# Simple electrical question **URL:** \ **Category:** Factual Questions\ **Created:** [October 16, 2003, 12:27am UTC](https://boards.straightdope.com/t/simple-electrical-question/207681 "2003-10-16T00:27:28Z")\ **Posts on this page:** 19\ **Page:** 3
**Author:** ![Kreekurmudgeon](https://avatars.discourse-cdn.com/v4/letter/k/9f8e36/32.png) [@Kreekurmudgeon](https://boards.straightdope.com/u/Kreekurmudgeon)\ **Post date:** [October 16, 2003, 5:02pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/41 "2003-10-16T17:02:19Z")
My bad. I meant “fuel solenoid” or “carburetor solenoid.” Now I can add “afterfire solenoid” to my jargon. Sorry. ---
**Author:** ![Kreekurmudgeon](https://avatars.discourse-cdn.com/v4/letter/k/9f8e36/32.png) [@Kreekurmudgeon](https://boards.straightdope.com/u/Kreekurmudgeon)\ **Post date:** [October 16, 2003, 5:05pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/42 "2003-10-16T17:05:26Z")
My bad. Of course I meant “fuel solenoid” or “carburetor solenoid.” Now I can add “afterfire solenoid” to my jargon. Sorry. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 16, 2003, 5:50pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/43 "2003-10-16T17:50:39Z")
O.K., this is a long shot. And it may not even make logical sense: Could it be that the switch is indeed NC, but that when installed on the mower, it is mechanically configured (using springs and what not) such that the switch is open when no one is sitting on the seat, and closed when someone is sitting on the seat? ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 16, 2003, 6:02pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/44 "2003-10-16T18:02:00Z")
I see what you’re getting at, **Crafter\_Man** , but that would require an external lever mechanism, obviously. But then, the switch should work the opposite way when it is removed from the chassis and operated manually, as the OP says he did. However, on inspection of the circuit diagram, it looks as though the switch MUST connect the circuit to ground in order for the mower to run, so your idea has merit. The diagram of the start switch is less than clear, but part of the ignition system is conected to it through the switch in question. So, I’m going to stick with my original idea that the switch wires must be connected together to bypass it. the OP was not clear if he did so, or not. If he did, and it didn’t work, then I give up. ---
**Author:** ![casdave](https://avatars.discourse-cdn.com/v4/letter/c/c6cbf5/32.png) [@casdave](https://boards.straightdope.com/u/casdave)\ **Post date:** [October 16, 2003, 6:13pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/45 "2003-10-16T18:13:24Z")
Hmmmmm, looks like there has to be slightly chewed through insulation somewhere. Try disconnecting the seat switch wire from either the PTO switch, if that is where it is terminated, or at the clutch brake switch if it is terminated there, it will depend upon the wiring harness. Does it then start ? Another option might be to take the ends from the disconnected seat switch and jury rig them into a simple on off switch and see what happens, does this behave in the way you expect? Very odd things can happen with stray earths, it may seem completely unrelated, but I would also make a point of checking the headlights, and the connector to them. Lights are notorious for producing partial earths. One last thing, sometimes you get a redundant N/O changeover contact connection in this kind of switch, does this have a third unused terminal ? ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 16, 2003, 8:05pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/46 "2003-10-16T20:05:33Z")
**Q.E.D.** : I’m not knowledgeable on automotive/tractor electrical systems, so forgive my ignorance on this, but what is the deal with the wire between the armature and terminal 4 on the ignition switch? It is also tied into the seat/PTO/clutch interlock system, but I’m not sure how the whole thing works. At first I thought the ignition switch was using it to supply +12VDC to the armature. But that can’t be right, otherwise the seat switch would be shorting it to ground! So if you have any idea on what’s going on with this part of the system, please let me know. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 16, 2003, 8:23pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/47 "2003-10-16T20:23:35Z")
> [@](#): > > \*Originally posted by Crafter\_Man \* > \*\*\*\*Q.E.D.