Owatonna / Mustang 770 wheel loader weak hydrostatic drive

[Woodtractor1]

Hi,
I have a 1978 omc mustang 770 loader that barely creeps up the slightest incline or up a small curb.

I just replaced the Ford 172 diesel engine with a Ford 172 gas engine.
I drained the hydraulic fluid amd replaced with tractor hydraulic fluid from the local hardware store. I didn't replace the filter at this time. I couldn't find one. Went to 5 stores today and ended up with a part number and a special order that will be here in a week. The current filter looks relatively clean.

The power was lacking before the engine swap but not nearly as bad as now. I ordered a replacement filter and will do another hydraulic oil change next week. The vacuum gauge sits somewhere around 2 when warm to 5 when cold, so not very high from what I understand.

The loader has a series 23 sundstrandt hydrostatic motor and similarly looking sundstrandt hydrostatic pump but I haven't laid eyes on the pump nameplate for the model.

1. How do I troubleshoot whether it's the engine's clutch, the hydrostatic pump,or the hydrostatic motor.?

The engine revs high and does not bog or stall when I am trying to go over a small bump or incline, but there's just no power. It will make it if I back up and get momentum. Engine oil pressure is 45-50. All my other hydraulic functions seem to have plenty of power.

2. If I need to install gauges where would I install a pressure gauge to the sundstrandt pump or motor?

3. Do I need to bleed the system? How do I do that?

I drove the 770 for a week and still had the same symptoms.

Thanks so much!

 

[used red MN]

Hi,
I have a 1978 omc mustang 770 loader that barely creeps up the slightest incline or up a small curb.

I just replaced the Ford 172 diesel engine with a Ford 172 gas engine.
I drained the hydraulic fluid amd replaced with tractor hydraulic fluid from the local hardware store. I didn't replace the filter at this time. I couldn't find one. Went to 5 stores today and ended up with a part number and a special order that will be here in a week. The current filter looks relatively clean.

The power was lacking before the engine swap but not nearly as bad as now. I ordered a replacement filter and will do another hydraulic oil change next week. The vacuum gauge sits somewhere around 2 when warm to 5 when cold, so not very high from what I understand.

The loader has a series 23 sundstrandt hydrostatic motor and similarly looking sundstrandt hydrostatic pump but I haven't laid eyes on the pump nameplate for the model.

1. How do I troubleshoot whether it's the engine's clutch, the hydrostatic pump,or the hydrostatic motor.?

The engine revs high and does not bog or stall when I am trying to go over a small bump or incline, but there's just no power. It will make it if I back up and get momentum. Engine oil pressure is 45-50. All my other hydraulic functions seem to have plenty of power.

2. If I need to install gauges where would I install a pressure gauge to the sundstrandt pump or motor?

3. Do I need to bleed the system? How do I do that?

I drove the 770 for a week and still had the same symptoms.

Thanks so much!

Looks to me like this is what you need.
Mustang 770 service manual
Does it even have a clutch? You just swapped the engines so you should know. Not sure why it would need a clutch with a hydrostatic drive.

 

[Woodtractor1]

Looks to me like this is what you need.
Mustang 770 service manual
Does it even have a clutch? You just swapped the engines so you should know. Not sure why it would need a clutch with a hydrostatic drive.

Yes, it has a clutch. I have the manual but it isn't very informative. Basic stuff.

 

[used red MN]

Yes, it has a clutch. I have the manual but it isn't very informative. Basic stuff.

An operators manual or the service manual? It describes it as 210 pages.

 

[Jim.ME]

Hi,
I have a 1978 omc mustang 770 loader that barely creeps up the slightest incline or up a small curb.

I just replaced the Ford 172 diesel engine with a Ford 172 gas engine.
I drained the hydraulic fluid amd replaced with tractor hydraulic fluid from the local hardware store. I didn't replace the filter at this time. I couldn't find one. Went to 5 stores today and ended up with a part number and a special order that will be here in a week. The current filter looks relatively clean.

The power was lacking before the engine swap but not nearly as bad as now. I ordered a replacement filter and will do another hydraulic oil change next week. The vacuum gauge sits somewhere around 2 when warm to 5 when cold, so not very high from what I understand.

The loader has a series 23 sundstrandt hydrostatic motor and similarly looking sundstrandt hydrostatic pump but I haven't laid eyes on the pump nameplate for the model.

1. How do I troubleshoot whether it's the engine's clutch, the hydrostatic pump,or the hydrostatic motor.?

The engine revs high and does not bog or stall when I am trying to go over a small bump or incline, but there's just no power. It will make it if I back up and get momentum. Engine oil pressure is 45-50. All my other hydraulic functions seem to have plenty of power.

2. If I need to install gauges where would I install a pressure gauge to the sundstrandt pump or motor?

3. Do I need to bleed the system? How do I do that?

I drove the 770 for a week and still had the same symptoms.

