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post #1 of 17 Old 09-06-2019, 03:27 PM Thread Starter
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420 Front PTO engagement issues

I was mowing a couple of days ago and noticed the battery light had come on. A little bit later the front PTO disengaged an would not reengage. The PTO engaged light lit when the switch was in the up position, but the PTO itself would not actually engage. The battery light only comes on when the PTO switch is engaged. Based on looking through different posts here, I have a feeler gage set and a multimeter on the way. Once the air gap is correct, if the PTO won't engage where should I start taking readings and what should I be looking for? Based on the symptoms, it seems likely it is electrical and not the air gap. Although I haven't taken readings on the battery yet, it seems fine. The tractor starts just fine. Thanks for any help.
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post #2 of 17 Old 09-06-2019, 04:41 PM
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Re: 420 Front PTO engagement issues

My first suggestion is to make sure you battery is fully charged, then try it. Lot's of times a low battery will cause it not to engage. Next is to check the Voltage regulator, while engine is running you should see about 13.6 - 14.7 VDC at the battery. The PTO circuit pulls a lot of load when turning on so if those are both good you should be set.

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post #3 of 17 Old 09-06-2019, 11:36 PM
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Re: 420 Front PTO engagement issues

It sounds like your VR is going bad. Do like PA said and check the voltage coming from the VR to the positive battery terminal with the engine running. It should be over 13 v. Also, a bad battery could cause it if it will not take a charge. I would check the VR voltage first then put a load meter on the battery to see if it is good. Just checking the battery voltage is not enough to tell.
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post #4 of 17 Old 09-16-2019, 11:22 PM Thread Starter
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Re: 420 Front PTO engagement issues

Thank you for the replies.

I've set the air gap correctly at .018" all around and still have issues. The PTO will engage when I first start the tractor, but then not again after it has run for a while and shut off then attempted to be re-engaged. If I either tap the clutch cover or give the adjustment nuts a slight turn (less than 1/8 turn) the clutch will engage. I'll check air gap clearance tomorrow after it cools down and see what it is cold with that slight turn

The battery light stays lit while the PTO is engaged but clears as soon as it is disengaged. At full throttle with the PTO disengaged I am getting 13.7 to 13.8 VDC at the battery. With the PTO engaged it drops to 12.5 at the battery. Readings on the input to the VR are around 57VAC from the stator and output from the VR is 13.7VDC at full throttle. I have not load tested the battery.

I did disassemble the clutch and everything looks clean. Field winding coil looks good. When I put a wrench across the field winding and flip the PTO switch with key in run a wrench sticks, but doesn't need a whole lot of effort to take off. I want to check the resistance and see if there is an internal ground on the coil, but I'm not sure where to measure.

If anyone has thoughts I would appreciate them.
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post #5 of 17 Old 09-17-2019, 07:59 AM
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Re: 420 Front PTO engagement issues

I hate to say it, but it sounds like you need a new clutch coil - and likely just an entire new clutch assembly.

I suspect your coil is going bad. This is causing two things to happen:

1. It has marginally enough strength to pull in the clutch. When cold, it has slightly more power (because the coil windings are cold, they conduct just slightly better and when it's hot, you are just below the threshhold of power to pull in the clutch.)

2. The coil is pulling more current than it should. This is pulling down the system voltage - which is why your battery light comes on.

If you have a multimeter that can read resistance, some one on here can probably tell you how many ohms the coil should read. With an ammeter, you could also read current, but you'll need to mess with some wiring to get your meter in series.

I say that you might need a need clutch assembly because if the coil is weak, it may also be allowing the clutch to slip while in use. If that's been happening, then the clutch face might be pretty worn. You can inspect that if/when you pull it off the tractor.
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post #6 of 17 Old 09-17-2019, 10:05 AM
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Re: 420 Front PTO engagement issues

I believe pto coil should be 3.7 to 4 ohms. Disconnect plug going to coil. Check wire to wire (if 2 wires) or wire to pto case (if single wire. Bob
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post #7 of 17 Old 09-17-2019, 10:56 AM
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Re: 420 Front PTO engagement issues

I've got an electric GEM car and I have had issues with batteries that check good by voltage but fail under load. Before I spent the big bucks on a new coil I would slap the battery out of one of my autos into the 420 and give it a go. Remember the JD motto, "Always do the cheapest remedy first".
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post #8 of 17 Old 09-17-2019, 11:21 AM
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420 Front PTO engagement issues

Either your not getting enough charge or your using too much charge . It could be that as the coil heats up it is shorting inside using more current than alternators can provide


