# John Deere LT155 won't shut off



## warren anderson (Dec 10, 2019)

So this lawn tractor has the usual phalanx of safety switches. I've done nothing to try to defeat any of these switches although I'll admit to being constantly aggravated by them, but I digress....

When I started the tractor after a long period of dormancy, I found that I could not shut the tractor off and that none of the usual safety switches that would normally kill the engine worked either. Specifically, turning the key off has no effect, getting off of the mower (which would normally trigger the seat switch) has no effect, going in reverse with the mower engaged has no effect (normally, this would kill the engine also). The only way I can kill the engine is to pull the spark plug wire off of the spark plug.

I wrongly assumed the problem might be in the ignition switch so I purchased a new one. That didn't change anything; same behavior as before.

It seems odd that a relay/switch could fail in such a way as to keep the engine running no matter what, but that sure seems to be the case.

Does anyone have any ideas which component might be to blame?


----------



## Fedup (Feb 25, 2014)

Admittedly no expert on lawn and garden, but in my experience most air cooled engine ignition systems are magneto type. They are self energized and use a simple ground wire to kill the spark. In the "off" position the key switch USUALLY sends that wire straight to ground. Should that grounding circuit somehow become disconnected from the mower wiring harness or the path between the ignition switch and ground (and all the safety switches as well) then no combination thereof will stop the engine. 

I would look for a small (probably black) wire coming out of the engine cowling somewhere and going into the engine harness leading to the key switch area. If you find it, experiment by temporarily grounding it for effect.


----------



## warren anderson (Dec 10, 2019)

Fedup said:


> Admittedly no expert on lawn and garden, but in my experience most air cooled engine ignition systems are magneto type. They are self energized and use a simple ground wire to kill the spark. In the "off" position the key switch USUALLY sends that wire straight to ground. Should that grounding circuit somehow become disconnected from the mower wiring harness or the path between the ignition switch and ground (and all the safety switches as well) then no combination thereof will stop the engine.
> 
> I would look for a small (probably black) wire coming out of the engine cowling somewhere and going into the engine harness leading to the key switch area. If you find it, experiment by temporarily grounding it for effect.


That sounds very plausible, thanks! I'll look for that [broken] ground connection.


----------



## Bob Driver (Nov 1, 2017)

You didn't mention what the OEM is on the mower engine, but most LT155's came with a 15HP Kohler. Fedup is correct there is usually a black wire coming out of the blower housing that grounds the magneto to kill the engine. All ignition switches have a terminal marked "G" (ground). When the key is switched to the off position, contact is made between the "G" and the "M" (magneto) terminal and grounds the magneto. If a Kohler won't shut off, it's usually because the main key ground circuit is open. 

Here's a simple 5-prong switch that JD uses.... G-Ground, L-Lights, B-Battery, M-Magneto, S-Start. 
*Off* -- continuity between G&M. 
*Run* -- continuity between B&L. 
*Start* -- spring loaded momentary continuity between B&S










Most OEM's like to use a "Frame Ground" for their electrical system, John Deere is no exception. Once you find the problem with the main key ground lead, do yourself a favor and move the main ground point directly to some place on the engine. Aluminum based ground contacts don't rust/corrode. All of the important electrical components (Starter, Mag, Stator, fuel shut off) are mounted directly to the engine, not the frame. Grounding the entire electrical circuit to the primary mounting source for the major components eliminates a whole host of problems a bad frame ground generates.... Like the engine won't shut off


----------



## kelley1 (Jun 26, 2021)

Bob Driver said:


> You didn't mention what the OEM is on the mower engine, but most LT155's came with a 15HP Kohler. Fedup is correct there is usually a black wire coming out of the blower housing that grounds the magneto to kill the engine. All ignition switches have a terminal marked "G" (ground). When the key is switched to the off position, contact is made between the "G" and the "M" (magneto) terminal and grounds the magneto. If a Kohler won't shut off, it's usually because the main key ground circuit is open.
> 
> Here's a simple 5-prong switch that JD uses.... G-Ground, L-Lights, B-Battery, M-Magneto, S-Start.
> *Off* -- continuity between G&M.
> ...


i appreciate your input. My old jd 116 started acting up and now i've spent more on it than it's worth. New coil, ignition switch, diode, and two new vacuum fuel pumps (for some reason i decided my carb is okay). Anyhow, it's running okay again, but it won't shut off. The guy that installed my coil said i shouldn't have power to the white wire (from the switch) when it's running. i grounded it to the shroud, to no effect. I know it's something simple, but i'm not bright enough to sort it out. Thanks for taking the time to read this.


----------



## kelley1 (Jun 26, 2021)

kelley1 said:


> i appreciate your input. My old jd 116 started acting up and now i've spent more on it than it's worth. New coil, ignition switch, diode, and two new vacuum fuel pumps (for some reason i decided my carb is okay). Anyhow, it's running okay again, but it won't shut off. The guy that installed my coil said i shouldn't have power to the white wire (from the switch) when it's running. i grounded it to the shroud, to no effect. I know it's something simple, but i'm not bright enough to sort it out. Thanks for taking the time to read this.


it's a b&s twin, 402707, btw


----------



## Bob Driver (Nov 1, 2017)

You mention changing a diode..... The 402707 runs two diodes in the wire lead going to the magnetos (part # 844547)








Did you perhaps install a single diode at the #2 cylinder lead? If so, it sounds like you may have the "bias" in the wrong direction. Diodes usually have a band on them that tells you the bias direction. Bias through the diode is anode to cathode









The wire coming from your key switch connects to ground with the key turned off. The diode bias is designed to protect the magneto(s) from a 12VDC power surge coming from the key switch. Bias ring should be toward the key switch (connects to ground). The most common ways to burn a diode (usually #2 cylinder because its 1st in line in the wiring harness) is to hook the battery cables up backwards (even momentarily), or install the wrong key switch.

If you did in fact install a single new diode, flip it over and see if it works. If that doesn't work, you've probably got the wrong key switch installed. The terminal where that white wire comes into switch should only have continuity with one other terminal with the key OFF.... That will be the ground. If you're lucky enough to get a switch with letters at the terminal base it would be G to M (Ground - Magneto).

If you did buy the wrong switch, don't go out and buy another one.... Just shove a flattened paperclip into the little square on the plastic connector and that will release the tang on the terminal so you can move the white wire to where it connects with the ground terminal with the key in the off position. More that likely it is connecting to the L terminal (lights) and only has 12VDC on it when you turn on the lights. Otherwise, you would have burnt the diode again.....


----------



## kelley1 (Jun 26, 2021)

The diode I replaced was in a red wire under the battery. I'm not aware of where the other one is.
The switch is the correct one. Purchased it at JD and it's identical to the old one. 
I have a wiring diagram, but I'm not smart enough to sort it out. I have all the safeties (3) looped, which should eliminate any white wire problems, but I guess not. I appreciate your response and time. Thanks!


----------

