Yikes... Reversed Jumper Cables!!!

M

Mark Williams

Ok... I have lots of excuses... I'd been working 70 hours a week for a
month... it was 6 am... it was foggy... it was dark and my flashlight
batteries were dead... my jumper cable red and black are too close... but
the bottom line is I messed up.

I have a 1996 Subaru Outback that had the dome light left on for days, and
therefore a very low battery. My wife has a 2000 Subura Legacy Sedan. I
thought I had been very careful, but I hooked the cables up backwards. The
Outback's power was totally shut down. I was dumb enough to start her car.
When this did not make a difference, I got out and happened to touch the
jumper cable, which was very hot. I knew instantly and confirmed my error.
I pulled them off and inspected batteries, fuses, wires, etc. for damage.
Everything looked okay. I hooked the cables back up and jump-started the
Outback. Both cars have been working fine for over a week... batteries have
been charging and holding a charge even in very cold weather... all
electronics are still working.

I am still very afraid of the damage I may have caused. Is there any safety
feature that Subaru puts in place to prevent stupidity and protect the
electronics? If not, what possible damage might have been done and how
should I check for it?
 
Mark Williams said:
Ok... I have lots of excuses... I'd been working 70 hours a week for a
month... it was 6 am... it was foggy... it was dark and my flashlight
batteries were dead... my jumper cable red and black are too close...
but the bottom line is I messed up.

I have a 1996 Subaru Outback that had the dome light left on for days, and
therefore a very low battery. My wife has a 2000 Subura Legacy Sedan. I
thought I had been very careful, but I hooked the cables up backwards.
The Outback's power was totally shut down. I was dumb enough to start her
car. When this did not make a difference, I got out and happened to touch
the jumper cable, which was very hot. I knew instantly and confirmed my
error. I pulled them off and inspected batteries, fuses, wires, etc. for
damage. Everything looked okay. I hooked the cables back up and
jump-started the Outback. Both cars have been working fine for over a
week... batteries have been charging and holding a charge even in very
cold weather... all electronics are still working.

I am still very afraid of the damage I may have caused. Is there any
safety feature that Subaru puts in place to prevent stupidity and protect
the electronics? If not, what possible damage might have been done and
how should I check for it?
Click to expand...

Since you DIDNT blow a fuseable link or fuses in the fusebox, you may have
gotten lucky and the battery sank all the voltage. The resistance of the
jumper cables saved your ass by dissipating the power as heat.
Watch for telltale signs, alternator whine in the radio, watch the voltmeter
for strange readings. Check your alarm system and see if it survived.
The reverse current was also dissipated by the 6 diodes in the alternator
and the stator. Also hit the "Keep-alive" circuit in your radio and alarm
system.Check the electronic clock too.
If you didnt turn the key you probably saved yourself a LOT of money.
You got your Christmas present early.....
Problems might not surface for a while.
 
Mortimer Schnurd said:
Since you DIDNT blow a fuseable link or fuses in the fusebox, you may have
gotten lucky and the battery sank all the voltage. The resistance of the
jumper cables saved your ass by dissipating the power as heat.
Watch for telltale signs, alternator whine in the radio, watch the
voltmeter for strange readings. Check your alarm system and see if it
survived.
The reverse current was also dissipated by the 6 diodes in the alternator
and the stator. Also hit the "Keep-alive" circuit in your radio and alarm
system.Check the electronic clock too.
If you didnt turn the key you probably saved yourself a LOT of money.
You got your Christmas present early.....
Problems might not surface for a while.
Click to expand...
That's my take on it, too. Sometimes it is better to be lucky than to be
good ;-)

If you have access to a digital voltmeter, measure the AC voltage across the
battery with the engine running. Damage to the diodes in the alternator will
show up as AC voltage above about 0.1 volts.... typically 0.5 VAC or more.
If that's good it's time to stop worrying about anything but the battery.

I haven't seen delayed damage from reversed cables (except to the battery
life) like I have seen with lightning damage. You should be okay - except I
wouldn't trust the battery that was already discharged and then jumped
backward. I predict it won't last the winter.

Mike
 
Mark said:
Ok... I have lots of excuses... I'd been working 70 hours a week for a
month... it was 6 am... it was foggy... it was dark and my flashlight
batteries were dead... my jumper cable red and black are too close... but
the bottom line is I messed up.

