My Tractor Forum banner

1 - 20 of 108 Posts

·
Registered
Joined
·
55 Posts
Discussion Starter · #1 ·
Hey -- I just replaced the PTO clutch on this 94 STX38 with an 8TEN aftermarket unit but now it won't engage when connected to the tractor. Here's what I tested:

-Ran a jumper to straight to the battery leads - The new 8TEN replacement PTO clutch clicks on at battery voltage about 12.5v.
-Tested the blue wire at the PTO switch - it has battery voltage.
-Tested the connector coming from the harness with the PTO switch 'on' and the key on 'Run' - it has battery voltage at the tractor connector but the PTO doesn't engage. If it has power, the PTO should engage, right?
-Tested continuity of the 8TEN unit. It has about 2.8ohms. Which is actually low - the JD tech manual says it's supposed to have 3 - 10 ohms

Still, no action on the PTO when the mower is running and the unit is connected to the connector from the harness. The connector on this 8TEN unit is garbage and fell apart, so I had to bypass it and connect the pins directly to the tractor connector. The 8TEN install guide says it needs minimum 13.5V to engage, which is close to what the mower is producing when running. But it clicked on when hard-wired to the battery at battery voltage of +/- 12.5v, so I don't get why it's not clicking on when it gets the same voltage from the tractor through the connector or when it gets the 13.5v that the tractor produces when running.

The air gap on the new unit is supposed to be factory-set.

The deck is on the tractor and the belts are on the PTO - even if it weren't that really shouldn't make a difference far as I know. I'm slapping myself for not doing all this testing BEFORE putting on the deck.

I'm stumped. If the problem were the low (2.8 ohm) resistance across the 8TEN PTO connectors, then it wouldn't click on when jumped from the battery. I hope I didn't cross poles from the 8TEN PTO to the JD connector when the 8TEN connectors came out of the connector housing. If I did, what damage could that cause?

Any help appreciated...grass is getting tall with every day that old tractor sits in the garage with no PTO...

Van
 

·
Administrator
Joined
·
21,520 Posts
Just a thought, doesn't a wire come off the PTO and go to an engine mount bolt for a ground?
 

·
Registered
Joined
·
55 Posts
Discussion Starter · #4 ·
Just a thought, doesn't a wire come off the PTO and go to an engine mount bolt for a ground?
There's only the connector, no other ground wire. It's basically a plug-and-play install, just that one connector.

You have to start somewhere...double check that gap
That's reasonable advice. Still, I'm not sure I get how the gap relates to the power issue and why the PTO engages when fed 12.5v direct from the battery. If the gap were off, wouldn't it just not fire up even when jumped from the battery?

I was reading around and came across this: STX38 Electrical Issue

That user found that the PTO requires +14v to engage, and that the stator/rectifier doesn't kick in until the battery voltage drops below a certain threshold. So the PTO will not engage immediately after starting the engine. The engine has to run a while to draw down the battery before the rectifier starts cranking juice back into the system. Once that happens and the voltage reaches the +14v required for the PTO to engage, then you can hit the switch to engage it. If you hit the switch before the rectifier has cranked the juice to +14v, you get nothing.

This would actually be in line with what I've experienced with that tractor, and it would also be inline with what the 8TEN manual says about requiring +14v to engage. There have been times in the past when I engaged the PTO shortly after starting the engine, and got nothing. I assumed it was because the battery wasn't sufficiently charged, so I put it on the Battery Tender overnight to top it off and tried it the next day, and the PTO engaged. I'm thinking maybe it the reason it didn't engage was not because the battery voltage was low and needed to be topped off, but because I hadn't waited long enough for the rectifier to start juicing.

I'm going to try this out tomorrow, i.e. let it run for a while and test the system voltage then try the PTO when it reaches +14v. I can't explain why the new PTO engaged when jumped directly from the battery at 12.5 v but doesn't engage when receiving 12.5v from the harness connector. Maybe just the act of starting the tractor reduced the voltage below the PTO's engage threshold...

That tractor has been around for over thirty years and I didn't see any mention of this behavior in the tech manual ... can't anyone confirm this from first-hand experience?
 

·
Super Moderator
Joined
·
9,590 Posts
I have had a bunch of these machines and never had this experience..there is another member, @Whirly who has had more of them than I and is quite knowledgeable...I hope he has an answer
 

·
Registered
Joined
·
55 Posts
Discussion Starter · #6 ·
I have had a bunch of these machines and never had this experience..there is another member, @Whirly who has had more of them than I and is quite knowledgeable...I hope he has an answer
OK, thanks. Maybe I misread that post - but the 8TEN doc is clear about the voltage requirement being greater than what even a fully charged 12V battery would produce. It says:

"With the PTO switch in the ON position, the voltage on the clutch should be between 13.2 and 14 VOC when a voltage meter is connected to the connection coming from the Mower. If the voltage was different, there is potentially a problem with the mower's electrical system. PTO ON = Voltage is 13.2 VDC minimum."

