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I have been performing DIY small equipment repair and maintenance for many years, but have no experience with hydro-static drive transaxels until now. I recently picked up a Craftsman ZT7000 with a Kohler 20HP drive. The engine runs great, but I’m experiencing serious and uncontrollable lurching when trying to move forward or backwards. I have read many threads that have lead me to check for a missing/broken spring and to check the oil levels in the transmissions. I have modified both transmissions with 1/8” drains and drilled the tin to get access to the top oil fill plug, and drained and changed the oil with recommended 10W50 Diesel grade engine oil. I have adjusted the coil spring tension on the brake control shaft between the two transmissions to different tensions within the range of adjustment, but none of the work has corrected the lurching. The drive belt tension seems adequate so I don’t suspect drive belt slippage and the drive belt is new. Anyone have any direction for me before I condemn the transmissions?

Maybe try doing the air bleed procedure on the hydro units. Could be trapped air in the system.

ILENGINE: Your suggestion is interesting. Could you please give me a procedure to do this? Murray transmissions (left) 1727806SM and (right) 1727807SM. I dont find any openings or bleeders in the trans case besides the filler plug and the vent tubes on the top. Thanks for your input

This is the basic procedure no matter what brand or style of hydrostat unit. Looks like the wire style bypass rod coming off the top of the hydro units and then lock into a tab on the rear of the mower.. May be hidden under the rear of the mower. Looks like a pull and lock wire style bypass.

PURGING PROCEDURES

Due to the effects air has on efficiency in hydrostatic drive applications, it is critical that it is purged from the system. These purge procedures should be implemented any time a hydrostatic system has been opened to facilitate maintenance or any additional oil has been added to the system

.Air creates inefficiency because its compression and expansion rate is higher than that of the oil approved for use in hydrostatic drive systems .The resulting symptoms in hydrostatic systems may be:
1. Noisy operation.
2. Lack of power or drive after short term operation
3. High operation temperature and excessive expansion of oil

Before starting, make sure the transaxle/transmission is at the proper oil level. If it is not, fill to the specification.

The following procedures should be performed with the vehicle drive wheels off the ground, then repeated under normal operating conditions.
1 With the bypass valve open and the engine running, slowly move the directional control in both forward and reverse directions (5 or 6 times), as air is purged from the unit, the oil level will drop.
2. With the bypass valve closed and the engine running, slowly move the directional control in both forward and reverse directions (5 to 6 times). Check the oil level, and add oil as required after stopping the engine.
3. It may be necessary to repeat Steps 1 and 2 until all the air is completely purged from the system. When the transaxle moves forward and reverse at normal speed purging is complete.

CaptainL said:

drained and changed the oil with recommended 10W50 Diesel grade engine oil.

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Hmmm. Are sure about this. Most Hydro Gear hydrostatic transaxles uses 20W50 conventional engine oil. And 15W50 Synthetic can subbed on new units but with older units it is better to stick with the conventional oil.

IF you used 10W50 standard or synthetic that might be part of the problem as it is too thin as it would take longer to build to working pressure. The only time you might want to use 10W50 is in the extreme cold areas.

What ILENGINE is referring to is the release valve that you release to move the mower when you want to push it. Jack the mower up and place jack stands under it so it is secure. release the valves and start the mower and swipe the control arms forward and backward. I do it about 10 times. As to the lurching check the control brackets to make sure they are tight on the trunnion shaft. The bracket the control arms move on the hydros.

StarTech said:

Hmmm. Are sure about this. Most Hydro Gear hydrostatic transaxles uses 20W50 conventional engine oil. And 15W50 Synthetic can subbed on new units but with older units it is better to stick with the conventional oil.

IF you used 10W50 standard or synthetic that might be part of the problem as it is too thin as it would take longer to build to working pressure. The only time you might want to use 10W50 is in the extreme cold areas.

Click to expand...

Regarding the oil grade, I am sure of nothing. I just followed advice from another.

hlw49 said:

What ILENGINE is referring to is the release valve that you release to move the mower when you want to push it. Jack the mower up and place jack stands under it so it is secure. release the valves and start the mower and swipe the control arms forward and backward. I do it about 10 times. As to the lurching check the control brackets to make sure they are tight on the trunnion shaft. The bracket the control arms move.

Click to expand...

Thank you for the clarification.

