Measure the battery/system voltage (between battery positive and a ground) while the engine is running at maximum RPM and the PTO not engaged.Your battery voltage at this time should ideally be near 14 volts.
Then measure battery/system voltage with the PTO clutch engaged.
If the voltage drops down below 12 or so, you need to test the alternator and regulator, if those turn out OK, you need to check the clutch.
I am leaning toward this being a problem in the charging system because usually if a clutch itself has a problem, it either will not engage at all, or it begins to blow fuses.
Some clutches do have adjustable air gap. On those, wear can increase the disengaged air gap to a point where the clutch is reluctant to engage, but if it engages, it usually stays engaged until you disengage it.
The problem you describe sounds more like a low charging system output.

Thanks for quick reply. Well I did what you said. Started it up n got 14v on the battery n 14v when the pto was engaged at the connector going to clutch.

Do you think I just need to replace the clutch? Last week I adjusted the gap a 1/4 turn on each nut. Thanks

It is hard to say offhand that you should try a new clutch, those are pretty expensive to be using in "guess work".
Because your clutch is of the adjustable type genre, I suggest you get a copy of the Warner PTO clutch service guide (free at the Warner website) and do the electrical tests to determine if the clutch may be faulty (open or short).
Trying to ID an intermittent open would be the most difficult problem, as those generally are affected by vibration in operation.
If your Cub uses a PTO relay, try a new relay before arbitrarily replacing the clutch.
If your Cub does not have a PTO relay, but instead relies on a simple PTO switch to carry the PTO current load......and the machine has some hours on it, the switch may have developed high resistance causing enough voltage drop across the clutch that it loses its magnetism when it heats up (low voltage = high heat production in coil).

Here's a link to the guide-
It may not be your specific clutch, but it should still give you an idea-

Here is a link that might be useful: Clutch troubleshooting

OP - Pls provide the full model number found on the machine ID decal located in the cavity under the tilting seat..

Thanks for replies. I did also replace the PTO switch since it was only $10. The full model number is 1515 13A-201F100.

Ok.....this is strange. I disconnected the seat contacts. They must be in series with the reverse because the pto stays engaged in reverse n when no one is on the seat.

The strange thing is if I engage the pto when idle it will stay engaged. Same for going in reverse. I cut my lawn in reverse and not once did it disengage. The really strange thing is if I ran it in reverse 20 seconds than I went forward it would stay engaged forward for 20 seconds.

My neighbors must have thought I was nuts because I was going forward n reverse the entire time I mowed the lawn. Lol.

Any thoughts?

You might have a fault in the RMC module (electronic box that controls all the "safety parameters").
You can get to a free online service manual that might help you diagnose that component.
It seems (to me) that everything is working fine so long as you have selected "Mow in reverse", but something is happening that causes the RMC to shut off the PTO when mowing forward.

Can I just replace the rmc module?

I think I maybe ready to throw in the towel and send it out to be fixed. I hate giving up but the grass is growing so fast this time of year and work is crazy.

Thanks for your help everyone.

OP - you can take that full model number and the serial number for you tractor, go to the link below, enter that info... and look to download the section titled "understanding the PTO switch". Lotsa good info there on how that whole system ticks... whether you fix it or you have someone else fix it - you'll be more in the know.

Here is a link that might be useful: archive

Well I took it in and they pulled the clutch n cleaned it up. Charged me $75. I mowed today n the clutch held better than before but a few times if I took my foot of the gas pedal n slow down it disengaged. Ran it for about half hour and as long as I went at a normal speed it was fine but slow down around corner n had to re engage.

Think it might be a short somewhere?

Adjust the spacing of clutch

***" I took my foot of the gas pedal n slow down it disengaged."***
Really, the GAS PEDAL? Your tractor has a GAS PEDAL that slows down the engine? REALLY?
I hope you are actually referring to the hydrostatic transmission control pedal.
If it is the hydrostatic drive pedal you are referring to, then the problem might be a maladjusted reverse travel sensing switch.
Have you tried the reverse mowing switch "trick" since you got the tractor back from the shop?

And what trick might that be?

Your trick that you described earlier (quote)

***"The strange thing is if I engage the pto when idle it will stay engaged. Same for going in reverse. I cut my lawn in reverse and not once did it disengage."***

Are you enabling the mow in reverse mode on the key switch console when you do this?

The ignition switch is 4 position. On/off/ lights on/ start. Don't have a mow in reverse mode. There are 2 contacts under seat that controls mow in reverse.

Hogan - your machine has safety interlocks that are designed to shut off the PTO automatically if the tractor is moved in reverse... and there is no override feature on that machine.
Also, your foot pedal that controls forward/reverse motion is like a rocker - and if you even MOMENTARILY rock that pedal back into it's reverse function (or if it bounces into same when you quickly pull your foot off), the PTO will cut off... and you will have to push in and re-pull out the PTO switch to get the deck blades going again.
Try paying attention to how you are treating that foot pedal while operating the tractor and see if it helps.

As to defeating that safety feature, I for one will not provide any advice/instructions on how to do so. This is a public forum and there are too many legal sharks about, imho.

Well that's the funny thing.....it does mow in reverse now. Never did.

dunno what sort of relationship you have with the shop that popped you for 75 bucks to clean/adjust your non-adjustable PTO, but would be wondering just they really did do - mebbe play with the PTO reverse kickout switch that operates off the forward/reverse foot pedal linkage....?
Anyhow, if PTO in reverse started only after they worked on it, would take it back and tell them to make it right - @ their nickel.
JMHO.. Like I said before - safety issue. Would hold their feet to the fire.

Well he removed the clutch and adjusted the spacing. I know he hammered me. Said charges $25/hr. I know he didn't spend 3 hrs on it.

in digging deeper, turns out that tractor IS equipped with an adjustable PTO clutch, which changes the current diagnosis - either that clutch's bearing's are freezing up, disallowing normal disengagement or the air gap adjustment was done incorrectly, causing the the pto clutch to be engaged nearly fulltime.(See Bill Kapaun's earlier link for PTO adjustment details.) Or someone has been playing with the safety circuits... ?
Can only recommend you take machine to qualified CC shop... or mebbe another poster can help. Good luck.