Battery Cable Grounding Warning

[DSSA]

I just picked up an '01 E53 4.4 this afternoon, and when I arrived to look at it, it had a dead battery for sitting for a couple of months.

We went to put jumpers from my truck (Dodge Cummins) on to the battery and the previous owner was removing the cover out of the way when i saw some sparking. We stopped moving everything and I carefully dug around the positing battery cables, paying attention to the wires themselves and looking for a split in the shielding. As I was looking, I picked up the lug that they use to join the two pieces of the main cable together, about 18" off of the positing terminal. I looked down while it was in my hand and noticed that the factory had used thick heat-shrink to cover it and prevent it from grounding where it lies on the floor of the truck. This heat shrink had split from age/improper application/a rough edge on the lug and the back side of the lug was completely exposed. It had actually arced enough over time that there were some pin holes burnt through the sheet metal where it had arced several times over who knows how long.

Once the car is towed over tomorrow night, I'm going to recover it with two layers of heat shrink and then wrap it in electrical tape.

However, if you're having electrical issues or are getting in at the battery for one reason or another, you may want to inspect this to make sure that the heatshrink covering it is good repair.

If I'm able to (per boards rules), once I get the car over to my shop I'll snap a couple of pictures to show you what I'm talking about.

Josh

[jdstrickland]

So, you had the Positive cable shorting out to the body?

[DSSA]

Correct.

See also "DC welding body repair system".

Maybe it's part of the "Convenience Package"

[Qsilver7]

In case you didn't know...you can jump start or charge the battery using the terminals BMW designed for this located in the engine bay:

(the illustration below shows how the positive battery cable runs throughout the e53)

[DSSA]

Originally Posted by Qsilver7

In case you didn't know...you can jump start or charge the battery using the terminals BMW designed for this located in the engine bay:

Yes sir, I did know that, but line loss all the way back to the battery I've always had better luck with deeply discharged batteries by going directly to the post (The end all of power and ground in the car). Found this out back when I first got my 2000 528iT and it had a nice draw issue going on with it. The lug I'm talking about that the heat shrink had failed on I believe was the line going from the batter to the front of the car. There's a metal, in-line connection lug on that line about 18" away from the battery terminal.

[z168]

Originally Posted by DSSA

...but line loss all the way back to the battery I've always had better luck with deeply discharged batteries by going directly to the post

Point of jump starting is not to revive the dead battery but to start the car and get the alternator charging. And with engine bay terminals being nearer to the starter it makes sense to use that neat option


back to topic - glad you discovered that before anything more serious happened. I will check mine when I re-do my battery cutoff switch

[DSSA]

Originally Posted by z168

Point of jump starting is not to revive the dead battery but to start the car and get the alternator charging. And with engine bay terminals being nearer to the starter it makes sense to use that neat option


back to topic - glad you discovered that before anything more serious happened. I will check mine when I re-do my battery cutoff switch

The idea that the underhood terminal is "nearer to the starter" is correct only in that it's physical location is in fact "nearer to the starter". In wire length, it's is actually almost twice as far from the starter in that it runs from that terminal back to the battery, and then the electricity has to flow from the battery back up front to the starter.

The under hood post is essentially a jumper cable back to the battery.

[DSSA]

Originally Posted by trader4

What evidence do you have that it's wired that way? I don't have
a wiring diagram in front of me, but from the way the cables appear
to run, I would think the routing is:

alternator - starter - Terminal at top of engine compartment - battery

It wouldn't make any sense to run a whole separate wire all the way
back to the battery. And that wire would have to be about the same
current carrying capacity as the main one, because it would have to
carry the full load of jump starting the car. What would the purpose
be of doing it the way you suggest, instead of the simple, direct way?

My evidence is basic 12V electrical theory.

A starter alone getting power will not make your car run. You can turn it over that way, but unless it's a < 1980s diesel you won't accomplish much more than that. Every other system needs to be energized as well.

Your main grounding point alone, is at the battery (not at the chassis like some people believe).

The jump start aspect under the hood is fine if you either have a slightly discharged battery, or have a lot of time to wait to charge up a severely dead one until they start wiring cars with zero resistance wiring and overcome voltage drop over distance.

The jump start terminal is there for convenience (because BMW likes to make batteries inconvenient to access--for several good theoretical reasons), not because it's a better connection.

[LVP]

Thanks for sharing this. I needed another excuse to undo the back end to get to the battery

[DSSA]

If you have other reasons to get back there, certainly check it while you're in there and maybe proactively give it a quick wrap in electrical tape while you're there for some extra coverage.

I'm guessing that this is a pretty odd occurrence with these cars though, and isn't really reason for everyone to quickly tear into the wheel well.

I just thought I'd post it if someone was trying to track down a strange electrical gremlin, as a place to check for an issue.

My 528iT wasted enough of my time in that realm, if the info helps one person, it was worth typing it out.

Looks like the car isn't being picked up until tomorrow, so I'll have to wait until that point to get a photo of what happened.