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Discussion Starter #1
Long time lurker first time poster! I just bought an 86 420 with a 60" deck and it ran and worked fine when I purchased it. I had to take the deck off so I could transport it home. Once I got it home I had to take the seat pan off so I could make modifications to slide the seat back. I put it all back together and hung the deck. It started but when I hit the pto switch and the 20 amp fuse blew. After doing that 5/6 times it wouldn't start with the key switch. The seat switch has been bypassed by the PO. Now, if I hot wire the pto from the battery, it works. I hot wire the fuel pump on the ignition plug, it works. I hot wire the starter on the ignition plug, it runs. The light on the dash won't work ( I changed it also). Nothing works with the switches. Only the kill switch. What's wrong with it?
 

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Any possibility that you pinched a wire somewhere while you did your work on it that could have caused some insulation to come off and that it is touching the chassis and shorting somewhere?
 

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I'm thinking Mark is correct. If you pinched the seat switch wiring and created a dead short, this will give you want you've got! Also, attempting to start with a short may have damaged the circuit breaker and ignition switch! Grab a meter and start checking. Bob
 

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Discussion Starter #4
I've taken the seat pan back off since then and moved the wiring around. If it was the circuit breaker, then why would I be able to hotwire it just by using a jumper wire from the power side of the ignition plug to any of the other spots on the same plug? I've even tried a different ignition switch.
 

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Bob is not saying that a blown circuit breaker is causing the problem...he is saying that by continuing attempts to start it with a short could cause the circuit breaker to become damaged...also the power side of the ignition switch is before the circuit breaker...breaker has nothing to do with your original problem...just could be a result of attempts to start with a dead short
 

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Did you check 20 amp fuse? This supplies power to your oil light. If no oil light, bad fuse, ignition switch, or circuit breaker. Do you have a wiring schematic? Bob
 

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Discussion Starter #7
I do have a schematic. And the 20 amp fuse is the one I've changed over and over.
 

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If you have to change a fuse more than once, it indicates there is a problem with the wiring or something that the wires connect. Since it might be an intermittent, this can be very difficult to find. I would use an Ohm Meter on the output side of the fuse and ground, but 12V/20A equals 0.6 Ohm, which is VERY hard to measure accurately.

If I understand what you wrote, the PTO works correctly and doesn't burn up when you apply 12V to it directly. Same for the fuel pump (or is the solenoid for the carb) and the starter.

That leaves the wires, the switches and the control module. Any one or more of them could be broken. Since it was sort of working before, there are likely more than one issue. You will need to follow the troubleshooting steps in the Service Manual. If you don't have one of those, you can follow the electric circuit diagram you do have. It is possible the key switch is no longer working, but I would expect it to just be one of the problems you find.

I have had an ignition switch go bad before. I used a bolt meter to measure the voltage on the various terminals. There is typically a 12V in terminal, a switched power out, a start, and a accessories/lights one. Most JD tractors have a control module for all the safety switches. The engine won't start if the safety switches say it is not safe to start, so you need to check all of them, seat, PTO, parking brake as I recall. The schematic should show you how they are wired. Make sure each switch is working correctly. If the key switch and all the safety switches are working correctly, and you can see the signals correctly at the module, that leaves the module at fault. If you are not used to work on electrical stuff, it can be difficult to solve these kinds of puzzles.
 

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Discussion Starter #9
The PO added an electric fuel pump. It's also been repowered with a 20 hp v-twin kohler. So no telling how bad the wiring's messed up. But it worked perfect when I bought it.
 

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Discussion Starter #10
He also couldn't figure out how to connect the kill wire to the ignition switch. So there you go! 😕
 

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You've got a dead short "someplace", and now you need to find it! The 20 amp fuse goes to TDCM, then pto switch, then headlight switch.Disconnect plug from pto switch and light switch. Turn key on. Does fuse blow? If so, disconnect 8 pin connector from TDCM and try again. If fuse still blows with plugs disconnected, one of the wires are shorted!

Another check with both wires disconnected from light switch is to verify light switch is "Off". If switch happens to be "On", turn "Off", reconnect plugs and see if fuse blows. If it doesn't, problem is in wiring probably going to rear tail lights.

Check stuff I listed and report back, Bob
 

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Mark, possible but not probable! I'm thinking tail light wiring, but can't be sure until checked. PTO switch is another likely candidate! Bob
 

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Discussion Starter #14
Well I finally found my fluke. I tested the PTO switch and the resistance is way too high on four of the pens when the switch is in one position. So I'm going to say the switch is bad and hopefully that's the problem.
 

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The bad switch could be causing the not starting issue. But, that won't blow the fuse. You need to find the short or the new switch could end up like the other one.
 

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Discussion Starter #16
• If the fuse blows after PTO switch
is moved to ON position, the short
is either at the PTO clutch or blue
wires running between the PTO
switch, clutch and lamp. The short
is NOT in or before the TDC
module.

That's what was happening initialy. I ran a continuity test on those wires but without the switch. It tested good, 0.00 ohms.
 

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Continuity is not the check you want for a short, unless you are checking between the wire and ground. If you did the latter test and you got 0 ohms, you found your short (unless you are measuring the ground wire).

So, did you fix the problem or not?
 

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Discussion Starter #18
No just messing around with it until I get a new pto switch. The first part of my last post is copied and pasted from the manual
 
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