For questions - and answers - concerning auto electrical systems.

13 responses total.

My daughter's 1999 Subaru Outbacks's battery exploded last Thursday
several miles after she had to have a jump to start. She got a tow and a
new battery OK, but the "fusible link" was also blown into little pieces. 
My questions are:

What makes car batteries explode? Was this related to the car needing a
jump to start after it had been parked for two weeks? 

What circuits does the fusible link protect? 

When would a fusible link rather than a fuse blow? 

Why would an exploding battery cause the fusible link to blow? 

(N.B. I just learned that the tow truck person first tried to jump the
battery, not realizing that it had exploded - that might cook the battery
further, but could it have blown the fusible link?) 

Are fusible links available for all cars at places like Murray's? 
(Autozone, where they towed the car, did not have one for the Subaru so
the tow crew wired up a wire cable jumper to replace it so the car could
be driven.) 

(N.B. They did not actually *tow* the car, as it is AWD, but carried it on
a transporter.)

Batteries generate hydrogen gas when they're charging, especially if
they're charging quickly.  Any short circuit or bad connection in or
near the battery that causes a spark can ignite the hydrogen and make
the battery explode.

Usually the fusible link is designed to prevent a fire if a high-current
part of the car's electrical system shorts.  In many cars there are
circuits that are basically unfused, other than the fusible link.  This
usually includes the starter and alternator circuits.  On my VW, which
doesn't have a well-designed electrical system, the instrument panel,
clock, and engine stop solenoid are also unfused!

I would replace the fusible link ASAP, because with a wire jumper across
it you may destroy the entire wiring harness or set the car on fire if
whatever caused the fusible link to blow happens again.

Your warning was already in my mind - we replaced the fusible link ($12)
yesterday. Whew. It's a little strange to have the "fusible link" a
bit of wire with connectors on the end rather than a common cartridge
fuse. Apparently (from my reading) it is meant to be "slow blow", but
slo-blo cartridge fuses are available too. 

The starter is not on the fusible link, but full generator current might be.
I wonder if the tow guys reversed the jump and blew the link themselves?

Hmm.  Could be, I suppose.  If so, the link may have saved a lot of
electronics in the car.

I'm not sure why "fusible links" are used instead of cartridge fuses,
but I'd guess it's because they're cheaper.

Nothing else in the car is "cheaper".....  8^}

My daughter phoned me yesterday a.m. because the car described in #1, with
a new battery, wouldn't start. There was 'dim' power for the interior
lights but not enough to operate the windows. So, with her on her cell
phone at the car and me at home, we tried to diagnose the problem.

There was some liquid around the battery support, but the car had just
been hosed off. We tested it for acidity with 'baking soda' - no fizz; not
acid.

But there was a rhythmic clicking noise coming from the vicinity of the
radio (not from the speakers), even with the key off. This might be
something that drained the battery so we sought the source by checking
fuses. The "clock" fuse under the hood turned off the clicking (good news)
but - not the clock! (bad news). The clock in part of the radio so we
tried the radio fuse under the dash. This stopped the clock but not the
clicking! Weird.

So, our diagnosis was the battery was drained by the mystery clicking thingy,
so take out both the "clock" and the "radio" fuses and get a jump to start,
and take the car in when convenient.

Then my daughter called back and said some workmen had come over and poked
around under the hood and found the + battery cable clamp was loose. She
tightened it and that solved all problems: car started - and "clicking"
went away. 

The moral is, when all else fails poke around. The *qauestion* is, what
was the clicking?

Arcing in the battery clamp?  Doesn't fit your description, though.

My daughter (and I - she held the phone up to the  radio) thought it sounded
like some motion of the casette tape mechanism. But why wouldn't that be
using radio power, not the mysterious "clock" power? And, with the battery
properly connected - no clicking at all. 

Perhaps the "door open" buzzer making a pathetic attempt to sound
without enough power?

It was definitely coming from the radio (compartment). It would appear to
be some electromechanical component that is powered on even when the key
is off, and trying to latch or whatever but failing to do so because of the
poor battery connection. Does anything do something like that?

My cassette player automatically ejects the tape when the key is turned off.

Does that mean that the ejection mechanism is continually powered on when the
key is off? That could explain the observations.

The actual starter circuit is usually unprotected by any sort of fuse -
basically, the whole goal is to take *all* the current the battery can
produce and dump it into the starter.  A fuse just gets in the way, and
doesn't protect you from anything that would be bad news anyways.
Effectively, the coils in the starter *is* your fusible link there.
Otherwise, fusible links are used for components where a short is not
commonly expected, and the unreliability of the fuse itself would be a
concern.  Fuses do age and fail all on their own.

Some radios have a reed relay in them -- could be for power or the
antenna.  In either case, it's possible that fluctuating power from the
radio was doing something weird to that.

Response not possible - You must register and login before posting.

- Backtalk version 1.3.30 - Copyright 1996-2006, Jan Wolter and Steve Weiss