2005 Jetta Electrical Storm!

My son returns from Tasmania in a few weeks after being gone for a year. As a treat I thought I’d have the headliner in his 2005 Jetta that’s been sitting in our driveway for the duration replaced. I did and it looked great, but the battery was dead when I picked it up. Battery indicator light had flickered on the way to deliver the car, but I thought maybe the battery was low and needed to charge up a bit.

The gentleman who’d done the work had a spare battery we used to jump start the Jetta and it started with no problem. As soon as we disconnected the clamps, however, the car ran for about 30 seconds and died despite the fact I was compressing the accelerator in a lively manner. Now I’m thinking very dead battery in tandem with an alternator that’s not charging and causing the electric fuel pump to die. Off to the parts store and back with a new fully charged battery (old one was indeed shot, but was from the factory and nearly 10 years old; full marks on the battery VW).

Here’s where it gets wild. Hooked the new battery up and as soon as the connection was made it seemed like everything in the car energized: fans, wipers, headlights, the works. None were switched to the on position, though, and there was NO KEY IN THE IGNITION! Additionally, when the key was inserted it wouldn’t turn over.

So, twistoes and twistetts, do you think that:

A) East Tennessee rodents have chewed through the wiring harness over the winter in a place I can’t see (like behind the dash) leaving me with an electrical short powerful enough to melt that 110 amp fuse on the battery (which it did) and a 25 amp engine control fuse (which it also did).

B) The nice old guy that did a bang up job replacing the headliner accidently mashed some wires together in the process.

C) The alternator/voltage regulator is shot, but not before it managed to fry things.

D) None of the above. This is just proof that no good deed goes unpunished.

I’ve checked for the dreaded coolant migration into the harness wiring and there appears to be none. Ideas?

I can see no reason other than the battery was hooked up backwards or a short. They only way to find the short is to start unhooking things and use a multimeter set to ohms to find the short.

Things seemed to be pretty reasonable until the battery was replaced. I tend to think the same thing as @knfenimore, the battery was connected reverse polarity.

You need to read this.


Look at what can happen when a battery is disconnected from a VW.


Sounds like the 10 year old battery, discharged from sitting, likely fried the alternator.

I also agree with @knfenimore that the battery might have been connected reverse polarity.

Where in East Tennessee are you? I have a spare multimeter you can borrow, and am in Johnson City.

I also have a Harbor Freight coupon for a free multimeter I can mail you.

I don’t have a wiring diagram, but IIRC that 110 amp fuse is connected between the battery and the alternator. That means if the alternator diode pack isn’t shorted, polarity reversal of the new battery is the next most likely cause.

You guys are great! Heading back over right now to check and see if I may have tried to do something that stupid. I’ll let you know what I find.

Thanks for the offer, LewisCannon. I’m in Dandridge and work up your way once or twice a week. This may turn out to be too simple, though. . . and embarrassing.

Okay, I’m a little embarrassed. I’m 57 years old and ran a large peach orchard for about 13 years after college. Lots of trucks, tractors and equipment with 12 volt batteries. I NEVER made the mistake of hooking up (or trying to hook up) a battery in reverse. Guess there’s a first time for everything.

Flipped the disconnected battery around in the slot this morning and fastened leads to the correct post. Everything is back to normal. . . almost. Car cranks, but won’t turn over and then I remembered the 110 amp fuse. Off to Knoxville to find a VW fuse! Film at 11:00. Thanks guys!

Don’t feel too bad, I think almost all us need a good dope slap at some point in time. At least I need one now and then.

Did you check all the other fuses?

I don’t think the blown 110 amp fuse would keep the engine from firing up. But as I said, I don’t have a wiring diagram.

Good luck.

Replaced the 110 AND the 25 amp engine control fuse. Fired right up and drove it directly to have the alternator checked as the battery light was still on. Alternator was reading “0”. Lets see, thats less than 13.8V, right? Today’s Knoxville trip will be to have it rebuilt, I guess!
All this, of course, is so I get the good nursing home when it gets time.

You may be better off getting a remanufactured unit if you can find one that has been rebuilt well. Be aware that just because the alternator has no output it doesn’t mean that it is really the trouble. It is possible that the wiring to it has a problem and that can cause no output from the alternator also. I assume the shop made the correct checks to verify that the trouble is really with the alternator itself and nothing else. Since the warning light seems to be working correctly chances are a replacement alternator will fix things up.

The big lesson here is, never make any battery connections until the connections have been double checked.

When a car battery is reverse-connected it often damages the alternator diodes. There may be a few other problems remaining, but hopefully once the alternator is replaced the car will be drivable.

For folks who like to worry, here’s another thing to worry about: years ago I purchased a brand new battery for my truck that was itself reverse charged. The side labeled “+” was actually the negative side. This is probably a very rare thing to happen, but I always check first for that condition with a DVM before I install a new battery.