\*\*: I’m not knowledgeable on automotive/tractor electrical systems, so forgive my ignorance on this, but what is the deal with the wire between the armature and terminal 4 on the ignition switch? It is also tied into the seat/PTO/clutch interlock system, but I’m not sure how the whole thing works. At first I thought the ignition switch was using it to supply +12VDC to the armature. But that can’t be right, otherwise the seat switch would be shorting it to ground! So if you have any idea on what’s going on with this part of the system, please let me know. \*\* It’s hard to tell precisely, because the electrical connections on the connectors and the ignition switch are unclear, but it looks like the ignition switch might shut off the engine by grounding out the armature windings. When the ignition switch is in the START or RUN positions, this conection is open. That’s what I think is happening there. ---
**Author:** ![olefin](https://avatars.discourse-cdn.com/v4/letter/o/d78d45/32.png) [@olefin](https://boards.straightdope.com/u/olefin)\ **Post date:** [October 16, 2003, 9:04pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/48 "2003-10-16T21:04:07Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > \*\* > > However, on inspection of the circuit diagram, it looks as though the switch MUST connect the circuit to ground in order for the mower to run, > > So, I’m going to stick with my original idea that the switch wires must be connected together to bypass it. \*\* I don’t see how you get that from the diagram? To me it shows the NC switch completing the circuit to ground the armature and kill the engine. I have tried connecting the wires together… it kills the engine as it should. ---
**Author:** ![Jurph](https://avatars.discourse-cdn.com/v4/letter/j/50afbb/32.png) [@Jurph](https://boards.straightdope.com/u/Jurph)\ **Post date:** [October 16, 2003, 9:36pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/49 "2003-10-16T21:36:01Z")
This discussion sounds remarkably like the [story about magic](http://www.everything2.net/index.pl?node=A%20Story%20About%20Magic) in the Jargon File. I’m wondering if perhaps the **seat** (or one of its supports) is part of the switch, so that it’s not the switch, but the metal body of the mower (or the seat support, or some other metal part) that is doing something to add to the switch’s functionality? What intrigues me is that the dashed line denoting the seat-switch includes a section of red wire. It very explicitly includes it. I’m wondering if it doesn’t just rely on ground, but relies on some other screwy principle, for instance: 1. when switch is opened, resistance is very high (but finite and set at some design value) 2. when switch is closed, current running to ground doesn’t just run to ground, but also induces a current somewhere in the red wire 3. there is loose insulation somewhere, so that there’s enough ground to keep the engine from starting, but not enough ground to stop it once it’s running…? I’m baffled, too. Try installing a More Magic switch. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 16, 2003, 10:14pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/50 "2003-10-16T22:14:12Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > \*\*It’s hard to tell precisely, because the electrical connections on the connectors and the ignition switch are unclear, but it looks like the ignition switch might shut off the engine by grounding out the armature windings. When the ignition switch is in the START or RUN positions, this conection is open. That’s what I think is happening there. \*\* Sounds good to me. I am also of the opinion that the PTO switch is being shown in the _PTO engaged_ position. Here’s the reasoning: The mower should not be allowed to start if the PTO is engaged, correct? Well in the diagram the NO contacts are in series with the starter coil. Also note that the NO contacts are shown open. This would indicate to me that the PTO switch is being shown in the _PTO engaged_ position. By a similar line of reasoning, it is my opinion that the clutch switch is being shown with the pedal up. (In order to start the mower you must first push the clutch pedal down.) So the PTO switch is being shown in the _PTO engaged_ position and the clutch switch is being shown in the _petal up_ position. Now if the PTO is _not_ engaged, and if your foot is pushing the clutch petal down, then seat switch is completely taken out of the circuit. So here’s what I would try for diagnostic purposes. With the mower initially off: a) Turn off the PTO b) Raise your butt _off_ the seat c) Push the clutch petal all the way in d) Try starting the mower According to the wiring diagram, it _should_ start. If not, something is screwed up. ---
