Battery Cable Grounding Warning

[mam4.6]

When the explosive battery cable ends disconnect, new ends get spliced in like that sometimes.

[jdstrickland]

Originally Posted by mam4.6

When the explosive battery cable ends disconnect, new ends get spliced in like that sometimes.

Is that for my benefit? I asked about the tape because it does not look like factory material.

If the explosive thingy did its job, doesn't that mean that the vehicle was in some sort of crash that was significant enough to set it off? That should be a significant impact, possibly an event that could total the car.

[mam4.6]

Originally Posted by jdstrickland

Is that for my benefit? I asked about the tape because it does not look like factory material.

If the explosive thingy did its job, doesn't that mean that the vehicle was in some sort of crash that was significant enough to set it off? That should be a significant impact, possibly an event that could total the car.

As I said, mine had the same problem, and mine was never wrecked, as far as I know. Not saying anything about yours, just putting it out there. If I'm correct, they don't have that thing in there from the factory, which is why I replaced mine so I have an unspliced connection all the way. No idea...

[DSSA]

Originally Posted by jdstrickland

Is that for my benefit? I asked about the tape because it does not look like factory material.

If the explosive thingy did its job, doesn't that mean that the vehicle was in some sort of crash that was significant enough to set it off? That should be a significant impact, possibly an event that could total the car.

As stated in the several of the earlier posts, the tape was done when I discovered the issue while looking at the car pre-purchase.

The vehicle is 100% factory paint, there is 0 evidence that any stereo equipment was modified, and BMW uses the same heat shrink many other places in the car.

I know a lot of BMW enthusiasts hate to hear such statements, but VW/Audi/BMW (I would possibly add Mercedes in here, but I haven't worked on enough to profess a valid opinion on them) electrical systems are horrid at best.

Fortunately for them, they build a chassis that's quite enjoyable to drive.

[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.

[DSSA]

Originally Posted by trader4

The starter alone doesn't get powered. You have a heavy, battery cable size conductor going from:

alternator - starter - terminal in engine compartment - battery

Connecting to *any* point on there powers it all.

By back-feeding through the system to the battery...

Originally Posted by trader4

I don't understand the comments about grounding either. The battery
is grounded to the car at the battery. Everything else is grounded
to the car wherever it's located, using the car metal as the conductor.
As for voltage drop, the loads that

Incorrect, an it's the basic premise of 12V systems. The chassis is *grounded to the battery, not the battery grounded to the chassis*. Unfortunately, this is what a lot of people believe.

Look at it this way--if your theory was correct, pull the negative terminal off of your battery and leave the positive hooked up. The car should still start if what you're saying is true.

To which you'll answer (using your theory): "Well, the battery isn't grounded to the chassis so it won't work".

Okay, so disconnect the negative terminal and connect a jumper cable (positive only) from another car. *THAT* battery is grounding to the other car (applying your theory) so it should "work", and it's now feeding 13+ volts to your car which all other systems are "grounded to the chassis".

Does the car start?

Are the frames on my '47 Lincoln Continental and '56 Jaguar XK140 built with different steel than our BMWs? Because they run off of a positive ground system. Once again, applying your theory that the chassis is the ground, somehow it's built to produce a 6 and 12V Positive ground.

Chassis do not create ground--the battery does.

Originally Posted by trader4

are needed to start the car, eg starter, ignition, computer, etc are all
located at or near the engine, not back where the battery is. The
voltage drop will be less with the power source connecting to the
terminal in the engine compartment than it is when starting normally.
Not that it matters, because the conductors are sized to the load and
the voltage drop is small.

BMW uses 2 gauge wiring. Applying the information above, and now realizing that power must make its way back to the battery, look up voltage drop for 2 gauge wire. It's rated (in perfect condition) for about 20 feet (not as the crow flies, but wire length) until you hit unacceptable voltage loss (less than 3%).

Originally Posted by trader4

For the record, I just looked at the Bentley manual and it clearly shows
only one large conductor running from front to back, as I described
above. Yes, the terminal is there for convenience. But it's not
wired in the bizarre way that you claim. A separate battery cable
size conductor running from the terminal all the way to the battery
in the trunk is not required, would be a considerable cost, and it
would actually work less effectively to jump start the car, you'd have
the power flowing 2x the distance.

Perhaps that wire does not go directly back to the battery, but the voltage does, through the system. Now making the aspect of having to feed back through the system to the battery (if not directly) then an even further path.

[jsoto]

Just found this post and figured I'd post some things I came across

BMW does make a repair kit for the + cable so you don't replace the entire thing...
I suspect this was the case and what the OP saw was such -- just not well done.


--------

The Tesa tape off by the compressor mounts - that is OEM, stock, normal...
Downstream on that electrical is not. It is continuous 1 piece cable.
I suspect the ~repair cable was utilized~


BTW, for those looking a the + WD in this post, I don't believe it's accurate. At least on prefacelift 6 and 8 cylinder models.

[jsoto]

Just for the X5 Wiki

This is what the repair cable looks like - similarly
The pic is not for the X5 but it is one of those BMW repair cables