By K Mcgiver (Kmcgiver) on Wednesday, August 29, 2001 - 09:13 pm:

Hydro's
Harry would be proud of this post. I have started to do this several times but have been pulled away for one reason or another. Sooooo here it goes......As discussed many times on here, the hydro linkage is the culprit of erratic hydro operation. When those possibilities have been exhausted, it is time to go internal and find the true cause of the issue. Before the first bolt is removed from the hydro unit, one must bear in mind this statement "you must work clean". Any debris at all inside the hydrostat could cause severe damage. Many times people feel that erratic operation is caused by the charge pump, no pressure, etc. The charge pump merely supplies fluid for the hydrostat operation. (Unless equipped with hydraulic lift or power steering.) Excessive charge pump pressure can caused erratic operation, we will cover that a little later. Usually the condition is herky, jerky operation or sudden movement of the unit and the hydro control lever has not been touched. The first possible cause of these conditions would be if the roll pin(ref.52) have moved out of their correct position. This can cause slight movement between the trunion shaft and the swash plate. (I have seen this several times.) As most of you know, I cannot stress enough how careful you must be when you remove a trunion shaft. We all know what happens if one of the roll pins in the trunion shaft is driven in too far. This is where a service manual is a must. I have another way to remove the roll pins that is less tedious. For those of you who would like to know this process, you can e-mail me, but I am not responsible for boo boo's and oh shi***'s. The next culprit could be that the rotating pump block(ref.16) and pistons are worn from use and sometimes the pistons slightly hang in their bores. In this case, the only remedy is to replace the pump block/piston assembly...big$$$$$$. The last culprit is related to the charge pump. I'm sure many of you have noticed that when the deck is fully raised or lowered, the tractor may begin to move as you're fully extending the hydraulic cylinder. This is caused from the center section(ref.27) and/or the input shaft(ref.8) being worn in the area that is marked red. What happens is the implement relief pressure fluid is being shot directly into the pump block causing slight movement. Again, this is huge$$$$. Personally, I have learned to live with it. This issue, can be controlled, but not cured completely like many diseases. The charge pressure can be lowered all the way to 90 psi. The way you do this is....there are 2-1/8" pipe plug in the top of the hydrostat. Remove the rear one and install a hydraulic gauge and run the unit until the fluid reaches operating temperature. Remove the charge pressure relief cap(ref.34) found on top of the hydrostat. Remove shims(ref.36) as necessary to obtain the desired pressure. It may also be necessary to grind a bit off the relief spring(ref.33). This will limit the amount of pressure leaking into the rotating pump group when the tractor is running and no hydraulic implements are being used. If your tractor has hydraulic lift but no power steering, you can stretch the limit by reducing the implement relief pressure all the way down to 300psi. The implement relief valve is found on the right hand side of the center section about halfway down. It looks identical to the charge pump relief valve, except for the fact, it has a stronger spring. You adjust this by stroking the hydraulic cylinder fully and allowing the implement relief valve to operate. Hold the lever so that the valve is continually relieving. Note the pressure on the gauge. Remove shims or trim spring as necessary. This will stop implement relief pressure from being dumped into the rotating pump block. There are other possible causes, but these are the most common that I have seen. Moral to the story...If you can live with it, don't worry about it.

Here's another graphic from McGiver - the proper dimensions for the trunion slot:

Click here for even more hydro information...

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