I'm almost embarrassed to ask this... - Page 2

andrewwynn · #11 07-05-2018, 01:07 PM

Definitely use the special nut. I didn't see mine until the 4th time I needed it (bad battery that tested good).

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X53Jay4.8is · #12 07-05-2018, 02:12 PM

Quote:

Originally Posted by andrewwynn

You can also apply power at the B+ under the hood that's easier than crawling through the car.

Oh yes this is a good way to do it.

oldskewel · #13 07-05-2018, 02:47 PM

Quote:

Originally Posted by andrewwynn

You can also apply power at the B+ under the hood that's easier than crawling through the car.

I'm in this situation (on purpose) on my 2001 3.0i. In the middle of a head gasket (or more) repair, I have disconnected the battery in the back.

I found that hooking up either a battery charger, or a battery and charger in parallel to the B+ under the hood DID NOT WORK to let me open the rear hatch.

I really expected it to work, so when it did not, my son crawled in to do the manual release. And then I tied a string from the release thing to the driver's door seatbelt, so now I can just open the driver's door with the key, and then pull on the string to open the back.

I really don't know why the electrical method did not work, but it definitely did not work for me. With a weak battery, I have had it work. But with the battery disconnected, it did not. EDIT - also, yes, I used the "special nut."

Qsilver7 · #14 07-05-2018, 02:54 PM

What did you ground to? Did you attach the negative cable to the "special nut" on the left fender?

wpoll · #15 07-05-2018, 05:28 PM

There is a real risk in applying power to the jump terminals when the battery is disconnected.

Where is the battery positive terminal?

If you only disconnected the negative terminal, then there is no real chance of a problem.

If you have completely removed the battery or only removed the positive terminal, where is it sitting? If it is touching the chassis or the negative lead, Bad Things (tm) will happen if you apply a high current source to the jump terminals....

audiophool · #16 07-05-2018, 05:34 PM

It’s not that hard to crawl in from the back seat. You’d have to do that for manual release anyway...

cn90 · #17 07-05-2018, 08:05 PM

- If you don't want to crawl in, then use a spare battery or another car and jump the terminals under the hood.

andrewwynn · #18 07-05-2018, 08:51 PM

B+ goes through the BST. If that is blown you won't be able to power anything from the B+/B- under the hood but you CAN power from the cig lighter port or the B+ on the trailer power connector (brilliant place to apply a battery tender)

SAGE advice from oldskewel and wpoll. Ensure no closed circuit before attaching a 1100A battery you will weld/melt anything in the path of conduction.

When in doubt check with Ω meter before you apply a high current source. If you're positive the battery terminals aren't shorted it's fair game to apply 12V across the jump terminals.

I'm curious why that didn't work when it didn't. I'm pretty sure the battery terminals shorts between high and low current sides so I would suspect unless BST blown this method should work.

(I have had cases where buttons didn't do as expected eg. Couldn't open door from exterior latch but the trunk button on the dash worked so that could have just been a computer hiccup..

andrewwynn · #19 07-05-2018, 08:55 PM

One more food for thought since sometimes the battery is dead and a person is stuck outside the car.

B+ is directly attached to the starter solinoid under the car. I've heard of people jumping the car from there to get in if their key cylinder is hosed and they only get in with the key fob but the trailer power is much more elegant you just need to be careful with current lest you blow the fuse. (Eg use a charger set to 2a for example)

oldskewel · #20 07-05-2018, 10:19 PM

Quote:

Originally Posted by cn90

- If you don't want to crawl in, then use a spare battery or another car and jump the terminals under the hood.

Just to make sure I'm clear here - that's what I did when my electrical opening failed. To answer + clarify, related to my experience (and it's still in this state, so I can go and test anything anyone wants):

- battery is in place in the back, with the negative terminal disconnected
- my first attempt was to attach my battery charger to the B+ port and the special nut in the engine bay. That did not work, but since I realized my battery charger may be a little too smart here to just apply 12V when there is no battery in the loop, I then ...
- connected a spare battery to the engine bay jump ports, and attached the battery charger to that spare battery to make sure it was at 12+V
- in this state, the key fob was completely non-op. And the hatch release button inside the truck did not work either.

On the other hand, with a battery that is low-voltage but still connected in the back, attaching a charger at the engine bay jump ports has been successful in making everything work.

I don't know what the answer is, and my "problem" might not apply to other E53's, but I'd at least recommend this as a warning to not count on it to work.

And basically one conclusion is that there is a difference between a dead battery that is still connected vs. a disconnected battery (at the negative terminal, if that matters).

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