Help, No forward motion!

[marrt]

One more thought on these repeated failures. The pump and wheel motor circuit is "closed." This means the oil in the circuit never returns to the tank and is NEVER filtered (ignoring makeup oil from the charge pump, which is minimal). So...if you have a pump failure and the motor circuit is contaminated (which is very likely), then the contamination will stay in the circuit and damage the "new" pump over time. More contamination in the circuit means much lower pump life. I'm simplifying here but this is the basic situation.

As a best practice, the wheel motor circuit should be filtered EVERY time it's opened (for example, to replace a leaking wheel motor or bad pump). At one time, PT even recommended periodic filtration of the wheel motor circuit and sold a kit for this purpose. I built a 3 micron filter kit, using a high pressure filter, and posted the design on here some time ago. When I added larger wheel motors and a new pump to my 425, I connected this filter inline at one of the wheel motors and ran the tractor, with the tram engaged, for several minutes (without going into math, 15 minutes run time is a good start IF you haven't experienced a significant failure...which you have).

Here's the bad news...if this circuit is badly contaminated, you will not be able to sufficiently remove the contamination with a filter kit alone. In general, the filter kit is used when there was no catastrophic failure...i.e., just replacing a leaking wheel motor or degraded but working pump. A catastrophic failure will deposit material in the lines that will be impossible to properly clean (industrial solutions exist for this but it's not practical for our small machines). From time to time, this material will "break loose" and recontaminate the filtered circuit. You "could" add a permanent high pressure filter to the circuit I suppose but it wouldn't be easy.

So, what would I do in your situation? You won't like the answer. I suspect you have a lot of contamination in this circuit and it's causing the multiple failures. Keep in mind...I don't know EXACTLY what's failing in your pump and I could be wrong. But if I'm right, you won't be able to easily solve this problem permanently without a very significant rebuild (new pump, new wheel motors and all new plumbing in the wheel motor circuit...plus, you'll need to properly prime the circuit before startup). Depending on your mechanical skill and patience, this is ether a very daunting job or a very easy, but expensive job. Either way, I'd do a lot more investigation on the root cause of these failures before I bought another pump (I guess your only option is to press Terry for his opinion on the subject). If you want total piece of mind, a new tractor purchase might not be a bad idea. Building an all new wheel motor circuit would cost something in the range of $6K or so...much more if you buy the parts from PT. Plus, if you don't do it right, you could damage the new parts too.

I'm not trying to scare you. But if you want a permanent solution, you've got figure out what is really going on, what's the total damage, and then make your decision.

 

[BobRip]

Marrt I thought the charge pump was about 4 gallons a minute. I am not disagreeing with what you said otherwise.

 

[marrt]

Marrt I thought the charge pump was about 4 gallons a minute. I am not disagreeing with what you said otherwise.

Hey Bob. I'm not sure of the flow rate of the charge pump on the 425...but does it matter?

To summarize for new owners reading this (not you Bob)...the charge pump is there to supply make up oil related to wheel motor bypass, pressurize the circuit to avoid/limit cavitation, provide a constant back pressure for the VD pump and provide some cooling for the VD pump under operation. On the PT, the charge pump pulls through the suction filter to ensure the oil is clean before it can be potentially introduced into the wheel motor circuit. Most industrial equipment uses a high pressure filter on the outlet side of the charge pump to avoid the potential problems I mentioned earlier (air bypass on the suction filter or cavitation from too much vacuum when the oil is cold). However, in that setup, care is needed to ensure the hydraulic oil in the tank is clean from all return sources. This requires additional filtration not present on the PT. Since the oil in the tank on the PT is returned from hydraulic cylinders and attachments, it could be dirty...so PT opts for the simpler suction side filter.

Anyway, back to the question above, most of the time very little oil is supplied the the wheel motor circuit and most of the oil is just returned to the tank. So, the charge pump is not supplying 4 gmp INTO the wheel motor circuit. This is just the flow rate necessary to create the 200-350 PSI charge pressure required for the specific pump design.

