I ran into the same problem before christmas. The engine would lug to the point of stalling in fwd or rev with auto-throttle and stall-guard active. Going to full throttle and putting the HST pedal to the floor would get past the stall point and it would drive me back to the shop.
I was near my next major service interval and I went to the dealer to pick up parts and fluids. I talked with the service manager - he said they were booked solid for the next 2 weeks, but after describing the issue he said the issue sounded electrical to him. On the models with the drive-by-wire servo actuated HST, he said the only failures he's seen are with the actual fwd/rev servo mechanisms. He said it typically happens around snow clearing season, especially with guys who operate along salt covered roads.
I told him I've been operating a lot in the rain and deep mud recently.
He told me that if there was a hydraulic pump issue I would be hearing bad noises and the engine wouldn't stall. The likely issue was that both fwd and rev hydraulic circuits were trying to be active at the same time and acting as an engine brake. If my pump was bad oil would simply blow by. He said these hydraulic pumps on the M59 are robustly built and he's never seen one fail.
He suggested checking and cleaning all the electrical contacts to start with, sometimes salt gets in and corrodes the contacts. At worst he said I may need to change out the servos, they had them in stock and they were only a couple hundred bucks. He gave me the parts diagrams and suggested I give it a try before booking a service call.
I bought some contact cleaner spray and cleaned the harness contacts from the servos and to the HST pedal. On the M59 the servo harness clips are a SOB to get at unless you have alien long skinny hands and fingers. I could only get at the top one off with a set of 18" forceps and a straw on the contact cleaner spray.
I let everything dry thoroughly, re-assembled and am driving around without any problems for over a month now.
Last edited:
