OK Listers. Below is a copy (No regard to copyright infringement. So
sue me.) of all the responses from the Shop-Talk list so far, to the
battery cutoff switch question. If you are pressed for time, stop reading
at the line below, and you will only get my opinion.
IMO on this is to break the grounding cable with a switch. That is the
negative cable on negative earth cars, the positive cable on positive earth
cars. Here's why. If the car is running when the switch is cut off, it will
probably keep running if you cut the non-ground cable. This is especially
bad for alternator equipped cars, in which case you will need to replace
the alternator. If the car is off when the switch is cut off, it will not
start
no matter which cable it cuts, but why take a chance.
For you competitive types, the switch MUST be in the grounding cable for
safety reasons. If a safety worker comes over after a crash, the first thing
he or she will do is cut off the power switch. If the engine keeps running,
what's the point? You will also be required to cut off power to the fuel
pump.
Now it's up to you to use this info and install your own switch any way
you see fit.
Allen Hefner
Philly Region SCCA Rally Steward
'77 MG Midget
'92 Mitsubishi Expo LRV Sport
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mjb@phase.cs.utah.edu (Mark J. Bradakis)
If you put the cutoff switch on the non-chassis side, the alternator or
generator can still be turning enough to keep the ignition system
energized, so the motor may keep running. If the cutoff switch has
connections for also breaking the alternator or generator circuit, then
it can go on that side.
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markm@tutsys.com (Mark Miller)
I respectfully disagree. Since there is most likely another connection to
ground from the charging system (bolt through the alternator, various
straps, etc.) having the cut off on either side (or even having two cutoffs
that would totally disconnect the battery) will not prevent the car from
continuing to run.
I believe that there is no great reason to put the switch on one side or
another, if its mission is to prevent a circuit being made while you are
working on the vehicle - like via a wrench on a hot terminal.
One reason to put it on the chassis side is that it is easier to put it
there - while you are removing the lead from the battery you won't be in
danger of shorting the lead to the chassis, as it would already be attached.
If you were removing the (+) lead on a negative ground car and had the
wrench hit the body you would need to go out and buy a new wrench. And we
don't want to go back to that craftsman/snap-on/husky thread again now, do
we?
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ryoung@navcomtech.com (Randall)
Breaking the circuit (on either side) between a running alternator and the
battery is NOT a good idea. The alternator is fully capable of generating
enough voltage to fry it's diodes (and possibly even you) when it's load is
suddenly disconnected.
IMO the big advantage of putting the cutoff on the chassis side is that then
you can short either battery terminal to the chassis (one at a time) without
damage. If the cutoff is on the hot side, then shorting the hot battery
terminal to the chassis will produce *sparks*.
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hamilton@cast.navnet.net (Bob Hamilton)
All my Sunbeam Talbots are POS ground and I have the cutoff switch
on the NEG side which is the hot side in this situation. Never had a
problem and don't understand why it would cause a problem. The ones I use
are the type that mount directly on the battery post and has a green knob,
CCW disconnects and CW connects.
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crothfuss@coastalnet.com (Chuck Rothfuss)
This is a hot topic on both the shop-talk and land-speed lists tonite. I
know what you mean about disconnecting the positive lead without also
shutting off the alternators field lead. I "saw the light" as I melted
every light bulb in the car down to their ceramic base and fried an electric
fuel pump when my alternator lost it's reference source. The solution is to
disconnect both the positive wire (neg ground system) and the field wire to
the alternator with the cut-off switch. No field current = no magnetic
field in the rotor and zero current production.
Chuck "Do you smell something burning?" Rothfuss
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ryoung@navcomtech.com (Randall)
> -----Original Message-----
> From: Chuck Rothfuss
> The solution is to disconnect both the positive wire (neg ground system)
> and the field wire to the alternator with the cut-off switch. No field
> current = no magnetic field in the rotor and zero current production.
That will work on _some_ alternators, although I'd still want to disconnect
the field connection first, wait for the magnetic field to decay, and only
then disconnect the alternator output.
BUT, most modern alternators get their excitation current directly, inside
the alternator ! Unless you modify the wiring inside the alternator,
nothing you do outside will disable it without harming it.
Why do you want to disconnect the battery from a running engine, anyway ?
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pethier@isd.net (Phil Ethier)
>Why do you want to disconnect the battery from a running engine, anyway ?
Because your car just crashed and the first worker on the scene wants to
kill the electrical system.
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mothra@netspace.net.au (Simon & Heidi Young)
cutoff needs to break the earth side of the circuit, simply taking the
battery out of the loop won't stop the engone as the alt/gen will keep it
running.
I believe there are systems available which also ground the coil killing
the spark.
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