Now that I've read Max's reply, I finally understand what Mike was trying
to do, and where it might have gone wrong.
Some time ago, battery cutoff switches were discussed on the list, and the
issue came up about radio presets. It was wisely suggested to put a small
fuse (1 amp?) across the terminals of the battery cutoff. Thus, when the
switch is turned off, juice will still flow through the fuse to maintain the
radio presets. But if someone tried to start the car, it would draw enough
current to blow the fuse and shut down the power.
Matt Kulka
Huntersville, NC
'74 B
-----Original Message-----
From: Max Heim [mailto:mvheim@studiolimage.com]
Sent: Thursday, March 25, 1999 1:41 PM
Cc: MG List
Subject: Re: Smoooke from the column...Fire in the 'B!
I'm not an electrical expert by any means (and perhaps I've failed to
grasp the subtlety of the situation), but it seems to me that this whole
concept of bypassing your battery cut-off switch to provide a constant
ground on one circuit is one of those "clever, but..." ideas. Assuming
that this whole circuit is perfectly electrically isolated and at no
point grounds to the chassis is a pretty big "if". And if it does have a
ground connection, say through the case of the stereo or the base of the
trunk light bulb holder or a worn speaker wire, and then you forget to
switch on the battery cut-off one morning, and go ahead and put in the
key and switch on the ignition, then the whole 20 amps or whatever of
starter juice is routing through your puny 16-gauge wire, which proceeds
to make like a glow worm while the insulation merrily sizzles (along with
any other small-gauge wires in the circuit). And in fact, this seems to
describe more-or-less what happened, no?
So, numero uno, I'd suggest putting some kind of low-amperage, quick-blow
fuse in that wire; something intended for stereo equipment, not your 15A
auto fuse. It shouldn't matter what gauge the wire is once it's fused.
You definitely do not want a heavy gauge unfused wire.
Secondly, I'd rethink your whole security issue. You mention persons
unknown getting into your car at night and leaving the lights on. Well, I
don't know where you live, but anywhere I can think of, the first thing
they'd do is steal your stereo, thus rendering the station presets
irrelevant. Leaving out expensive or impractical measures such as a
locked garage, high fence, or large dog on a long chain, you might
consider a car alarm system, or a motion-activated light on your driveway
(they make solar powered ones that don't require any wiring--you can just
stick 'em up on a post, porch or fence, and they're a lot cheaper than a
stereo--or a wiring harness).
IMHO, YMMV, FWIW...
>
>
>
>>On Tue, 23 Mar 1999, Mike Lishego wrote:
>>
>>Constant ground?? Not unswitched power? this will become an
>important
>>distinction...
>
>
>There's a reason for that. I have a battery cut-off switch that I
>use on my ground terminal. I disconnect it every night because a) I
>don't want my battery to drain for a silly reason, b) nobody can
>steal my car if it won't start, and c) nobody can crawl in my car
>and leave the lights on *been there, done that.* I ran a full-time
>ground to the radio only because I wanted to keep my stereo settings
>for more than a day.
>
>>> Along with this, the remote ground wire that I had run for my
>>> stereo melted too. I *ass*umed that a ground (especially to a
>>> battery post) didn't need wire that was too thick. I guessed
>wrong.
>>
>>No, you were right, if all that is being grounded is the radio.
>
>
>Actually, it's the radio and the circuit it's on. That would
>include the map light (Which only works through the switch, not with
>the doors), the cig lighter which is disconnected, and the trunk
>light. I don't think that's too much to ask from a circuit that
>sees limited use?
>
>The radio only takes it's power from the circuit when the car is on,
>but the whole set-up is gounded and has power when I had my
>permanent ground wire connected.
>
>Obviously, I get to spend some time buying
>>> thicker gauge wire for my stereo and fuel pump, along with some
>$$$
>>
>>Fuel pump, maybe. Stereo, no.
>
>
>Picked up some 10-ga wire from Autozone today. Gonna solder the
>connections and heat shrink them tomorrow...
>
>>By the way, on my stereo installations both the
>constant(unswitched)
>>power for memory backup and the switched power lead are fused.
