Alternator blowing fuses


#1

My 2002 Jetta GLI dies last Saturday. I roll started it and took it to the closest shop. They replaced the 6 year old battery which they said was the problem. On the drive home all the gauges went dead and came back to life with the airbag warning light staying on. Took it back and was told the 40 amp alternator fuse was blown. Just got a call that the alternator keeps blowing the fuse and that it must have a short circuit in it. They are replacing the alternator. The voltage regulator is internal to the alternator so it is plausible but has anyone heard of this issue before? Could there be something else causing this?


#2
Are you sure they said fuses and not diodes?

#3

they said fuse, a 40 amp fuse to the alternator.


#4

You may have had an old batery on it’s way out, but that wasn’t the cause of your problem. It’s very unlikely (but not impossible) for a bad alternater to cause the fuse to go. I’ll bet you have an intermittent short in your electrical system somewhere.

Sounds to me like your shop is doing “swap out trroubleshooting”. After they swap out the alternater you will likely still have the problem and then they will swap out something else until it stops happening.

Take it to someone who will troubleshoot the problem.


#5

The alternator can develop an internal short and blow the fuse in the electrical cable between it and the battery.
I hope that the shop has made sure that the electrical cable terminal, which attaches to the alternator with a nut (usually), isn’t touching the body of the alternator and creating the short. The terminal has some kind of insulator under and around it to prevent shorting the wire cable to the alternator body…


#6

Yes - I was too quick to say very unlikely - the rotor windings can short - I don’t believe they are behind the diodes.


#7

I believe the voltage regulator is internal to this alternator as well which could fail and also cause the problem. I was between a rock and a hard place so i had to accept the alternator but the car seems to be working fine so far.