Mower Blades Won't Turn or Engine Will Not Start

This information is not in the John Deere repair manuals because of product liabilities.


First of all we need to understand that product liabilities show it's expensive to keep someone living after an industrial accident, i.e. an operators limb is cut off or damaged and infection set's in. The hospital bills are a potential liability and responsibility of the manufacturers if the accident could have been prevented by a switch made to safeguard the possibility of injury or death. This is why those safety switches are needed and they cause problems as well as require some electronic board.


The combination of moisture, ocean breeze and humidity of 80 + % makes electrical connections oxidize and that causes more resistance to electrical volume flow (amps). The circuit board electronic relays are controlled by electrical energy from the safety switches. These switches can cause much time to be spent as well as frustration of the machine not working in the jungle with grass growing an inch a day. Breakdowns will cause grief in simple so if this is your final straw before the mower is used for fuel in a great big fire then follow the instructions.

Before you go into modification of the switch input system to the controller just know to never get off the mower without shutting off the blade switch and never get on and off the mower on the blade discharge side.


The information on the Mower Problems page at the bottom Pdf. is the system we are working on. Most all mowers have safety switches which are essentially the same in function.


If operator is not on the seat the mower will not start unless the parking brake is set and the PTO switch or lever is in the blade off position. The engine will start if the parking brake is unlocked and off if the operator is setting in the seat turning on the seat switch and the PTO switch or lever is in the blade off position. Either scenario if the PTO switch or lever is in the on blade position the mower will not start.

Here's the last resort "eliminate" the switches after spending many hours finding out the Power Take Off (blade drive) switch is not providing voltage and current to the PTO clutch drive wire.


Key Switch soldered to a circuit board style, most John Deere riding mowers.


The brake switch is also called the neutral start and the wire on the John Deere system is supplying voltage to this switch after the key is turned on to run and thru the 15 amp spade fuse on the circuit board. The switch is normally open meaning the wires are broken apart in the switch and this is written as N.O. in switch terminology.


If the brake pedal is depressed the switch closes and makes contact that allows current and voltage to flow thru the switch connections. This allows the operator to then start the engine without getting into the seat. The PTO will not work with the brake switch closed.


This is the first switch to test out and eliminate by finding out what it isn't as the cause. So on this system as shown in the PDF there are two connection points on the circuit board card that the wires attaching the key switch is soldered onto. These two connection points are called X2 and X3.

These two connection points on the circuit board have connectors that allow a meter probe to be pushed into the terminals from the back without removing them from the circuit board. This testing is done with all wires plugged in.


First of all your circuit board will have two small lights, a red one and a green one. If the Green one is lit it means that you ignition interlock is on and making the engine ready to start. If this light is not on you need to test two things in this order.


NOTE for this test set the meter to DC - 20 volt scale and place the black or negative meter probe into the negative post terminal opening at the battery so it is firmly secured on the ground post.


Now use the red or positive meter probe and touch the battery positive post and measure the battery voltage and write that down.



Take the red meter probe and locate the X3 terminal connector, the one closest to the key switch wires. The first switch in line for the ignition relay circuit is the brake switch.


The brake switch is the first inline and if the brake switch is activated by pressing the brake pedal down it will close and the contacts will then allow the battery voltage and current to flow into the circuit board Interlock Module (IM) connector X3 closest to key switch soldered wiring. This connector will have two yellow wires in it at one point of connection.


To test the brake switch operation without a meter and to identify which circuit is bad before going deeper look at the circuit board closely, there are two small red and green plastic button looking things. Those are LED's (Light Emitting Diodes) and when voltage is present on that circuit they light up.



Lift the hood and look at the circuit board of the IM, the two LED's will be off, turn the key to the on (run) position, not start but make sure the dash lights come on by the steering wheel. Do not set on the machine seat but push down the brake pedal and the green light should go on. Let off on the brake pedal and the green light should go out. If it doesn't' light up when the brake is pressed and held down the engine will not start and stop there, you will need to do a preliminary switch circuit test further down in the text here.



If the green Led lights up then release the brake pedal is shows that circuit brake switch is working. Next have someone set in the seat and watch the green light it should light up if the seat switch is good and getting voltage.


The green light let's you know that the brake and seat switch are functioning correctly, they are both input to the IM that control the ignition circuit. If the green light does not go on the engine will not rotate to start. That is the first circuit to make work.



Test this circuit (ignition interlock) by leaving the black probe in the negative battery terminal. On the circuit board connector touch the red probe to the top of the fuses to check for voltage, test all four test points on the fuse tops. If there is battery voltage there then go to the X3 connector on the circuit board IM and you will see two yellow wires and install the red probe into the back entry of the connector and make sure battery voltage is present at those wire ends.



