Kill Switch Wiring

There have been a lot of posts on here about wiring up a kill switch using the trailer break-away type switched that Denny's carb shop sells. I haven't actually used one of theirs, but I have used this type of switch. I am going to try to post a wiring diagram that shows how you can hook one up using a Single Pole, Double Throw relay that is available at most parts stores. This method does not just "ground out" your ignition to kill the engine. Since the SPDT relay has both a normally open terminal and a normally closed terminal, you can use it to kill the engine basically the same as turning the key off. Plus, the current going to your ignition doesn't have to run all the way to the back of the tractor and back up to the coil. This method will work on MSD type ignitions as well. You just connect the relay between the ignition key and the small red wire coming out of the MSD box. Let me know if you all have any questions and I will do my best to answer them.

PS. The Relay has both an 87 and a 87a terminal. The 87 is for a normally open application and the 87a is for a normally closed application. In this setup, the 87a is used and the 87 is not used.

If you really want to get fancy, you can hook up a small LED light on your dash and run it through the 87 terminal. It will light up if the kill switch is pulled. This is a kinda neat way to make sure the kill switch isn't out of its socket if your tractor won't start.

very nice diagram and very helfful. Thanks!

Though I applaud your schematic skills and do not want to take away from your idea I have a problem with this and other type circuits like "grounding out the points". The problem I have is these style of kills are not fail proof. What I mean is they have to work to kill the tractor, not to make it run.

The cheapest method (and a fail safe one) is to run the power wire from ignition to the coil to the rear of the tractor, loop through a simple 110 plugin, and then to coil or MSD box. It only takes about 5 feet of additional wire which would not make a measurable amount of voltage drop. I realize this would not work on a Mag setup, but you could adapt this concept to the internal wiring in a mag.

Just my thoughts....

Brian

This is true. I meant to add that all you had to do on a Mag ignition was to attach one wire from the break-away switch to a gound and the other to the ground lug on the mag. My method won't work with a mag.

This is true. I meant to add that all you had to do on a Mag ignition was to attach one wire from the break-away switch to a gound and the other to the ground lug on the mag. My method won't work with a mag.

My point was if you use a normally open switch as your safety switch and your wire has an open (or a brake) somewhere else in your control system, it will NOT kill the tractor.

But heck I wonder why we really have have them anyway....half the pulls I go to they are never hooked up....even saw when the sled guy should have killed a tractor because its back wheel was braking off and he admitted that he forgot about killing it.

Brian

I found some parts numbers if any of you are interested. The relay can be bought at NAPA as part number MPE AR272SB or as part number ECH AR272. They are available at Auto Zone under part number 733634. Advance Auto Part's crappy website wouldn't let me search for what I needed unless I had a specific vehicle, but I know they offer them too.

Another option to the 110v plug is a marine type lanyard kill switch. It is as fail safe as the 110v plug, but it isn't as bulky. One of my tractors has this type on it and it works well. If you are interested in that style, it can be found at most marine stores or at ACE hardware superstore.com under part number 8093056.

You can power put the coil wire on the relay from the switch. That is fine but in this case the + power from the battery should come from a good power supply like the battery to pin 30. The ignition coil would go on the output side. Pin 87A. JJ

As Hardline stated, the power should go in blade 30, if wired that way the indicator light will work. the other way the circuit stops at the blade and the switch reed of relay is dead, if the reed is hot it will power one of the two depending if the relay is activated or not.
The Bosch type relays are extremely dependable and the coil circuit takes very little to engage it, it draws very little current and takes the load off the ignition switch. I like to use them to power electric fans and water pumps as they were designed for heavy amp applications and save wear and tear on switches