Mounting a Master Quick Disconnect Switch on a 1946-7 Cadillac

[Bill Ingler #7799]

Here is an idea that was given to me several years ago. Mounting a quick disconnect switch on an older car is a must for me, even if I have just installed new wiring. Opening and closing these big hoods on the 40s Cadillac is a pain and it becomes more of a pain the older you get. So someone said put the master disconnect switch on the upper part of the plash shield behind the right front tire. So now instead of opening and closing the hood, just reach up on the splash shield back of the tire and turn the lever. Having helped install this switch on several friends cars, I took some pictures to show where this switch is mounted.

The switch can be bought at most auto stores or on line. Find the spot that will put the switch close to being over the battery, drill a small pilot hole, insert a sheet metal punch, just a tad bigger than the stem of the switch. Tighten the sheet metal punch and it will give you a perfectly round hole in the splash apron. Mount the switch, then get some 00 welding cable and make your two short cables from the battery to the switch. I elected to use the power side of the battery to the switch and then the other cable to the starter solenoid. I have also mounted the switch on a 41 in the same general location.
Bill

[John Washburn CLC 1067 Sadly deceased.]

Bill,

Excellent idea. But make sure you get the correct amp switch some have less amps capacity (the cheaper ones). So go for the high amperage switch. My opinion.


the Johnny

[Steve Passmore]

Couldn't agree more Bill on needing these switches an d I make it a standard thing on all the re-wires Ive done but I wouldn't mount any electrical item anywhere inside a wheel arch which is subject to the amount of water that gets in there. They will eventually corrode and be covered in gunk.  Perhaps we get more rain than you?
I have fitted dozens of these and I always find somewhere in the car to mount them out of sight.  In the event of any problems you need to get at them quickly.  My most successful place is just in front the drivers seat in the floor out of sight.  Just my 2 cents worth.

[Jay Friedman]

On the other hand, with a 6 volt system if the cables are any length at all aren't you risking enough voltage drop to cause starting problems?  I've heard of this, particularly with 49 and later OHV motors with higher compression. 

[Bill Ingler #7799]

Steve: I certainly agree that you get more rain than our average of 8 inches a year here in Phoenix and yes where I installed the switch could in the long run give me a problem. So far over after 15 years and many CCCA tours through all parts of the US, with plenty of days driving in the rain, the switch still works. I don`t think there is any one perfect place to mount the switch on our cars but regardless of where the switch is mounted, anyone who drives an older car should consider a cut off switch.

Jay: You bring up an interesting point about voltage drop, length of cable and starting with a 6 volt. Where I have the cut off switch mounted requires about 33 inches of cable for the longest cable. I use 00 welding cable, an Optima battery, ground from battery direct to a mounting bolt on the starter. Then the engine is grounded twice from the heads to the firewall and grounded at each front motor mount. I have never had any problems on a cold or hot start but this doesn`t mean that length of cable is not a factor in hard starting.    Bill

[Classic]

I was always taught that a master disconnect switch should be installed in the battery ground cable, not the hot cable.  It doesn't matter whether the car is positive or negative ground.  The reasoning for this is the same as for disconnecting battery cables.  You always disconnect the ground cable first, and reconnect the ground cable last.  This is so that if a wrench or other metal should happen to touch the battery hot terminal and the body or frame of the car at the same time, you won't have a potentially very dangerous short circuit (i.e. Fire).  This is because the cable from the battery hot terminal to the switch (if the switch is connected to the hot cable) is always hot!, regardless of whether the switch is on or off.

Like Steve, I also mount my disconnect switch on the floor in front of the driver.  It is close enough to the seat to be easy to reach in an emergency, but at the same time not visible to the casual observer unless they stick head down in front of the seat.

A suitable switch is a Cole Hersee 2484 available at O'Reilly Auto Parts and other auto part stores, as well as online.

Battery cable gauges for 6 Volt systems should be 00 (2/0) (0.3648" copper diameter) or even better 000 (3/0) (0.4096" copper diameter).  Please note that these sizes are not the same #2 or #3 gauge, which are much smaller.

[R Sotardi #11719]

 Six volt Cadillac starters draw A LOT of amps. Do not even think of those "green wheel" things. I used a Knife switch for a while, and still there was resistance. Now  I use  3/0 cable & ground strap, with no kill switch and my 50 cranks like a 12 volt. I believe I checked the amp draw quite a few years ago. It was 325 amps 6v. If the switch is not rated well above that ( at least 500amp) it will act as a current bottleneck. Ron

[Jay Friedman]

My feeling is similar to Ron's.  Have had bad luck with the "green wheel" thing and am afraid to risk hard starting on a hot day after a high speed run with long cables to a kill switch.