**Author:** ![Thin\_Ice](https://avatars.discourse-cdn.com/v4/letter/t/aeb1de/32.png) [@Thin\_Ice](https://boards.straightdope.com/u/Thin_Ice)\ **Post date:** [October 16, 2003, 10:29pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/51 "2003-10-16T22:29:06Z")
The diagram only shows one plug but if the switch has a two-pole plug for the wires so the seat can be removed with the switch intact then the plug could have an interior switch that connects the wires when the other half of the plug is missing. It would work like a headphone plug in a radio that cuts off the built-in speaker when you plug in the headphone jack. ---
**Author:** ![olefin](https://avatars.discourse-cdn.com/v4/letter/o/d78d45/32.png) [@olefin](https://boards.straightdope.com/u/olefin)\ **Post date:** [October 16, 2003, 10:35pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/52 "2003-10-16T22:35:28Z")
> [@](#): > > \*Originally posted by Crafter\_Man \* > \*\*Sounds good to me. > > I am also of the opinion that the PTO switch is being shown in the _PTO engaged_ position. Here’s the reasoning: The mower should not be allowed to start if the PTO is engaged, correct? Well in the diagram the NO contacts are in series with the starter coil. Also note that the NO contacts are shown open. This would indicate to me that the PTO switch is being shown in the _PTO engaged_ position. By a similar line of reasoning, it is my opinion that the clutch switch is being shown with the pedal up. (In order to start the mower you must first push the clutch pedal down.) > > So the PTO switch is being shown in the _PTO engaged_ position and the clutch switch is being shown in the _petal up_ position. > > Now if the PTO is _not_ engaged, and if your foot is pushing the clutch petal down, then seat switch is completely taken out of the circuit. > > So here’s what I would try for diagnostic purposes. With the mower initially off: > > a) Turn off the PTO > b) Raise your butt _off_ the seat > c) Push the clutch petal all the way in > d) Try starting the mower > > According to the wiring diagram, it _should_ start. If not, something is screwed up. \*\* You are saying what I already said about the PTO switch. It should be shown engaged **or** the NO and NC are reversed. As far as your diagnostic procedure… I already tried that… as I posted above… the engine will start and run with no one in the seat until the brake pedal is released. Releasing the brake pedal puts the seat switch back into the circuit. ---
**Author:** ![olefin](https://avatars.discourse-cdn.com/v4/letter/o/d78d45/32.png) [@olefin](https://boards.straightdope.com/u/olefin)\ **Post date:** [October 16, 2003, 10:39pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/53 "2003-10-16T22:39:13Z")
> [@](#): > > \*Originally posted by Crafter\_Man \* > \*\* > > I am also of the opinion that the PTO switch is being shown in the _PTO engaged_ position. > > So here’s what I would try for diagnostic purposes. With the mower initially off: > > a) Turn off the PTO > b) Raise your butt _off_ the seat > c) Push the clutch petal all the way in > d) Try starting the mower > > According to the wiring diagram, it _should_ start. If not, something is screwed up. \*\* You are saying what I already said about the PTO switch. It should be shown engaged **or** the NO and NC are reversed. As far as your diagnostic procedure… I already tried that… as I posted above… the engine will start and run with no one in the seat until the brake pedal is released. Releasing the brake pedal puts the seat switch back into the circuit. ---
**Author:** ![Jeff\_Lichtman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jeff_lichtman/32/1531_2.png) [@Jeff\_Lichtman](https://boards.straightdope.com/u/Jeff_Lichtman)\ **Post date:** [October 17, 2003, 8:01am UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/54 "2003-10-17T08:01:58Z")
The diagram shows a dashed line around the seat switch. Inside the dashed line are the words “seat switch harness”. The positive line from the battery is shown going through the box. Could it be that the battery wire is in a harness with the seat switch, and that by disconnecting the seat switch you’ve also somehow disconnected the battery? ---
