Simple electrical question

So simple but has throw-ed me! I’m no expert but do understand some electrical circuits. Many, many years ago I built our first color TV. It was a 25 inch Heathkit and we used it for 17 years.
This was part of a color tv course that I completed.

My question… is about a simple 2 wire (one wire is ground) normally closed push button switch. It is the seat kill switch on my riding lawn mower. When the operator is seated the switch is open and this allows the mower engine to run.

Decided to bypass the switch by unplugging the switch, this should work for then the circuit would be open. But no, the engine will not run without the switch and the switch is n.c.?

It makes far more sense (not to mention being electrically simpler) to have the switch be closed while the operator is seated, so that when he gets off (or falls off) the switch opens, interrupting the current to the ignition coil. Try jumping the two wires together, instead.

I think the switch in question is a spring-loaded plunger type switch (at least it is on my riding mower). When the operator is not sitting in the seat, the spring pushes the plunger up and opens the contact in the switch. If the operator is in the proper position for operation, his/her weight compresses the spring and closes the switch.

Bypassing this switch would defeat one of the safety features of your mower, and is not recommended.


First, I didn’t design it. :wink:

No, as I said it is a n.c. switch. Closed, kills or grounds the circuit.
It will not run closed. (or the wires together)

Well, you said it won’t run open either, so you’re overlooking something. Are you positive the motor is working? Did it run immediately prior to you disconnecting the switch?

Great… you have described a normally closed switch. :slight_smile:

I didn’t ask for your recommendation… maybe you should reread my question and post a solution if you have one. :slight_smile:

I have checked it every way I can think of.
No problem with the engine.
It runs with the switch in the circuit, in the open position.
It kills the engine in the closed position or with the switch unplugged from the wires??

Perhaps one of those wires is grounded to the chassis through the switch as a sort of failsafe? Try it with first one wire disconnected, then the other.

Hmmm. This is a strange one.

The system could be designed to work with either a normally-closed or normally-open switch. I agree with Q.E.D. that the designers would opt for a system that uses the latter, but for a different reason: using a normally open switch would make the circuit fail safe, in that a broken wire or disconnection would render it safe. (Well, it wouldn’t be able to start. Which I guess is “safe.”)

Are you 100% positive it’s a NC switch?

If so, then maybe one of the switch terminals is grounded, and one of the wires needs to be grounded in order for the system to work. When you disconnect the wires, neither is getting a ground, so the system doesn’t work. Not likely, but it’s a possibility.

Geeze, Q.E.D., we must think alike.

The switch frame is metal that plugs into a hole on the frame but I removed the switch from the frame and unplugged the two wires. One wire is about 6 inches long going to ground.
I tried it with the switch removed from the frame with the same results. I checked the switch with a ohm meter and the switch case is not grounded.

Sorry, I can’t retry any thing right now for I’m about hundred miles away from our other place.

I’m a little confused on this point. When you removed the switch, did the tractor start with the wires still connected to the switch?

I am 100% sure it is a n.c. switch.

One side of the switch goes to ground.

The n.c. switch grounds the circuit and doesn’t allow the engine to run. Therefore removing the wires opens the switch… just like pushing the n.c. switch. Simple… but it kills the engine to unplug the wires.

Yes, the engine would run only if the switch is open.

You say one wire goes to ground. Where does the other go?

Despite the fact that the switch has one lead connected to ground, it can still be a N.O. switch. Are you thinking it’s a N.C. because it’s grounded or because it says so on the switch’s case?

The other wire goes to (in parallel) the PTO switch and the clutch brake switch.

Remember, what I said in my first post, I am familiar with electrical circuits. :slight_smile:
I know you are just trying to cover everything.

No, first I checked it with an ohm meter, also the wiring diagram shows an n.c.

That’s all I wanted to know, because it’s acting like a N.O. Well, I’m stumped, then.

Is this what you’re saying?

  1. The switch is configured as “normally closed.” (This means that the switch is closed when no one is sitting in the seat.)

  2. You remove the switch from the tractor.

  3. You reconnect the wires to the switch. The switch is in your hand.

  4. The tractor is able to be started when you press on the switch.

  5. The tractor cannot be started when you don’t press on the switch.

  6. The tractor cannot be started when you disconnect one or both wires from the switch.

If the above is accurate, then I don’t know what to tell you. Seems rather impossible to me.