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Hi all,

I'm currently having an an issue on a JD 420 where the tractor will either show no response at all to the ignition key being turned, make a single click, or start after making atypical grinding noise and run until the parking brake is released ( so 10 seconds or so ). The tractor is refurbished and has a non stock onan t260 from a JD 930 and this issue only started after performing an oil change. The battery voltage seems fine and its been on a trickle charger and the tractor is more or less free of debris.The safety switches on the seat and transmission lever seem to be mechanically working but i should probably check them with a multimeter. Does anyone have any guidance as to how to troubleshoot this issue or have experience with a similar problem? Also does anyone know the location of the brake safety switch? Thanks in advance
 

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You are having several issues. One of which is related to the seat switch or its wiring.

The second is probably a poor connection somewhere. Either where the PO spliced wires, or the ignition switch are the most common causes.
 
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The key switches on these tractors can suffer from age and corrosion, particularly if stored out in the weather. Just replace it now if it is the original that came with the machine several decades ago... The power to the starter solenoid goes through the safety switches as you noted but not the seat switch (the seat switch is only involved in ignition cutoff if the operator leaves the seat when the tractor is in motion or the PTO is ON.) The cranking interlock safety switches are the neutral switch, the parking brake switch and the PTO switch(es)...if your tractor has no rear PTO there will be a bypass jumper at X5. Note that the wiring excerpt below shows that the power to the solenoid passes through he key switch TWICE...so it needs to be free of corrosion and contact wear.
Text Circuit component Passive circuit component Diagram Line


All of the starting symptoms you mentioned above indicate that the solenoid is not getting full voltage and current, so again the key switch is the prime suspect, but maybe there is a loose connection associated with your recent oil servicing activities.

The ignition cutoff is controlled by the TDCM, and the seat switch DOES figure into this when the tractor is moved out of neutral or the parking brake is released. Check the F2 fuse as it must be intact and the fuseholder making reliable contact if the seat switch is to be properly sensed by the TDCM. The wiring excerpt for this is shown below:
Text Technical drawing Diagram Drawing Line


If you had the fender deck off the tractor when doing the lube service, you may have pinched the seat switch harness and grounded a lead.

What is subtle in the wiring diagram above is that the connection at X23 pin 7 is not shown...it comes from the S2 terminal on the key switch on the first diagram and provides a bypass for the seat switch when the tractor is in the conditions needed to crank the engine...as soon as you release the brake or turn on a PTO the ignition will be killed if the seat switch does not sense operator presence.

The location of the brake safety switch is low on the tractor frame and actuated by the brake lever travel -- these parking brake mounting brackets have been known to get bent/broken if the brakes are not kept in reasonable adjustment the the brake pedal travel becomes excessive. This excessive brake wear and related pedal travel is usually due to over using the brakes to slow the tractor regularly rather than using the hydro ground speed lever as recommended in the operator's manual.

Let us know what you find on your tractor...

Chuck
 

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Be sure to inspect and clean the mating harness connections to the key switch -- if the switch contacts were getting to be a high resistance from internal wear/corrosion, the switch will have heated up and may have caused the spring contacts to become loose so check for that as well. A clean, tight fit to the new switch is needed...

Chuck
 

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Glad the new switch fixed it!!

That black wire is #130 on the schematic and the spade connection is X25. This wire and connector comprise the return circuit to ground for the rear PTO dash indicator lamp (which is not fitted to your tractor.) It does not need to be connected to anything, but if you are compulsive about such things you might sleeve it with clear tubing so it does not ground out any adjacent connections -- you can even put a note on it to remind you what it could be used for if you ever decide to add the rear PTO as would be needed for rear powered attachments.

Chuck
 
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