Automotive Electrical question/part

[Jason Roehl]

Our church recently had a new engine put on our ZT mower, but the new engine is a different make and electrical setup. I need to get some of the safeties working (especially the "dead-man's switch" under the seat). Here's the problem. When the seat switch is depressed (someone sitting on the mower), the switch is closed and allowing current to flow to something. When the switch is open, it doesn't kill the mower anymore due to the differences in the control/ignition systems.

There is a terminal on the new engine that, when grounded to the frame, kills the mower.

So, I'm thinking that I can re-wire the seat switch to a relay such that when the relay is powered (switch closed due to person on seat), it opens a circuit between that terminal on the engine and the frame, but when power is lost (person gets off seat), it closes that ground circuit, killing the engine.

Does anyone know where I might find such a relay? Or at least get me pointed in the right direction with correct terminology?

I'm thinking it's NC SPST, but specs seem to be kind of thin when I search.

Thanks.

[Dan Hintz]

No need for a relay, just choose a NC (normally closed) switch under the seat. While the person is sitting down, the switch is open... when they get up, the switch closes and grounds the system. Sounds like the switch currently under the seat is a NO (normally open)... odd, but possible.

[Jason Roehl]

No need for a relay, just choose a NC (normally closed) switch under the seat. While the person is sitting down, the switch is open... when they get up, the switch closes and grounds the system. Sounds like the switch currently under the seat is a NO (normally open)... odd, but possible.


That would seem to be the obvious solution, and I certainly thought of that, but then there's the whole physical engineering of the switch and installing it in place of the proprietary switch. Wiring is much easier.

[Ken Fitzgerald]

Jason,

Typically that type of safety switch would be wired to the primary side of the coil in the old pre-electronic ignition days.

Have you looked at the current switch and it's mounting? The odds of that switch being proprietary are slim. Like Dan, I think installing a new switch would be the easier route.

Does this mower have some type of battery on it to use to power a relay?

[Tom Stenzel]

Jason,

Have you looked at the current switch and it's mounting? The odds of that switch being proprietary are slim. Like Dan, I think installing a new switch would be the easier route.

Does this mower have some type of battery on it to use to power a relay?

^----This. The relay must have power to start the engine. Also the relay must be sealed well enough so that dust doesn't cause to hang or get in the contacts. If you're determined to go with the relay route and the mover is 12 volt, a place that services RVs would be a start.

Modifying the switch to a normally closed/open on sitting would be better.

Fundamentally the problem is that it's backwards in terms of safety. If a wire breaks or something goes wrong the safety then doesn't work and the mower still runs, where normal design is that if a safety system fails the mower shouldn't run at all. Just something else to ponder.

-Tom

[Tony Zona]

Remove the switch and inspect it closely. Sometime it will have an unused terminal that is opposite of the one being used.

If your luck is like mine that won't be the case. :)

[Jason Roehl]

I got the seat switch figured out. A trip to the local electronic supply store netted me a 5-terminal relay: 2 control terminals, and then the other 3 terminals are for either NC or NO operation, depending on how you wire it. The seat switch is a simple NO momentary switch, with 12V being fed through it when closed (someone on seat), so I tapped into the switched side of that to power the above relay in a powered-open configuration. That is, when I apply power to the relay, it opens the ignition ground/kill for the engine. If someone is not on the seat, the relay is not powered, closing the circuit that grounds out the ignition, killing the engine, or preventing it from starting.

Next up is to do something similar with the parking brake to the signal wire for the starting solenoid so that the engine won't start unless the parking brake is set, then to initiate a kill if the controls levers are swung into position with the parking brake set. I have the first part figured out, no problem there. The second part is going to take a little head-scratching yet. I may also figure out how to use the key switch to kill the ignition, as currently it's tied to the fuel pump, so turning off the key sometimes doesn't instantly kill the engine (but never more than a few seconds, it just doesn't feel right). I think this was mostly done with solid-state electronics originally, but those didn't work with the replacement engine.

I would have dug farther into it, but I didn't have a roll of wire with me like I thought I did, and I had a meeting, so I ran out of time. The important thing is that the dead-man switch is working, and the mower runs/cuts like it did when we first bought it (used) last summer with ~1150 hours on it. It's an '03(?) Scag Tiger Cub with a 52" cut--that thing can flat-out MOVE! It's rated for 23 acres/day (8 hours).

[Jason Roehl]

Remove the switch and inspect it closely. Sometime it will have an unused terminal that is opposite of the one being used.

If your luck is like mine that won't be the case. :)

Nope. Just two terminals. And it makes sense that you want the power running through the switch when someone is on the seat--that way loss of power is the same as nobody on the seat.

I'm attempting to do everything I can to make the mower function as designed, through other methods if necessary, but so that the functional outcome is the same. One of the kluges was that the choke lever operation is backwards from the original because of how the cable had to be routed around the engine, so it will just have to be re-labeled (hello, silver Sharpie!).