>That's how
>>they come in the factory (stereo factory) harness.
>
>
>Right, my stereo is like that. However, since the cig lighter
>circuit sees power at all times, my radio won't switch off with the
>ignition switch. I think I'm going to change this with the new
>wiring. I've got a mind to run a hot wire to a fused circuit on the
>fuse box that will shut off the radio with the key.
>
>Would I have any problem running a ground to the battery from the
>radio only in this situation? What about the trunk light (it is
>nice to open the trunk to turn on the battery and have the light
>come on for you...)
>
>My question is simple - why did the
>>> white wire melt?
>>
>>Well, it seems pretty clear you had a short-to-ground, which pulled
>too
>>many amps through the ignition switch. That's what melted *that*.
>
>
>Yeah, that fuel pump wire actually burned through itself to the
>ground...Bad news when that happens...
>
>>If you had a "Constant ground" wire that melted, it means that too
>much
>>current was supplied to *it*. If it was really a ground wire that
>means
>>something high current was using it *instead* of its regular ground
>path.
>>This implies an open ground connection elsewhere (such as, oh, the
>ground
>>strap on the block) and/or a short between this wire and hot.
>
>
>OK, the radio was a constant ground. I tried to not use the stereo
>without the battery connected to the main harness (ie - all the
>grounds), but I might not have a few times. That's where the
>problem probably started...
>
>>If it was a constant voltage, instead, it means that *it* was
>shorting to
>>ground, bypassing the stereo.
>>
>>I'm confused by the wire running directly from the ignition switch
>to the
>>fuel pump, instead of into the harness. This sounds like a DPO
>>modification. If it *is*, it implies there is something wrong with
>the
>>normal route through the harness. Like, say, a previous fire has
>melted
>>the insulation inside the bundle, shorting wires together (go
>ahead, ask
>>me how I know <g>).
>
>
>I'm not sure if this hack is due to the harness or the pump...I have
>an aftermarket pump on the car, but the white wire is also hanging
>down by the fuel pump. When I looked under there, it was freshly
>melted, along with a black wire coming from the wiring loom. I cut
>them both to where I could see separate wires again, then taped
>their ends off. Dunno what the black went to though...
>
>I'm looking at my hayne's wiring diagram right now...It *seems* that
>the fuel pump's white wire gets juice from the ignition switch and
>also from the fuse block. If I disconnected the white wire from the
>fuse block that runs to the fuel pump, this would solve any
>potential shorts, right? No, I take that back - it would still get
>juice from the switch. Judging from the diagram, I'd have to find
>the connection where the two wires meet on their way to the fuel
>pump and disconnect both of them. Anyone know where that is?
>
>The black wire intrigues me...I never knew MG fuel pumps weren't
>grounded to the body! Hmmm...I see that this wire also supplies a
>ground for the rear right-side lights. Could this disconnected
>ground be the reason my right turn signals blink slower than the
>others? I guess I'll see - once both white wires have been disabled
>at the fuse block, I should be able to connect the nblack to ground.
>
>>Summary:
>>The stereo wires shouldn't have melted if all they were attached to
>was
>>the stereo. The fuel pump wire doesn't have to be *that* thick, if
>all it
>>was powering was the fuel pump. Bypass wires tend to indicate
>harness
>>problems. Ground wires melt due to suddenly becoming the best
>>path-to-ground, hot wires melt due to the disappearance of proper
>load
>>(shorts to ground).
>
>
>Thanks for the tips...This wil definetly help me in my search for
>electrical salvation!!!
>
>>eh. Fun.
>>
>>--
>>John M. Trindle | johnt@tsquare.com | Tidewater Sports Car Club
>>'73 MGB DSP | '69 Spitfire H Stock | '88 RX-7 C Stock
>>
>
>
>"You have children - how would you feel if your kid caught on fire?"
>-My quote of the day, 3/23/99
>Michael S. Lishego
>http://www.mgb.bc.ca/virtualgarage/
>
>
>
--
Max Heim
'66 MGB
runs great, looks scabrous
mountain View CA
|