Automotive Circuit Protection - Part 1

[Garfield Willis]

On Fri, 14 Jul 2000 20:27:23 -0500, Tom Sharpe <twsharpe at mtco.com>
wrote:

><snip> isn't it enough to just make sure the battery does not become
>disconnected?? Get your power from the battery + and - ,,, then the alternator
>can't get to the ECU when disconnected.
>
>One more thought about jumping, the alternator sees (any) two batteries as one
>discharged battery and promptly attempts to charge it (max). Tom

Uhh, lemme get this right, Sharpie. You're replying to a message posted
3 weeks ago, and asking "isn't it enough to keep the battery connected"?

Wow, I can't possibly match your attention span! :)  But I'll try to
catch up some.  Lemme see, your first suggestion seems to be, "get power
from the battery, then if you disconnect the battery, the "alternator
can't get to the ECU when disconnected". Uh huh.

OKAY, yes well (in my best Montie Python/John Cleese voice), if you "get
power from the battery", then if you disconnected the battery, would you
imagine the ECU follows on like some dangling bit of kelp? No? Alright,
so you disconnect the battery, but the ECU stays connected to the main
bus? Probably. Rightee hoo then, but what THEN is it still connected
toooooo? Ahha, just possibly the ALT!

Well, THAT doesn't seem to have helped much, now has it?

OK, moving right along. About jumping, you mention that "the" ALT sees
the two batteries as a mostly discharged battery (I presume your point
is that a good battery and a dead battery in parallel still present a
pretty large load to an ALT? Right? Roight!). If my faculties are still
partially intact, I recall the point about jumping (which someone else
made, not moi) was that there are TWO ALTs possibly involved if BOTH
vehicles are running. This can be potentially BAD for the vehicle that
suddenly sees alot less load when the jumper cables are disconnected.
FINE for the vehicle that has the dead battery, it's ALT actually sees
an INCREASE in load, as soon as the good battery is unjumpered. BUT the
vehicle with the good host battery, IF you have it's
engine/electrics/ALT running, SUDDENLY can see a huge decrease in load,
when you disconnect the jumber cables. Quite possibly nearly as bad as
if you had disconnected it's own battery, relatively speaking. Remember,
"Load Dump" is a matter of relatives; like a dead battery (and it's
charging load) suddenly disappearing. That was why the recommendation to
leave the HOST/good-battery vehicle's engine (and ALT) OFF whilst
jumpering.

Nothing like a Vuja Day, but seriously, how did you come to reply to a
thread so dead and musty in the grave? Been time travelling again, have
we Tom? You've just GOT to stop using that infernal machine; look what
it did to the last guy who owned it! There was a whole MOVIE done on
him, ya know.

Garfarkle


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