datsun-roadsters
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Re: weird electrical problems

To: Daniel Neuman <dneuman@quark.sfsu.edu>
Subject: Re: weird electrical problems
From: "John F Sandhoff" <sandhoff@csus.edu>
Date: Mon, 23 Jul 2001 15:53:52 -0700
You commented:
> There was not any direct juice going into the old harness thru the old
> alt hot lead without the fuse in place.

Here's my thinking, FlameMan :-) (I almost bought you a set of
'eight ball' fuzzy air fresheners the other day)

Your new alt goes straight to the battery. You've probably hooked some
other goodies straight to the battery (perhaps fused but it doesn't matter),
but the car itself is still getting its juice thru the existing harness. From
the sounds of it, you've also got a fused wire (the 35 amp missing fuse)
that goes from the alt to the main car wiring, almost certainly bypassing
the ammeter.

The wiring, BTW, runs this way: From the battery, down to the starter.
>From the starter, a pair of wires feed the car: the 'w' wire goes to the
ammeter and from there the fuse block and the rest of the car; the 'gw'
wire goes thru a separate fuse to the hazard switch; and thru a 20 A
fuse on the fuseblock to the brake lights, brake check light, and horn.
Note that if the ammeter blows, you have power to the brake lights,
horn and hazard but nothing else.

If you have an additional wire (the 'missing 35A fuse' lead) that ties into
the main wiring PAST the ammeter, then you will have an additional
path for current when the fuse is inserted. No fuse: power flows thru
ammeter only. Add fuse: ammeter reading is severely reduced (the
resistance of the alternate-path wiring determines by how much) and
most of the current is bypassed.

If you look on the fuse block you'll find a heavy white (as I recall it has
a red stripe, but it's the heaviest) wire. You should see +12 on this
wire. I suspect you see zero with the 35A fuse removed and +12 with
it installed. This wire is the feed between the ammeter and the fuseblock.
If the ammeter has voltage going in (that heavy 'w' wire from the starter)
and it is still OK then it must have voltage going out (this wire at the
fuse block) - assuming the wires and connections themselves are OK,
of course.

The symptoms you describe just scream out 'bad ammeter!' If you're
really brave and aren't afraid of a few flames :-) you can CAREFULLY
try shorting out the ammeter with a jumper clip. But be CAREFUL, if
you pass much current thru a lightweight jumper you'll see for-real
smoke!

-- John
     John F Sandhoff   sandhoff@csus.edu   Sacramento, CA

p.s. Saw the 'Devil Ride' at a car show Saturday - a converted hearse.
He had a flame steering wheel - flames carved into the metal spokes!
I forgot my camera, alas..

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