I performed the purge procedure between rain showers today, but there has been no improvement. I inspected the linkage from the control levers to the transmission shifting pivot point and there doesn’t appear to be any lash or wear in the linkage. The drives seem to work smoothly while jacked up on stands both individually as well as together in both for and rev, but with the machine on the ground, it jerks and lurches forward and reverse.

You either still have air in there, not enough oil , a defective pump ( highly unlikely ) or a bad drive belt / tensioner or dragging brakes
Unless you pulled the hydros apart fully before you let loose with your Black & decker then I could guarantee you have swarf in the system so even if you get the lurching problem solved you would have wrecked the transmissions .
These things are made in a sterile dust free environment so why did you think it was OK to grab your drill & start making holes everywhere ?
A brain dead ego posting on Y-Boob ?

bertsmobile1 said:

You either still have air in there, not enough oil , a defective pump ( highly unlikely ) or a bad drive belt / tensioner or dragging brakes
Unless you pulled the hydros apart fully before you let loose with your Black & decker then I could guarantee you have swarf in the system so even if you get the lurching problem solved you would have wrecked the transmissions .
These things are made in a sterile dust free environment so why did you think it was OK to grab your drill & start making holes everywhere ?
A brain dead ego posting on Y-Boob ?

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Whew! Nice talk. Just to clarify, “..making holes everywhere” was actually one hole each in the bottom of the drives, to create a 1/8 NPT drain. The holes were drilled carefully with the cases full of oil still (as the only way to change the oil is to remove the drives and invert to drain through the vent hose or fill, which certainly would guarantee any sediment to flow all over the mechanism.) The oil in the case affectively flows the last bit of grinding out when the drill penetrated, helping to prevent what you suggest. Secondly, the lurching was a condition existing before I made this modification.
Its a nice concept about the drives being manufactured in a dust-free environment, but when they are in use in the field having a vent tube to the dustiest environment imaginable, its no wonder the oil gets to look so dirty. Coupled with the impossible serviceability, its simply planned obsolescence. Note how in later years these static drives were improved with external expansion/fill tanks displaying proper oil level and drain plugs encouraging maintenance oil changes.
Trying to navigate past your angry response, I had mentioned having adjusted the spring tension on the brake control shaft to different positions of tension with no change resulting. Can you or anyone offer a prescribed procedure for correctly adjusting the brakes or where to inspect for dragging brakes?

To do that I would need to know what transmission is actually in there
As you probably know if you repair mowers, no one will tell you what transmissions are fitted any more they just list them as hydros and all of the parts listings just have a part number
Even worse is I come up with 3 different units
One is a hydro Drive no model listed , one is a Murray ( so obviously rebranded from somewhere ) and the last is listed as a Craftsman wheel motor
The IPL's on parts tree ( who show the dives as Murray ) show what looks very much like an EZT with an external toothed parking brake .
And if that is the case then they can not drag it is either on or off .
If it is an internal disc then that is where all of the crud in the oil came from
Very rare to find a lot if dust from the air vent in hydro oil
They get hot and blow air out in use the when sitting still in their sheds get cool & suck air back in while sitting still, so little to no dust .

Rule of thumb when dealing with Cheap & nasty ZTRS ( and that is most of them ) with a Left & right combined pump & motor units is if it affects both drives then it is a problem with the engine , engine pulley or belt
If it affects one slightly more than the other then it is a slipping belt
If it affects one substantially more than the other then it is a drive problem

Now it would nice to had noted the problem existed prior to the work. When I Google ZT7000 I coming up with a Murray built unit (107.27772?) with Hydro Gear EZT (ZT-2200) hydros with has an external cog parking brake. The only adjustment is made to the brake is make sure it is clearing the cog when the parking brake is release.

Also depending how dirty the oil was when you drained it out the internal filter might be partially clogged. Also the OM last version Sear has post is from 2006. So those hydros probably have a lot of wear by now.

Getting to the filter requires disassembly of the unit as the center section has to be removed in order to change the filter. It is a nylon mesh type filter but still can clog especially if water has been present. While the center section you check wear on the mating surfaces for the pump motors.

bertsmobile1 said:

To do that I would need to know what transmission is actually in there
As you probably know if you repair mowers, no one will tell you what transmissions are fitted any more they just list them as hydros and all of the parts listings just have a part number
Even worse is I come up with 3 different units
One is a hydro Drive no model listed , one is a Murray ( so obviously rebranded from somewhere ) and the last is listed as a Craftsman wheel motor
The IPL's on parts tree ( who show the dives as Murray ) show what looks very much like an EZT with an external toothed parking brake .
And if that is the case then they can not drag it is either on or off .
If it is an internal disc then that is where all of the crud in the oil came from
Very rare to find a lot if dust from the air vent in hydro oil
They get hot and blow air out in use the when sitting still in their sheds get cool & suck air back in while sitting still, so little to no dust .