Unplug clutch and with engine off pto on measure restance of clutch cold and after it fails let us know




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Last edited by groomerz; 09-17-2019 at 11:27 AM.
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post #9 of 17 Old 09-18-2019, 12:22 PM Thread Starter
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Re: 420 Front PTO engagement issues

Thank you for all of the suggestions. I did try a car battery and got the same results so I think the issue must be in the coil itself. This is where I'm kind of stumped though. The coil itself looks pretty good. The insulation varnish is in good shape. No wear, cracks (other than some hairline surface cracks) or other indications of breakdown that would lead a decrease in resistance or ground. It looks pretty good compared to some of the pics I've seen on this site of failed coils. When I measure resistance wire to wire at the plug (this is a two wire model) I get 0.1 ohm resistance . Switching to the continuity "tone" mode on my multi meter gives me a solid tone which to me seems like an indication of a direct short. If that was the case, would I even be getting a magnetic field, and shouldn't I get a blown fuse as soon as I engage the PTO. On the other hand if I read resistance between either one of the leads and the case itself I get infinite resistance.
Is it possible that the two pin connector has an internal short or do I really have a coil that has failed so badly internally that it is shorting that badly?

Thanks for any more thoughts.
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post #10 of 17 Old 09-18-2019, 01:44 PM Thread Starter
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Re: 420 Front PTO engagement issues

If it is the field winding that needs to be replaced does anyone have thoughts on OEM field coil (~$320 - $350) vs the Xtreme replacement clutch assembly that is about $220?
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post #11 of 17 Old 09-19-2019, 02:54 AM
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Re: 420 Front PTO engagement issues

Mark, 0.1 ohm means coil is shot. I've never used the Xtreme products so can't comment...but have seen good recommendations here and on other sites.

Let me try to explain the readings you got and what's going on inside your coil. Your coil is a single piece of wire going in, winding around and around, and coming out. This wire has a VERY thin varnish insulation and expansion/contraction of the wire due to heating/cooling can crack the insulation. I'm not sure of the exact wire size in the coil, but I know it's small and will assume a 20 gauge wire. 20 gauge wire has a resistance of 10 ohms per 1000 feet and a good coil should have a resistance of 4.0 ohms, so it should have 400 feet of wire in the coil. Your getting 0.1 ohms resistance so the current is flowing in 10 feet, finding a crack in the insulation, jumping to the next wire, and coming back out to the plug...rather than traveling the entire 400 feet. It's still a single wire so your continuity "tone" will sound...unless the wire was burnt in two, but then you would have 0.0 ohms! Infinite resistance from either wire to case simply means the crack in the insulation connects wire to wire and not wire to case.

I hope this helps you understand, Bob
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post #12 of 17 Old 09-19-2019, 05:55 AM
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420 Front PTO engagement issues

I used extreme clutches anseem ok they have replaceable bearing which is ok but not a big deal as if your bearings are worn the clutch / brake plate is usually worn too. If you want a clutch to last I always engage and disengage at half throttle. Less wear and tear. Commercial guys usually wide open.
Here is some good info
https://ogura-clutch.com/downloads/P..._web.pdf<br />

It sounds like your coil is shorted. If you need Another test you could do “amp draw test”
Remove fuse You might need some alligator test clips or leads Set amp meter to dc amps 10 amp range. Make sure you move pos lead to 10 amp socket on meter. Insert leads where fuse was and you will read amperage and pto on Can be anywhere 4-6 dc amp normally depending on clutch

When your done remember to move red lead back to volts ohms socket on meter or you will blow meter fuse if you check voltage with plug in amps socket We have all done it so you have been told


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Last edited by groomerz; 09-19-2019 at 06:10 AM.
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post #13 of 17 Old 09-19-2019, 06:05 AM
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Re: 420 Front PTO engagement issues

After looking at the pic of winding it looks like it has burn mark on it from short coulbe a reflection. Plug it in as is and turn coil on I bet it gets hotter on that one side that is discolored

Btw that amp draw test can be done with just plugging coil in it doesn’t have to be reassembled or engine running


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post #14 of 17 Old 09-19-2019, 06:15 AM
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Re: 420 Front PTO engagement issues

Bob great explanation of coil. I like to correct you if “wire is burnt in two “ it is open and infinite reading


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post #15 of 17 Old 09-19-2019, 07:23 AM Thread Starter
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Re: 420 Front PTO engagement issues

Thanks all. New clutch assy ordered and on the way.
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