Like many others, the 2015 John Deere D125 Lawn Tractor (aka ride-on mower or garden tractor) has two annoying safety switches.  One stalls the engine if you mow in reverse without pressing a button.  The other stalls the engine as soon as you get off the seat (or lean too much forward or to one side).  This post is about disabling/bypassing the (newer) seat safety switches.

Unlike older models, the new seat safety switch now has four pins, and not just two wires, so things are a little bit more complicated than the ‘cut and short’ solution presented on many older YouTube videos.  ‘Cut and short’ will not work on the newer safety switches.

I conducted some research and found out that the switch itself is JD Part Number GY20073.  It is available for next to nothing to people in the US, but the price increases by a factor of about 500% when you try to get it in Australia.  Further research reveals that the (alternately-branded) Rotary 14246 Plunger Safety Switch is identical and much cheaper to source in Oz.

Having thus acquired a seat safety switch, I then used a multimeter to test the electrical continuity between all possible combinations of pins.  This was first done with the switch in its default ‘off’ position (spring-loaded pin all the way up) and then in its ‘on’ position (pin pressed down).  Here are the results:

• switch ‘off’ (pin up)
  • open circuit:  1-2, 1-3, 2-4, 3-4
  • closed circuit:  1-4, 2-3
• switch ‘on’ (pin down)
  • open circuit: 1-2, 1-3, 1-4, 2-3, 2-4, 3-4
  • closed circuit: nothing

What this tells us is that when no-one is sitting on the seat, the outer pair of pins are connected and so are the inner pair of pins, but when a person sits on the seat all of the pins are disconnected.  This is surprising, as what it means is that to simulate a person sitting on the seat (the pin being pressed down) all you need to do is make sure none of the wires are connected — and that would be as easy as just unplugging the connector from the safety switch!

This was such a surprise that I actually didn’t believe it.  It doesn’t make sense to build a ‘safety’ switch that can just be unplugged by an end user.

Closer examination of the switch with a magnifying glass revealed some imprinting:  63DH, DELTA SYSTEMS INC, STREETSBORO OHIO, US PAT NO 6,207,910.  Examining the patent online made for a couple of hours of painstakingly slow but interesting reading.

The patent for this switch describes it as a “low profile, two pole, plunger-type safety switch”.  The electrical testing confirms two pole behaviour.  What was really interesting, however, is that the patent also includes the design of the connector — and the interaction between the two is where some clever engineering magic happens.

As “it is desirable to prevent an operator from bypassing the operation of the safety functions provided by a seat mounted safety switch by simply disengaging the connector coupled to the switch housing” the engineers designed the connector to short-circuit the two middle wires in the connector itself when it was unplugged.  Sneaky devils!

As can best be seen in FIG. 12, when the connector housing 202 is engaged with the switch housing 112, the beveled edges of the triangular nubs 127 a, 127 b extending outwardly from the forward edge of the bottom cover 117 contact and deflect downwardly the arcuate portions 264 of the forwardly extending arms 262 thereby permitting electrically connection or nonconnection between the socket assemblies 210 b, 210 c to be determined by the position of the actuator 130

So, what seems to be happening is that the two middle wires (2 and 3) are being used to make sure that the connector is actually connected to the switch and also sense the position of the pin.  If 2-3 is closed circuit then either a) the connector has been unplugged, or b) the connector is plugged in but there is no-one on the seat (pin is up).  In either of these cases the motor won’t run.  If 2-3 is open then a) the connector must be plugged in and b) there should be someone on the seat (pin is down).

The outer two wires (1 and 4) only sense the position of the pin.  If 1-4 is closed-circuit then there is no-one on the seat (pin is up).  If 1-4 is open circuit then someone’s on the seat (pin is down).

It is thus almost certain that both 1-4 and 2-3 need to be open-circuit in order for the motor to run.

The utterly trivial conclusion to all of this is that, in order to bypass the seat safety switch in your lawn tractor all you need to do is cut the cable before the connector.  Simply cutting the cable and then insulating the exposed ends will ensure that no circuits are ever closed and therefore that all of them remain open — thus mimicking the electrical behaviour of the connector being plugged into the switch and the pin being pressed down.

Wow, just wow.

So, having done my homework and testing, it was now time to put the theory into practice.  I had a few hours of mowing to do and a bit of spare time up my sleeve, so I pushed the lawn tractor out of the shed, tilted the seat forward and unceremoniously cut the cable all the way through with some snips.  Brought the seat back down, hopped on the seat, applied the brake, pushed the throttle up to the choke position and turned the ignition key — the starter turned a few times and then the engine roared to life.  I let the engine warm up for a while and then, with the motor idling, just hopped off the seat… and… nothing changed!  The engine kept on running.  Success!

I then happily mowed for a few hours, hopping off frequently to open gates, move obstacles and pick up pieces of rubbish — and the engine kept running regardless of whether or not the mower deck was engaged or what revs it was pulling.

