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[GMCnet] Oh, did I mess up! [message #167395] Tue, 24 April 2012 08:05 Go to previous message
Richard Denney is currently offline  Richard Denney   United States
Messages: 920
Registered: April 2010
Karma:
Senior Member
So, I put new batteries in the coach.

It was cold and rainy, and I didn't get home until 15 minutes before dark.
Given the demands on the rest of the week and weekend, right up until time
to leave for Bean Station, I rushed out to make use of what daylight was
left. I installed the new battery in the Toyota (see elsewhere) so that I
could at least move that truck, and then I pulled the batteries from the
coach to replace them with fresh batteries I'd bought while at Costco on
the way home from work (a 100-minute commute from Baltimore--why I don't
usually have time in the evenings to work on things). So, I was in a hurry,
working with a flashlight, and getting lightly rained on. Too many stress
factors to maintain a proper attitude to work on the coach.

And I made the ultimate bonehead mistake. Yes, I installed the chassis
battery backwards. The only reason I didn't immediately know it was that I
have a master battery switch for that battery that I had turned off. After
installing the battery, I flipped the switch and stepped back into the
garage and plugged in the AC extension cord to the coach to power the
converter. I was about 10 feet away when I started hearing noises.
Awareness came simultaneously with the panic reaction, and I immediately
ran back and turned the switch back off and yanked that power plug.

Sizzle, sizzle.

I test for which wires were hot, and the wires leading to the combiner were
quite toasty, along with a 10-gauge lead from the boost solenoid to the
main power terminal on the firewall. That's the red wire coming out the
bottom of the main terminal in this picture:

http://www.gmcmhphotos.com/photos/electrical-wiring-concepts-for-73-230/p26980-chassic-wiring-center.html

None of the wires toasted long enough to really melt anything, or so it
seemed. They were hot, but not fried, though something certainly was
sizzling.

The combiner, having combined the house and engine batteries wire in
reverse, would have taken the brunt of the current flow, plus any circuit
that could find ground with the ignition switch turned off. It turns out
that the only circuit that is always on is the brake lamp/turn signal
circuit.

The combiner seems to be combined without there being charging voltage
present, though I did start the engine so it may have been surface charge
that was keeping it combined. Perhaps those long lead wires really did
limit current sufficiently to protect the combiner--more testing required.
I believe that's what' was sizzling, and in better light I'll know more. I
have a feeling it's melted. My combiner is an early Yandina from the West
Marine days (of the vintage that Ken Henderson cut apart and inspected on
its very unusual failure), and it is not the later model with better
protection.

And all switched circuits seem to be fine: the engine starts and runs, the
dash air runs, the house circuits work, the lights work, etc. I suspect the
alternator protection cable would have protected the alternator--the wire
to the alternator probably had no path to ground to draw current. My house
feed circuit from the up-front house battery to the house panel is
protected on both ends--in the front with a thermal breaker and in the back
with a magnetic breaker--but neither breaker was tripped (this surprises
me--I would have thought current would have tried to flow also through the
rear combiner to the genset start battery in the back as soon as the
converter came on and caused that combiner to combine). The only thing I've
found so far that does not work is the brake lamp/turn signal circuit. The
fuse on that circuit was blown, and a new fuse blew immediately on
installation. The load side of the fuse is a direct short to ground.
Perhaps the flasher unit (an electronic model) has blown up and shorted to
ground. The problem does not seem to be in the wires leading to the lights
themselves.

I am glad I have that master switch, even though I would have known my
mistake as soon as the clamp touched the terminal without it.

Moral to the story: Don't do simple things in a hurry--we take obvious
stuff for granted and then make bonehead mistakes.

Rick "I love the smell of melted insulation in the morning" Denney

--
'73 230 "Jaws"
Northern Virginia
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