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Discussion Starter · #1 ·
Have a John Deere 420 - 1983. Seems that the front PTO coil is getting excessively hot. One burned up in June & I replaced it. Now the new one is also getting hot. The unit runs for about a half hour before the smell becomes noticeable. I have checked and rechecked the gap on the friction plate so many times that the feeler gauge us wearing out. The original Onan engine was replaced with a Brigs about 10 years ago, and the shafts were not the same. The Briggs supplier provided an adapter sleeve and modified key to make it comparable. However, the Briggs does not have a shoulder cut on the shaft it act as a stop for the armature., I have installed bushings to keep the armature off the coil, however, the hot odor persists. At wits end, what could be causing this excessive heat?? At $600 each, not enthusiastic about buying another one soon.
 

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Skeeter, Check air gap and then voltage to coil.

Not sure how/where you are checking air gap. The instructions for my 314 state to check gap under the adjusting nits with pto engaged. This will be fine if everything is what/where it should be, but the lack of the step on the crankshaft may (??) be mis-positioning the rotor. Check gap between rotor & armature through slots. The armature not be fully disengaging and dragging on the rotor creating friction/HEAT!

Remove wire/connector from pto coil. Check voltage to ground with engine running and throttle set where you normally run...1/2, 3/4, WOT, where YOU normally run! V Regulator could be putting out to much and overheating coil.

One last thought, do you have specs for replacement pto? Verify voltage spec IS 12VDC. Best of luck, Bob
 
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This may or may not be your problem, but something to check. I did a Briggs repower on a 420 last year. I had been instructed to grind off the locating nubs on the back of the coil that would be used to center the coil in the counterbore of the Onan block. The Briggs block does not have this counterbore and the nubs will contact the Briggs block face (end plate) when the 4 coil mounting bolts are tightened against the tapped bosses. This can bend or possibly mushroom the coil "cup" allowing it to just touch the inside of the rotor. Metal to metal contact, even just so slightly will make friction to heat the coil potting and cause it to melt, smoke, and give that give off that stomach upsetting oder.

If you have the old coil, see if the nubs were removed from it. Look at the inside of the rotor to see if it has a shiny spot, especially at the bottom radius. Check the new coil for missing paint and burrs at the top of the "cup". If so, that confirms rotor to coil "cup" contact.

I decided to leave the nubs on the coil base and use .125" spacers between the bolt bosses and the coil base. That allowed the nubs to have about .015" clearance to the block. With the key removed, the rotor turned freely, or so I thought. Replaced the key, assembled the rest of the clutch, and gave it a try. Great, for about 30 seconds. I saw smoke, shut it off, and found the clutch was too hot to touch. After it cooled, I disassembled the clutch, and started looking for what stupid thing I missed. Finally came to me that the rotor was touching because of the spacers under the coil base. What I missed was the rotor needed to be spaced out at least .125" to allow for the coil spacers. Duh! Once the key was installed and the crank bolt was tightened, the rotor did rub, ever so slightly. So I made a spacer to slip over the adapter sleeve and bottom on the small flange on the sleeve allowing for the radius at the flange. I made it about .135" thick and same O.D as the flange. Made dang sure the rotor was not touching before engaging the clutch again. Problem solved. The coil did not get ruined, but it took a couple hours for my stomach to settle down. It probably did lose some of it's useful life though. Should have done what the kit provider told me to do in the first place. :bash:

tommyhawk
 

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Discussion Starter · #8 ·
Tommyhawk:

Thanks so much for the tip and suggested cure. Absolutely miserable weather day here, so not in the mood to tear this thing totally apart, but I can see where the back plate of the coil is out from the mounting plate in the Briggs block, and it is bowed out ever so slightly, so as soon as weather improves, I’m tearing that sucker apart and doing as you suggested - think the problem is solved. You’re right, the smell is stomach turning, especially when you can $600 bills flying out the window. At least your engine provider gave you instructions, more than I got, but that was 11 years ago. Thanks again - Skeeter
 
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