No idea about the seat switch (maybe a parts diagram for the mower will give you a hint how it comes apart). Maybe test what the voltage across the battery terminals are when the PTO is engaged (may be non-trivial to do), as it could be:
-bad battery (even if it's new, it can be bad)
-bad electric clutch (uses too much power)
-bad charging circuit (can't put out full power, perhaps has wrong parts on it, or parts have failed or installed incorrectly)

 

Anyone else?? Still stumped

 

I agree with Dave check PTO clutch it may be pulling to many amps and the charging system cant keep up.

 

Waht is the rated amp draw and how do I test it?

 

test the OHM resistance of the PTO coil to see if the coil could be defective

 

I did a resistance test on the clutch, it is 2.2 in the range it is supposed to be.
Did a load test on the battery, it only drops to 10.5 volts.
When running, I am getting 14.4 volts at the battery indicating the charging system is ok.

 

What's the voltage across the battery with it running and the pto engaged?

 

It dropped and held at 12.25 volts. When the pto was disengaged, it only went to 12.55 volts then

 

That kinda points to a problem with the charging system, in that it can't handle the load of the pto, and can't even recover afterwards with the pto off.

 

It might have a bad diode in the voltage regulator/rectifier or a Stator beginning to fail when hot, same with the PTO clutch magnet beginning to fail when it gets hot.
Check all of your electrical grounds and battery cables where they connect, make sure they are clean and tight.

 

Anyone know how to test the voltage regulator? I just took it off to check part numbers. It looks original. All connections look good so far.

 

watch some YouTube videos on testing

 

So I stopped out at my local Gravely Guru's (aka dealer) who are totally awesome to deal with. I had the regulator along with me. They said there is no way of testing it without it on the machine and running. They asked me a bunch of other questions about what I tested and did not test. They said it is the fuse holders. Got myself two holders, new fuses and a new seat switch all for $20. I put the new fuse holders on tonight. Tested it out after (just in the shop) and it seems to be working much better. Very little drop in voltage with PTO engaged (Went from 14.5 to 14.2) I will be testing it out tomorrow.

 

That sounds like your charging system is working again.

 

You can check the diodes with an Ohmmeter or a Diode tester, they are not hard to check with the regulator/rectifier unit on a bench.
To test the regulator part, it is easier to have it on the tractor and engine running to test the battery voltage.
They do make special tools to test the regulator while it is on the bench, but that tool is very expensive. It varies the input voltage to compare it to the 'Cut-Off' voltage to see if it is regulating and dumping off the excess voltage to ground or not.
That is good you got it fixed with the bad fuse holder, they do go bad and fail. Let's hope that was all it is, good luck with it.

 

The fuse holders were the problem. I ran it the other night and once in a while shut off the PTO and turned it back on. No problems. Did it again right before putting it away and it worked as it should. Its amazing how with those being sealed the way they are that they still go bad. Makes you wonder about all those style connections in cars and trucks that see way worse conditions than these mowers.