I have a 1996 Subaru Outback that had the dome light left on for days, and
therefore a very low battery. My wife has a 2000 Subura Legacy Sedan. I
thought I had been very careful, but I hooked the cables up backwards. The
Outback's power was totally shut down. I was dumb enough to start her car.
When this did not make a difference, I got out and happened to touch the
jumper cable, which was very hot. I knew instantly and confirmed my error.
I pulled them off and inspected batteries, fuses, wires, etc. for damage.
Everything looked okay. I hooked the cables back up and jump-started the
Outback. Both cars have been working fine for over a week... batteries have
been charging and holding a charge even in very cold weather... all
electronics are still working.

I am still very afraid of the damage I may have caused. Is there any safety
feature that Subaru puts in place to prevent stupidity and protect the
electronics? If not, what possible damage might have been done and how
should I check for it?
Click to expand...

You got lucky. There was probably enough
resistance in the jumper cables, and the
charged battery was probably old enough
that it couldn't push enough amps to burn
anything up.

I seriously doubt there will be any long-
term problems. I'm glad for you...
 
If you have access to a digital voltmeter, measure the AC voltage across the
battery with the engine running. Damage to the diodes in the alternator will
show up as AC voltage above about 0.1 volts.... typically 0.5 VAC or more.
If that's good it's time to stop worrying about anything but the battery.
Click to expand...

Kewl info!

....if you were showing >.1 VAC, could you throw
an external bridge rectifier on it and get by
that way, or does the condition actually cause
other stress to the alternator, that eventually
kills it (excess heat, etc)?
 
CompUser said:
Kewl info!

...if you were showing >.1 VAC, could you throw
an external bridge rectifier on it and get by
that way, or does the condition actually cause
other stress to the alternator, that eventually
kills it (excess heat, etc)?
Click to expand...

I looked at that years ago when I had a
shorted diode on my Datsun's alternator.
The alternator puts out 3 phase AC requiring
6 diodes to convert to DC. It's pretty
much impractical to kludge something up
to do that.

Back then, you could buy a new bridge
assembly and solder it in yourself. I
think it cost less than USD 20 and took
about 1/2 hour to do it.

Whether they are available now, I don't
know.
 
Jim Stewart said:
I looked at that years ago when I had a
shorted diode on my Datsun's alternator.
The alternator puts out 3 phase AC requiring
6 diodes to convert to DC. It's pretty
much impractical to kludge something up
to do that.

Back then, you could buy a new bridge
assembly and solder it in yourself. I
think it cost less than USD 20 and took
about 1/2 hour to do it.

Whether they are available now, I don't
know.
Click to expand...
Exactly so. The AC is a symptom of the diode bridge being damaged. I also
don't know if the diodes are available separately any more. They are inside
the alternator, on two plates (3 per plate; one for each phase at each end
of the windings.) I've never changed any myself. In all the bad alternators
I've seen the AC voltage was at least 0.5 volts, and the most recent one was
reading 1.7 VAC!

In modern cars a damaged diode bridge tends to get worse within a few days
or weeks, as the high current demanded by all the electrics and electronics
is pulled through two phases instead of 3. But if the AC is low now -
indicating all three phases are working - there is no reason to worry about
it getting worse from the jumper incident.

Mike
 
Ok... I have lots of excuses... I'd been working 70 hours a week for a
month... it was 6 am... it was foggy... it was dark and my flashlight
batteries were dead... my jumper cable red and black are too close...
but the bottom line is I messed up.

I have a 1996 Subaru Outback that had the dome light left on for days,
and therefore a very low battery. My wife has a 2000 Subura Legacy
Sedan. I thought I had been very careful, but I hooked the cables up
backwards. The Outback's power was totally shut down. I was dumb
enough to start her car. When this did not make a difference, I got out
and happened to touch the jumper cable, which was very hot. I knew
instantly and confirmed my error. I pulled them off and inspected
batteries, fuses, wires, etc. for damage. Everything looked okay. I
hooked the cables back up and jump-started the Outback. Both cars have
been working fine for over a week... batteries have been charging and
holding a charge even in very cold weather... all electronics are still
working.

I am still very afraid of the damage I may have caused. Is there any
safety feature that Subaru puts in place to prevent stupidity and
protect the electronics? If not, what possible damage might have been
done and how should I check for it?
Click to expand...