Rule of thumb when dealing with Cheap & nasty ZTRS ( and that is most of them ) with a Left & right combined pump & motor units is if it affects both drives then it is a problem with the engine , engine pulley or belt
If it affects one slightly more than the other then it is a slipping belt
If it affects one substantially more than the other then it is a drive problem

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I too have spent considerable time on-line to first identify and then troubleshoot the hydros and the problems. My machine appears to have the Murray hydros with the external gear cog. I have not found a number on either case, but have not removed these drives yet. My findings, based on my machine Model# 107-277720 with the 20HP Kohler engine, tells me these are the Murray hydros (Left) 1727806 and (right) 1727807. Non of the parts schematics have shown an external model number location for these hydros and if that’s available, I could go from there.
Thanks for your input.

CaptainL said:

I too have spent considerable time on-line to first identify and then troubleshoot the hydros and the problems. My machine appears to have the Murray hydros with the external gear cog. I have not found a number on either case, but have not removed these drives yet. My findings, based on my machine Model# 107-277720 with the 20HP Kohler engine, tells me these are the Murray hydros (Left) 1727806 and (right) 1727807. Non of the parts schematics have shown an external model number location for these hydros and if that’s available, I could go from there.
Thanks for your input.

Click to expand...

Also, your advice regarding both drives affected; that makes good sense the way you explain it. I will look into this further. Thank you.

CaptainL said:

I too have spent considerable time on-line to first identify and then troubleshoot the hydros and the problems. My machine appears to have the Murray hydros with the external gear cog. I have not found a number on either case, but have not removed these drives yet. My findings, based on my machine Model# 107-277720 with the 20HP Kohler engine, tells me these are the Murray hydros (Left) 1727806 and (right) 1727807. Non of the parts schematics have shown an external model number location for these hydros and if that’s available, I could go from there.
Thanks for your input.

Click to expand...

Usually the label is on the axle housing facing rearward. It will start with ZC or ZD. I would attach the generic IPL for these units but at 3.5 meg it is too big to upload.

But you can try the following temporary link.

ZC,ZD Generic IPL

Sorry if I came on a bit strong
I bought this repair run 11 years ago from a bodge artist of the first order and as such I have spent years repairing things properly

And yes he had a habit of drilling holes in hydros to drain the oil and another hole in the body to access the filling port rather than pulling the hydro out and doing it properly.
Very annoying as most come out in less than 1/2 hour .
To a one, every drilled hydro has failed and on some of them he had used flathead Harley oil ( 40w70 ) in a couple had had packed it in shortly after the initial "repair"

There are really only a few things that cause hydros to surge
1) air in the system
2) insufficinet oil
3) pump running too slow

IF you look carefully at the IPL Star has made available to you then you will notice that there are NO INTERNAL OIL SEALS .
Thus the entire system requires a careful balance between viscious friction of the oil and the surface finish & space between the various surfaces.
I have attempted one pump repair by machining the grooves out of the valve plate and lapping it against the valve block to a mirror finish .
I was chuffed when I spun the drive shaft with my fingers and the wheels turned .
In the mower, it ran great for 10 minutes then slowed down to a stop .
Lesson learned , it is a job for those who know what they are doing .
The specialist hydro repair workshop owner was good enough to tell me what I had done wrong , surface finish was way to fine ,
But I decided then & there that I would never split a box again
Down here a drop in pump/motor unit is $ 600 ( Aus ) wholesale which makes a repaired box $ 1000 ( for a 100 series JD ) against $ 1300 for a brand new box with a 3 year warranty .

As I assume you are repairing then reselling mowers then this is my "test" of hydros.
I have a tilting trailer that I collect mowers with
When I unload it . I engage the drives and with the engine turned off give the mower a push
IF I have to push it hard to get it off then the hydro is perfect
IF it rolls a little but stops when I stop pushing, again it is useable
IF it slowly rolls off after a push then we might get another 2 to 4 years with an oil change
If it takes off under its own weight, and the belt is good then the hydro is stuffed