MTFCA Forum

BATTERY CUT OFF SWITCH--I'm installing a knife type battery cut off switch. Reading the description of installation, it says that it should be installed on the negative side. Since the T is negative ground, I planned on stalling it on the positive side. Am I correct??

No. You should still use the negative. Not as much chance for sparks that can ignite hydrogen gases. This type has a post to connect the negative braided strap or cable to. Jim Patrick

I tried installing one on our '21 Touring but it was too tall and the floor board would not set back down in place. I might try a thinner piece under the battery.

Battery cut-off switch should always be installed on the Ground side of the battery, whether its positive or negative ground for the reasons stated above.
I recently bought this one from Snyders with the bracket but have not figure out where it mounts. I'll have modify it .

I have several - all negative cable connected. Touring is mounted in the rear floor board corner, Racer's is in the steel floor panel at the bottom near the front seat panel.

My 26 touring came with one like jsaylor posted. It is mounted on the seat riser between the drivers legs. It saved me from tearing up my starter and ring gear teeth when the cheap repo starter button shorted out and the starter would not stop. Flipped the disconnect switch off, then I disconnected the starter lead, flipped the disconnect switch back on, hand crank started and was on my way again.

Addendum: I use the "heavy duty" type that John posted the photo of above - I wouldn't use anything that could possibly produce a spark unless connected to an Optima battery ! I have several mounted all connected to a grounded cable and in the floor near the base of the front seat panel.

My 2c worth is to make sure it is easily accessible. On the front of the seat upright is a good place.

I have one of the rotating switch versions on the truck mounted on the wooden battery/tool box:
When the horn button jammed by the time I got out of the truck, ran around and turned it off I came very close to fire in the wire! that was a close shave. (This will start a discussion on master fuses as well....)

Just as a suggestion, you might want to try one of these from HFT (I know, I know it’s HFT)


as they can be mounted below the floorboard or behind the front of the seat remote from he battery. The only piece visible when you remove the key is the tip of the black pipe, so it doesn’t interfere with the appearance of the vehicle.

I’ve had one installed on my truck for probably 15 years with no issues. I always remove the key from the ignition switch and the battery disconnect at every show.

The removable key also prevents anyone from messing with your machine, potentially turning on lights or other switches.

When top clearance is minimal, they make battery cutoff knife switches with a sideways hinged knife. Jim Patrick

I also use one similar to jsaylor's but mine is a 4 pole switch. I mounted it next to the starter switch with a small hole in the floor board for access (discreetly covered by the floor mat). It drops the power before the starter switch.... but also disconnects the the power to the bottom of the coil box. So it won't run / start on mag. Bill

Thanks everyone. Negative side it will be.

I've installed several of these over the years on vehicles. I've never heard of putting them on the ground side so I always put them on the positive side. Here is the one I installed on my Delivery. Later I flipped it around so the swing arm of the switch is de-energized when open. I placed this one under the vehicle not far from the foot starter switch. I figured I had to get under there to turn the gas on and off so do both at the same time.

I prefer the 1/2 inch wrench cut-off switch, low cost, always works, takes up no additional space. Always disconnect the ground side first, if the wrench slips and grounds out it will do nothing, but ground out the positive terminal and sparks will fly. Those cut-off switches are adding risk and an additional point of failure, putting something like that on the hot side of the battery is dangerous, and if it should fail while driving you will ruin your generator.

A word of caution! Be sure to keep the contacts clean and tight. Corrosion is common if it is installed near the battery, or in a place where it will get wet driving during a storm. If it opens while driving, it could cause you to burn up the generator. So keep it clean and good contact. I know this from experience.
Norm

I work on fire apparatus , and all of the trucks that I service have two switches. One for the batteries, which is a positive connection and one for the ignition. And some of the older ones still use the colherse switch which is also a positive connection. In thirty years I have not seen any issues with a positive connection.

I don’t doubt or question your experience, but the switch directions recommend installing the switch on the negative side for a reason. A knife switch that is installed on the battery post, like many of them are, if connected to the positive post, can cause a hot spark which can result in an explosion if there is hydrogen gas emanating from and remaining in the area of the battery. Most times it is not a problem, but if conditions are right, it only takes one time. Negative mounts are less likely to cause a spark. Of course, if it is installed on the cable, away from the battery or the gas tank, either side will work. Jim Patrick

I'm not an electrician, but if you put the switch on the positive side, wouldn't anything upstream of it still be "hot", which you might want in some applications such as large vehicles with multiple accessories that can be disconnected without disabling the whole vehicle.