Brilliant, just brilliant.  Makes mowing so much more efficient and enjoyable!  🙂

Side note:  On the JD D125 the seat safety switch cable that you cut is enclosed in a springy plastic sheath that runs down into a hole in the chassis of the tractor.  When you tilt the seat forward this stretches and is thus under tension.  When you cut through the cable the bottom part will probably withdraw down through the hole and quickly disappear from view.  Don’t bother trying to fish through the hole with your fingers, pliers, or a wire.  Kneel down next to the right, rear wheel and look just above and forward of the wheel — you will see the cable/sheath clamped to the frame.  The part protruding up from the clamp will be the loose end you just cut.  Use cable ties to secure this loose end somewhere so it won’t rub against anything metal or collect water.  If you are particularly paranoid, or often drive your tractor through puddles, consider even taping or plugging the end with something to seal it.

Remember that the wires in this loose end need to always remain open-circuit.  If they are rubbing against metal parts or bridged with a droplet of muddy water, then they can close-circuit and — as far as the electronics of the engine are concerned — signal that the operator has left the seat and result in the engine stalling (or not starting).  If the contact is intermittent (as it would be if vibrations were bouncing the wires against a metal part) then they could send conflicting signals multiple times per second to the engine which would result in erratic engine operation (symptoms similar to an engine being starved of fuel).  Secure and/or seal the loose end to eliminate this problem from developing down the road.

According to greenpartstore.com the switch is used in (and thus this bypass will work for) all the following models:

• 102, 105, 115, 125, 135, 145
• D100, D105, D110, D120, D125, D130, D140, D150
• E180 (Thanks Jim!)
• L100, L105, L107, L108, L110, L111, L118, L120, L130
• LA100, LA105, LA110, LA115, LA120, LA125, LA130, LA135, LA145, LA155
• LT150 after serial number 040,259
• LT160 after serial numbers MOL160H035097, MO160C035447, MO160D415192
• LT170 after serial number 035,001
• LT180,  LT190
• LTR180 after serial number 500,001
• G110
• X110, X125, X130R, X145, X155R

It is likely that many other brands of lawn tractors or garden tractors (e.g. Husqvarna, Sabre, Scotts) use a similar, if not identical switch and can be bypassed in the same way.

Disclaimer

I am neither young nor stupid.  I’m never in a rush when I mow.  I pay attention to where my feet are.  My property is flat and the soil is sandy.  The D125 comes to an automatic halt if you take your foot off the pedal.  In my particular situation the negatives of a seat safety switch far outweigh the positives.  Your circumstances are probably different.  Use your brain.  Safety switches exist to protect you in a variety of situations where things can go wrong.  It is generally not a good idea to bypass them.  If you are young and usually rushing to cut the grass on a sloping site with slippery clay soil with a mower that doesn’t automatically stop when you take your foot off, then you’d have to be insane to bypass the seat safety switch.  Accept responsibility for your own actions and don’t blame others if you screw up.

Final Thoughts

The reason why such a safety switch design is so effective actually involves psychology.  The average person, when faced with an annoying safety switch, may simply try to unplug it.  In their mind all the ‘clever stuff’ is being done by the switch itself.  They think that by just unplugging the ‘dumb cable’ from the ‘clever switch’ they can stop the ‘clever stuff’ from happening.  When this doesn’t work, they are probably a little bit surprised, then plug it back in and are just relieved that the tractor still fires up.  Nothing broken — whew!

The average person doesn’t have the technical knowledge or confidence to pursue the matter further.  Based on a lifetime of prior experience, connectors are just dumb bits of plastic so there’s no way the connector could have anything to do with the tractor not starting up — they think that an unplugged cable behaves the same as a cut cable (i.e. no electricity goes anywhere).  That means ‘the problem’ must be somewhere back in the engine bay itself and, well, that’s all just too hard.  The simple strategy failed — give up.  People with that mentality are probably the ones that designers are trying to protect from themselves — and rightly so.

Those that do a bit of research, however, find that in order to effectively bypass the switch, they need to actually damage the machine (i.e. cut wires or, alternatively, rip out the shorting member with pliers).  A lot of folks won’t be willing to do that because the idea of inflicting permanent, irreversible (in their minds) damage to something they paid money for is a mental obstacle too high for them to overcome.  Since this issue is likely to arise when the machine is newly purchased, that makes it even harder.  Deliberately damage a brand new tractor — are you crazy?

By moving ‘intelligence’ from somewhere obvious (the switch) to somewhere obscure (the connector), the designers thwart the annoyed masses trying to bypass the seat safety switch.  By making the only workaround one that causes damage, the designers further discourage those that are precious about their tractor.  About the only folk left are the hackers who just don’t give a damn.  If you’ve made it this far you’re probably one of those.  I hope you found this post informative.  Happy hacking!

• Update (2021-01-10):  Disabling Lawn Tractor Safety Switches (Part 2)