It would be real interesting to know exactly how much flow enters the wheel motor circuit. Of course this will vary greatly depending on the condition of circuit components. A new PT should have very little oil entering the circuit and, therefore, a higher charge pump pressure. A damaged circuit will have a lower charge pump pressure due to bypass (less resistance, more flow). This is the reason one measures charge pump pressure as a first diagnostic step to access the condition of the wheel motor circuit (particularly the VD pump).

 

[BobRip]

I notice that all of the oil from the oil cooler goes back to the oil tank. If this has very little flow then there will not be much cooling. I don稚 see the charge pump in the diagram that I have. Just curious here. I do think that the main problem here is material left in the wheel circuit. It would be good to see what Terry says.

 

[MossRoad]

I read somewhere that the charge pump is 4GPM. It's a fixed displacement pump, so it's volume can only change with RPMs. It constantly pump and supplies makeup oil to the variable volume pump as oil slips past the seals as well as back pressure, keeping the closed loop of the VVP and wheel motors full of oil all the time. My guess is, anything that is not injected as makeup oil gets routed back to the tank, and that constant flow probably supplies a cooling function as well. gotta go tornado warning.....

 

[MossRoad]

Well it's gonna miss us....

My guess is that's why the oil cooler is plumbed in line with the charge pump.

 

[BobRip]

I agree.

 

[MossRoad]

Anyone notice how much heat these things build up just sitting still? I noticed it my first winter. I started it up and let it run for about 20 minutes at 1/3 throttle while I hand shoveled the steps and narrow walkways and brushed off the cars. I went back and the cooling fan was on in the dead of winter. Not only the charge pump moving fluid around, but the main and aux PTO pumps are always running, circulating fluid and generating heat.

 

[coolconnection]

As a best practice, the wheel motor circuit should be filtered EVERY time it's opened (for example, to replace a leaking wheel motor or bad pump). At one time, PT even recommended periodic filtration of the wheel motor circuit and sold a kit for this purpose. I built a 3 micron filter kit, using a high pressure filter, and posted the design on here some time ago. When I added larger wheel motors and a new pump to my 425, I connected this filter inline at one of the wheel motors and ran the tractor, with the tram engaged, for several minutes (without going into math, 15 minutes run time is a good start IF you haven't experienced a significant failure...which you have).
You "could" add a permanent high pressure filter to the circuit I suppose but it wouldn't be easy.

Thank you everyone, I have the tram pump off and had it sent back to PT. I talked to Terry and he said your not even close to be in panic mode. The wheel motors do eventually drain back into the holding take he said. I was raised to respect peoples thoughts and make my own decisions and keep them to myself. I have read what everyone has posted and came up with some ideas. At this time I really don't want to buy a new tractor but it is really fun to look. I just can't get past how versatile this tractor is for me. The standard tractors just don't measure up to my tasks.
I do like the idea of some type of filtration in the wheel motor system at least for awhile. Marrt I am go to try to see what you did for that inline filter if I can find it. Tomorrow Terry should give me a call since the tram pump was suppose to be there today according to ups tracking. I am going to bring this up to him. I called a place that would basically do what an inline filter would do and it was really expensive. Money don't grow on trees around here but I really like that idea of another filter in that circuit even if it is just a small one. I don't know if it could be permanent, does the oil go both directions in the lines going from forward to reverse. I am replacing all the oil with fresh (old oil is really dark now) and the reservoir is drained and completely wiped down. Anyone know how to get all or the majority of the oil out of all the lines. Blow it out?
Again thanks for all the replies. I will update you all as things start to happen.

 

[MossRoad]

As I understand it, and this could be incorrect, the oil goes from the tank, through the existing filter, to the charge pump, to the variable volume pump as makeup oil. Any oil that slips past the VVP goes to the oil cooler, then back to the tank.

The VVP itself is reversible, depending on the angle of the swash plate. Neutral when pedals are centered. Press the forward pedal and fluid flows out to the wheel motors. Let up and it stops. Push down on the reverse pedal and the fluid flows the other way.

I don't see how you could filter the wheel motor circuits under operation, as it would backflush the filter each time you reversed direction.

You could filter them with a temporary filter placed in-line right at the input or output of the VVP and running the machine up on blocks in one direction only.

At least, that's the way I understand it.