IMHO

to the car, connecting the batteries in series would just look like you
shorted across the battery.
there would be no excessive currents in the cars' circuits to blow
fuses or links. with the
batteries in series you are taking power out of the car (out of both
cars) not putting it in.

to the jumper cables, it would look like you connected them to a
battery and then shorted
the other ends, times two.

that said, if you were to jumper your 12v battery to a 24v battery,
either in series or parallel,
bad things would happen.

it is unlikely that shorting the battery for a short period would
damage the alternator.
this short would simultaneously reduce the current available for the
alternator's field. the alternator
rectifiers do not burn up when you drive on a low battery. a lead acid
battery has a low internal
resistance, this is what allows it to dump the large currents required
for starting. when a battery
is in a low state of charge, say below 10 volts, it is going to draw
pretty much the same current
from the alternator as if it were a dead short. in addition, I believe
alternators are designed to
operate into a dead short, that is, the field current is limited to a
maximum that will not allow fire to
occur in either the field or stator windings. it is only logical to
install rectifiers with a rating that
can readily handle the maximum output.

and finally,
when you engage your starter motor, this is probably only a few ohms
resistance. in other words
nearly a dead short. so again, it is unlikely you caused any damage to
either battery, either.
 
IMHO

to the car, connecting the batteries in series would just look like you
shorted across the battery.
there would be no excessive currents in the cars' circuits to blow fuses
or links. with the
batteries in series you are taking power out of the car (out of both cars)
not putting it in.

to the jumper cables, it would look like you connected them to a battery
and then shorted
the other ends, times two.

that said, if you were to jumper your 12v battery to a 24v battery, either
in series or parallel,
bad things would happen.

it is unlikely that shorting the battery for a short period would damage
the alternator.
this short would simultaneously reduce the current available for the
alternator's field. the alternator
rectifiers do not burn up when you drive on a low battery. a lead acid
battery has a low internal
resistance, this is what allows it to dump the large currents required for
starting. when a battery
is in a low state of charge, say below 10 volts, it is going to draw
pretty much the same current
from the alternator as if it were a dead short. in addition, I believe
alternators are designed to
operate into a dead short, that is, the field current is limited to a
maximum that will not allow fire to
occur in either the field or stator windings. it is only logical to
install rectifiers with a rating that
can readily handle the maximum output.

and finally,
when you engage your starter motor, this is probably only a few ohms
resistance. in other words
nearly a dead short. so again, it is unlikely you caused any damage to
either battery, either.
Click to expand...

You REALLY need to re-read the statement AND learn simple electrical
theory!!!
Connecting a healthy charged 12 volt battery to another vehicle BACKWARDS
results in a lot more then *YOU* think.
Re-read it again and think about what was asked and done....

Also..you know nothing about alternators.......
 
Mortimer said:
You REALLY need to re-read the statement AND learn simple electrical
theory!!!
Connecting a healthy charged 12 volt battery to another vehicle BACKWARDS
results in a lot more then *YOU* think.
Re-read it again and think about what was asked and done....

Also..you know nothing about alternators.......
Click to expand...

My God, you are INCREDIBLY smart. Could you please take a few minutes
and tell us EVERYTHING you KNOW about ALTERNATORS.
 
Chuck.K said:
My God, you are INCREDIBLY smart. Could you please take a few minutes
and tell us EVERYTHING you KNOW about ALTERNATORS.
Click to expand...
Hey Chuck!!! some BOZO named Norm thinks I am you!
Amazing....
If you really DO want to know about the "innards" of Alternators..I used to
rebuild them :)
Would be happy to share the info if you need it.
 
Mortimer said:
Hey Chuck!!! some BOZO named Norm thinks I am you!
Amazing....
If you really DO want to know about the "innards" of Alternators..I used to
rebuild them :)
Would be happy to share the info if you need it.
Click to expand...

I know Norm and while he doesn't know much about alternators he does
know usenet jerks. If he says that you are me then that is good enough
for me. By the way Mortimer I have been having trouble with my
esophagus, how about you?
 
Chuck.K said:
I know Norm and while he doesn't know much about alternators he does
know usenet jerks. If he says that you are me then that is good enough
for me. By the way Mortimer I have been having trouble with my
esophagus, how about you?
Click to expand...
Has been fine! ate a whole pizza without chewing it last night.
 
Chuck.K said:
I know Norm and while he doesn't know much about alternators he does
know usenet jerks. If he says that you are me then that is good enough
for me. By the way Mortimer I have been having trouble with my
esophagus, how about you?
Click to expand...
Try a lighter weight oil. If that doesn't do it you might need new seals or
a rebuild. Or you could check a wrecking yard for a low mileage one.
(Actually, I do have trouble with my esophagus, and it really hurts when the
spasms get going!)

Mike
 

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