Small Engines (Lawn Mowers, etc.): Sears lawn tractor with torque loss, shaft drive, hydrostatic transmission

Question
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Followup To
Question -
I have a lawn tractor that is 6 years old now.  Today while mowing, I noticed that I have suddenly lost power going up any type of hill.  The engine is running fine, the blades are spinning fine, and when going on a flat area or down hill it is fine.  Just going up hills there is very little power.  I have a hydrostatic transmission with a Brigs 19.5 V engine.  Any ideas?  I went to the Sears site but they only list parts, no help with this.
Answer -
Hi Rick,

Does the engine slow down or does only the ground speed slow?  Does it happen the same, with the blades disengaged?

Michael

The engine and blades continue with no loss of power.  There is a straining sound from under the seat that grows as the tractor slows to a crawl and stops.  I then put it in reverse and back up and find a location that is not a steep.  Again, the engine stays at full speed.

Thanks for asking and appreciate any suggestions or ideas.

Answer
Rick,

Without a doubt, the problem is in the hydrostatic transmission.  You might luck out with a simple fix like a loose belt that drives the hydro. (Although most have a shaft drive.) Low hydro fluid or a clogged filter.  Or... an internal transmission problem.  Hydros are complicated and require special service not usually attempted at most repair shops.  They could send it out for repair or replace it.

Let me know how you make out.

Good Luck,
Michael