One yellow wire goes to and through the PTO switch in the dash and onto the brake switch input terminal. Go to the X3 connector and touch the red probe of the meter to the Purple wire #710 and there should be no battery voltage present without pressing the brake. When the brake is pressed there should be voltage at this wire. If there is no voltage present make sure the PTO switch in the dash is off (down). If it is down and still no battery voltage at 710 Purple test the voltage at the yellow wire on the PTO switch and then go across to the purple wire an see if the battery voltage is going through that part of the PTO switch and if it is then replace the brake switch. You really don't need the brake switch to mow, it only allows a person to start the mower off the machine without setting in the seat. When the brake switch is on it does not energize the PTO circuit and the blades will not rotate.



The other yellow wire that has voltage when the key switch is turned on (closed) at X3 goes directly to the seat switch and comes back into the X2 connector of the IM as Pink #800. As the seat switch closes the green light should come on at the IM if voltage is present at the Pink wire 800. If the green light does not come on and there is voltage present at the key switch and the Pink wire end in the connector X2 then replace the key switch and the IM (they come attached together). Green led on means mower is ready to start according to the Interlock Module. There are tests further described however for now we are testing the IM switch input circuits controlled electrically.



To test the circuit board by using a bypass (elimination) method make sure the black probe of the meter is firmly stuck into the slot in the terminal at the battery post. This assures that when voltage is measured by using the red probe at the IM that the grounds are working properly. Make a short wire piece from some wiring and strip both ends about a half inch back to expose copper wire. Stuff one end of the copper wire into the slot at the positive + battery terminal on the post of the battery. Use the other stripped end and touch the copper wire to the Pink wire in X2, just stick it in the back side without taking any connectors off the IM board. This should make the green light go on right away and if it doesn't then the mower will not start and needs the entire IM replaced. make sure during the tests that the key is on the run or on position.



The elimination method for the brake and seat switch is simply snip the yellow wire near the PTO switch connection in the dash. Strip it back a half inch and stuff the end in the Pink wire connector at X2. This completely bypasses the first two switches keeping the mower engine from rotating with the starter motor after rotating the key switch to start.


Below is a simplified circuit drawing reduced from the actual service tech schematic which has no color wires and is the complete safety system which is hard to follow as well contains more information than needed to do this quick field test sequence. The arrows in red are the flow of +12 volts DC from battery and back to ground. 


In the drawing above you can see the flow of voltage that originates from the battery (source) and continues to the red wire at the Ignition Key Switch, with the switch in the on position (not start) the + voltage from the source continues a path into the Interlock Module (IM). From inside the circuit board components the + voltage travels in the direction indicated by the red arrows and stops at the switches.



The seat switch is normally open (N.O.) so the wires are broken apart inside the switch when open and no voltage can go further. The brake switch is also open (N.O.) so no voltage can pass into the IM circuit board terminal connectors (Purple X3). The brake circuit is needed to set the ignition interlock green led on to start the engine and allows the engine to run when the PTO switch is turned off (pushed in).



Below is test #2 the ignition circuit and when activated to start the engine lights up the green light emitting diode (LED) on the IM circuit board.

As the green light (LED) goes on the mower engine should rotate with the starter, if it doesn't then go to our Electrical Repair page for a better understanding of the system involved when the green light is on and the engine still will not rotate and start.



Below is the final test to verify that all circuits are working that will enable the PTO switch interlock to activate, the green LED is on and then when the PTO switch is energized the red LED will light up also as exampled. 

Test #3 exhibits the circuit switches that are critical to have working to enable the PTO circuit and make the blades turn without shutting down the engine or preventing it to start. As the PTO switch energizes at 10. it enables the PTO to turn the blades.


The cut lines were necessary to make a clear path for you to follow in the drawing and there are a couple wires you could cut and re connect to eliminate the switches.


Eliminate Seat Switch : Cut 451 Yellow at the IM circuit board connector X3 to a length of 3" from the connector X3 on the circuit board. Strip it back a half inch and push it into the connector X2 where the Pink #800 wire goes into the connector so the wires make contact with each other. Or just strip a little from the Pink #800 wire and attach the yellow 451 and Pink 800 together. This energizes the ignition relay and turns on the green light.



NOTE: The PTO switch will get electrical energy after the seat switch circuit is closed (operator on seat) and then energize the Pink wire #805 at the X2 connector terminal. The PTO switch has to be in the off position so the Pink wire 805 going into the connector at the bottom of the PTO switch in dash connects (closes) the Pink wire with the two Purple wires directly next to the Pink wire. Those two Purple wires attached together at the PTO Switch connector have to get voltage before the  red PTO light (LED) will come on at the IM circuit board.

To eliminate the PTO switch as a problem at this point when the green LED lights up there will be a slight clicking noise coming from the bottom of the carburetor, that is the fuel shutoff solenoid that is energized by the #805 Pink wire in X2. The voltage travels to the PTO switch connector Pink. Test the voltage at the PTO connector switch and use a jumper to connect the Pink to the two Purple wires at the PTO Switch connector.