No. If either the negative or positive is disconnected, the power to all systems is disabled. You need both positive and negative connected to have power. Jim Patrick

If the switch is placed at the battery, yes that would be true.

I prefer an enclosed switch to limit corrosion as well as any likelihood of an exposed spark.

As long as the switch controls the primary cable and thus everything from that battery post - either ground or "hot", I can't see how it could make any difference whether that is + or -. Can you explain why it should matter?

I see the reasoning for the switch being placed on the negative post in the vicinity of the the battery, sparks could occur causing battery gas’s to ignite. When I installed the switch on my 27 touring I placed it on the frame near the starter switch.

The negative battery post is a little smaller than the positive post.

If the switch size was designed for the negative post, some extra force might be required to put the connections together all the way.

I use the Cole Hersee master disconnect switch on all my T's. Drill the hole for the ground in the frame and grind off the rust & paint on the frame and connect the switch then connect the ground cable to the switch. It will be hard to get to but no one will notice it. We forget there are two sides to a electrical circiut positive & negitive both will only ark if connected or disconected under a load! Yes Some modern Fire appuratus master & ignition switches are on the positive side of the circiut but control the $300 relays located in a sealed inviroment that switch the current. According to Occam's razor the simplest solution will most likely be correct.

Regardless of how its wired a kill switch, as they are called, may (likely to) "spark" if some down stream object is in the ON position to complete the circuit when the switch is turned on. Given that situation it is best to use an enclosed switch where the spark is contained. Secondly, if an exposed switch is used then it should be connected to the same ground (positive or negative) of the vehicle to prevent accidental sparking or shock (note it will spark if some down stream object is in the on position). Also: If the exposed switch was connected to the positive terminal of a negative ground vehicle ( or vice versa) touching the copper surface would give one a shock to remember or worse, if the user is touching a grounded surface. When connected improperly, anything making a contact with it, including water (rain, snow washing the car), would cause either cause a shock, spark or drain the battery, nothing needs to be turned on in that situation since the electrical connection is completed at that point. Hope this helps

I'm surprised that no-one has mentioned the voltage drop you get with these things.
For those of us still using 6v (and preserving our starter motors), we need all the volts we can get.
The knife type is probably better than those with face contacts, but there will still be some resistance.
My car had one of these switches. I removed it.
But I do have a 'main fuse'.

I understand that subject is the reasoning for attaching such a switch to the ground side as it does not reduce the draw from the starter.

Of all the people that have added a cut-off switch to their Model T's, I would like to know how many have added them to their modern cars too.

Just my opinion about the battery switch.

As it is a safty device, it should be installed on a safe place not near the battery or the gas tank, good visible and easy accessible.
Best is to install it in the ground circuit of the car to disconnect the ground connection.
The switch should be from good quality and closed (not the plastic ...) that can stand the maximum current in the circuit. In the model T it should stand about 400Amp ( 6V starter).
In a modern car you can bridge the switch by a 5Amp fuse to keep the "Keep Alive Memory" of the microprocessor working. I used it a lot in modern farm tractors and harvesters.

Andre
Belgium

If you use one, please hide the thing. They are UGLY. I've seen many otherwise nice cars (and trucks) with their looks ruined by a battery cut-off switch.

My 26 Touring had this bracket on the frame with a hole so I mounted my cutoff and new bracket on it. Not sure if this bracket is original or not. I can reach through the rear wire wheel spokes to turn on and off without lifting the floorboards. Don't intend on driving in weather. Just an idea for mounting.

Mike

I really appreciate all the interest in the cut off switch. I have learned a lot by listening to experience. I never thought the the initial question would evolve to be like the famous "which oil is best or which polish to use on brass or kevlar is best or cotton for sure". This has been fun. Thanks again everyone for good reading on a dreary couple of days. Dick C.

The battery cut off switch in my truck is fully enclosed, even the cable connections and is attached to the positive side, away from the battery. It is rated at 1000 peak amps, 300 amps continuous. These can be picked up at most auto parts stores.