**Author:** ![Uncommon\_Sense](https://avatars.discourse-cdn.com/v4/letter/u/f9ae1b/32.png) [@Uncommon\_Sense](https://boards.straightdope.com/u/Uncommon_Sense)\ **Post date:** [October 17, 2003, 1:17pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/55 "2003-10-17T13:17:38Z")
I`ve got nothin`. I guarantee that it`s going to be a fundamental oversight by the time it`s figured out. I studied the drawing too, and it should work the way **olefin** says it does (or doesn`t). I Googled for this problem and didn`t come up with ANYthing similar. Me-thinks an OOPS will be along anyday now. The only thing else I would do, is check to see if all the assumed grounded parts of the chasis are in fact at the same ground potential. IOW, make sure you have continuity between the grounds. And test to see if the seat frame is grounded with the seat up and with the seat down, not that there is some mechanical grounding continuity (seperate from the switch) that we are overlooking. ---
**Author:** ![olefin](https://avatars.discourse-cdn.com/v4/letter/o/d78d45/32.png) [@olefin](https://boards.straightdope.com/u/olefin)\ **Post date:** [October 17, 2003, 1:42pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/56 "2003-10-17T13:42:06Z")
> [@](#): > > \*Originally posted by Jeff Lichtman \* > \*\* Could it be that the battery wire is in a harness with the seat switch, and that by disconnecting the seat switch you’ve also somehow disconnected the battery? \*\* The answer is no. Once again… I have never had a problem with starting after disconnecting the seat switch. The battery cable going to the starter solenoid is shown in the drawing. ---
**Author:** ![olefin](https://avatars.discourse-cdn.com/v4/letter/o/d78d45/32.png) [@olefin](https://boards.straightdope.com/u/olefin)\ **Post date:** [October 17, 2003, 1:56pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/57 "2003-10-17T13:56:00Z")
> [@](#): > > \*Originally posted by whuckfistle \* > \*\*I guarantee that it`s going to be a fundamental oversight by the time it`s figured out. Me-thinks an OOPS will be along anyday now. > > \*\* 😃 I agree. It has to be something simple… That was what I kept telling myself. Next week I’m going to solve it. :smack: ---
**Author:** ![NutMagnet](https://avatars.discourse-cdn.com/v4/letter/n/ea5d25/32.png) [@NutMagnet](https://boards.straightdope.com/u/NutMagnet)\ **Post date:** [October 17, 2003, 3:07pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/58 "2003-10-17T15:07:31Z")
When the switch is in the NC position (no one on the seat) is there continuity between the switch body and the frame (ground)? How about between the plunger and the frame? How about between one of the wires and the frame? When the switch is open, what else changes in continuity, besides the break in continuity between the two wires? What I’m wondering is, despite the diagram, could the switch really be a two-way device? Closed - continuity between the two wires, AND no continuity between (the switch body, or switch plunger, or one wire) and the frame Open - no continuity between the wires AND continuity from (one wire or the switch body, or the switch plunger) and the frame. When the switch is removed entirely, and you’ve manually connected the two wires you have performed one function - connecting the two wires, but not performed the other function. That must be either connecting or disconnecting one of the wires from whatever’s left in the device - the body or plunger. I don’t know that I have the answer yet, but it just seems there must be two things going on when the switch is activated. Other than this, some other part of the circuit may have been altered/compromised from the diagram. ---
**Author:** ![NutMagnet](https://avatars.discourse-cdn.com/v4/letter/n/ea5d25/32.png) [@NutMagnet](https://boards.straightdope.com/u/NutMagnet)\ **Post date:** [October 17, 2003, 6:59pm UTC](https://boards.straightdope.com/t/simple-electrical-question/207681/59 "2003-10-17T18:59:04Z")
Oops. You confirmed **Crafter\_man** ’s questions: \* 3. You reconnect the wires to the switch. The switch is in your hand. 1. The tractor is able to be started when you press on the switch.\* I missed that previously. The only thing I can think of is some circuitry in the switch that’s not connecting the two wires at zero impedence, but that sounds pretty esoteric for a kill switch. [Previous page](https://boards.straightdope.com/t/simple-electrical-question/207681.md?page=2)