Is the nut under there loose? Flip up the melted cover and tighten it up. If loose it could get hot.
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10-16-2020, 06:33 PM	#3
relative4 Major 612 Rep 1,272 Posts Drives: '08 335xi E90 MT stage 1+ Join Date: Feb 2019 Location: Denver, CO iTrader: (0)	Just checked the nut, it was snug. The pattern of melting in the long skinny section makes me think a huge amount of current has been passing, or at least passed at one point, through the fusible link to the main cable, but I have no idea how to go about verifying that or identifying a cause.
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10-16-2020, 08:23 PM	#4
Mustafa.e92 Lieutenant 206 Rep 498 Posts Drives: 2009 bmw 335i Join Date: Feb 2020 Location: Canada iTrader: (0)	That doesn't look good. Use a volt meter see if the alternator is over charging
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10-18-2020, 12:06 PM	#5
relative4 Major 612 Rep 1,272 Posts Drives: '08 335xi E90 MT stage 1+ Join Date: Feb 2019 Location: Denver, CO iTrader: (0)	The voltage is about 14.3 at idle. When the engine is running, the temperature of that terminal is just above ambient, and also there is no heat damage on the compartment cover, so it's not an ongoing problem. I suspect this happened when I was doing some engine bay work with the B+ terminal uncapped and shorted it to ground with a wrench. Nice loud pretty spark, and louder profanity. Seems like a lot of heat to dissipate from a 1/10 second discharge, but it's all I got. Ordered a used distribution box from eBay and will see if problem repeats. If not, will install a new box from FCP. Last edited by relative4; 10-18-2020 at 12:23 PM..
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