Maybe I'm just the first unlucky dude in history who has ever started a huge troubleshooting marathon just because he hit the PTO switch too soon after starting the engine.
 

·
Super Moderator
Joined
·
9,590 Posts
I am wondering if it could be just the switch...is it moving back and forth smoothly?...It is reasonably easy to get to ..I had one get jammed and was able to get it apart, unjam and lube it and put it back together
 

·
Registered
Joined
·
55 Posts
Discussion Starter · #9 ·
I am wondering if it could be just the switch...is it moving back and forth smoothly?...It is reasonably easy to get to ..I had one get jammed and was able to get it apart, unjam and lube it and put it back together
I think the switch is fine. It worked before I put in the new PTO clutch and I sprayed it with Deoxit. Mechanically it's smooth, plus I'm getting OFF = 0V and ON = battery voltage at the PTO connector, as expected.
 

·
Registered
Joined
·
55 Posts
Discussion Starter · #12 ·
Had the old PTO clutch failed?...why did you replace it
There was a loud rumbling noise coming from it when engaged and the blades weren't cutting the grass, but rather just kind of skimming over it. The belt was fine and I had the deck pulleys and bearings checked over last year. The JD dealer said the PTO was shot, and that matched all the descriptions of failing PTO bearings that I'd read about. When I pulled the old one, there was a scary amount of play in the bearings. The technician at the dealer said when these things seize up you can have a **** of a time getting them out, so I figured it's about time to replace. It was the original from 1994, so I figured for the $120 for the 8TEN unit, it was worth it to just go ahead and replace.
 

·
Super Moderator
Joined
·
9,590 Posts
How did you do getting the old one off?...they give a lot of people fits...if you hit a dead end...I know I have a new switch out in my shed...somewhere...I can run some continuity and resistance tests on it and give you te results and see if your switch has same readings...let me know...they are fantastic little tractors
 

·
Registered
Joined
·
55 Posts
Discussion Starter · #14 ·
How did you do getting the old one off?...they give a lot of people fits...if you hit a dead end...I know I have a new switch out in my shed...somewhere...I can run some continuity and resistance tests on it and give you te results and see if your switch has same readings...let me know...they are fantastic little tractors
That's a good question. I ride and work on older motorcycles and drive a 94 Ford Explorer, so when I hear something change in the way something runs, I stop using it immediately and find out what caused the change. When I heard the PTO start to rumble end of last season, I took it straight to the JD dealer and he said told me not to run the tractor until I fix the PTO. So I didn't. That was good advice because all I had to do was put an impact wrench on the bolt that holds it on and it came right off. Like I said, if I had gone on running it after the thing had completely failed and seized up.... probably a whole 'nuther story.

I'm going to try again today to get the voltage up to spec for that 8TEN unit. If that doesn't work, I'll definitely continue the saga here. But I am pretty sure that switch is OK. We'll see...

I agree, this is a great machine and hope to get many more years out of it. I can't stand the newer anything -- too many chips and boards and other garbage that just gets in the way.
 

·
Super Moderator
Joined
·
9,590 Posts
I have used that relay on a bunch of different tractors and it is like magic......however the clicking is an indication that you have some connections that could use a little cleaning up....which COULD also be contributing to the PTO loss of enough power...if you need any help with the relay ...just holler....it takes literally 10 minutes to do it
 

·
Administrator
Joined
·
21,520 Posts

·
Premium Member
Joined
·
5,660 Posts
Yeah, I would suggest disconnecting/cleaning/reconnection all the main cables between the battery/frame/starter/starter solenoid (if present)...
 

·
Registered
Joined
·
55 Posts
Discussion Starter · #19 ·
Yeah, I would suggest disconnecting/cleaning/reconnection all the main cables between the battery/frame/starter/starter solenoid (if present)...
Thanks for the help folks - this tractor was inherited in not-so-optimal condition and spent some years in a backyard. I disassembled and cleaned the carb and it fired right up, and I've been using it ever since without much in the way of TLC, except replacing that stupid little grommet that prevents the steering column from flopping around. So a once-over on all the contacts and that relay upgrade is definitely long overdue. Just picked up a relay at NAPA and paid the local lawn guy to to the first mow while I get this all worked out.
 
1 - 20 of 108 Posts
Top