Thanks so much!

To be clear this picture of a 770 Mustang OMC wheel loader is similar to yours?





From your post it sounds like you are doubting the engine clutch is good. Is it an actual mechanical clutch on the fly wheel between the engine and hydrostatic pump? You posted you just changed the engine, how did the clutch disc and pressure plate look? Worn to the rivets? How much pedal free play?

How long have you had this machine?

 

[Woodtractor1]

Thats it but mine is green.

I've had the Machine 14 years. It was barely able to climb onto my gooseneck trailer a few years ago. But since the engine switch, the power is way down. Full throttle gas 172 ford engine.

It is an actual clutch on the flywheel. I didn't take the pressure plate off because I was preoccupied with trying to get the engine running and installed. It just didn't cross my mind. The pressure plate and throw out bearing looked great.

The clutch pedal is usually only used when starting the engine. I don't use the clutch at all because the cable is shot. I guess the clutch helps when you starting it in cold weather and the hydraulic oil is thick. I live in Arizona. Not a problem usually.

Above is the hydrostatic pump

Above is the hydrostatic motor.

Can I attach a hydraulic gauge somewhere? If so, where?

Can I block off fluid from the pump to the hydrostatic and see if it stalls the gas engine? That would eliminate the clutch and pump. I just don't know it that will damage anything.

 

[DoubleO7]

Yes, it has a clutch. I have the manual but it isn't very informative. Basic stuff.

What did the hydrostatic fluid look like and smell like when drained?
Might want to open up the old filter to see how plugged it is.

All my other hydraulic functions seem to have plenty of power.

Does it have another hydraulic pump and controls that are independent of the hydrostatic tranny?

 

[Jim.ME]

Thats it but mine is green.

I've had the Machine 14 years. It was barely able to climb onto my gooseneck trailer a few years ago. But since the engine switch, the power is way down. Full throttle gas 172 ford engine.

It is an actual clutch on the flywheel. I didn't take the pressure plate off because I was preoccupied with trying to get the engine running and installed. It just didn't cross my mind. The pressure plate and throw out bearing looked great.

The clutch pedal is usually only used when starting the engine. I don't use the clutch at all because the cable is shot. I guess the clutch helps when you starting it in cold weather and the hydraulic oil is thick. I live in Arizona. Not a problem usually.
View attachment 67723
Above is the hydrostatic pump
View attachment 67724

Above is the hydrostatic motor.

Can I attach a hydraulic gauge somewhere? If so, where?

Can I block off fluid from the pump to the hydrostatic and see if it stalls the gas engine? That would eliminate the clutch and pump. I just don't know it that will damage anything.

Thats it but mine is green.

I've had the Machine 14 years. It was barely able to climb onto my gooseneck trailer a few years ago. But since the engine switch, the power is way down. Full throttle gas 172 ford engine.

It is an actual clutch on the flywheel. I didn't take the pressure plate off because I was preoccupied with trying to get the engine running and installed. It just didn't cross my mind. The pressure plate and throw out bearing looked great.

The clutch pedal is usually only used when starting the engine. I don't use the clutch at all because the cable is shot. I guess the clutch helps when you starting it in cold weather and the hydraulic oil is thick. I live in Arizona. Not a problem usually.
View attachment 67723
Above is the hydrostatic pump
View attachment 67724

Above is the hydrostatic motor.

Can I attach a hydraulic gauge somewhere? If so, where?

Can I block off fluid from the pump to the hydrostatic and see if it stalls the gas engine? That would eliminate the clutch and pump. I just don't know it that will damage anything.

If there was a definite drop in power when you installed the engine, without checking the clutch, that makes that clutch a suspect item. Is there free play in the clutch linkage so you know the throwout bearing is not contacting the pressure plate fingers and holding the clutch partially disengaged?

If it was weak several years ago, the pump and drive motor likely have some wear. You need the service manual for the loader and/or the correct Sunstrand manuals to have the proper testing procedure. I will say do not try to block (deadhead) the pump; you may cause even more damage. It may take more than one pressure gauge and also a flowmeter maybe required to properly diagnose that drive system.

 

[Woodtractor1]

What did the hydrostatic fluid look like and smell like when drained?
Might want to open up the old filter to see how plugged it is.

Does it have another hydraulic pump and controls that are independent of the hydrostatic tranny?

Yes there is a small Cessna pump coming off the gearbox. I believe it operates the other hydraulic functions, cylinders etc. Or does it also have something to do with propulsion? The hydraulic cylinders have plenty of power.

The old filter isn't bad at all. The oil appeared normal

 

[DoubleO7]

Yes there is a small Cessna pump coming off the gearbox. I believe it operates the other hydraulic functions, cylinders etc. Or does it also have something to do with propulsion? The hydraulic cylinders have plenty of power.

The old filter isn't bad at all. The oil appeared normal

Many Sundstrand hydros have a thru put output shaft directly connected to the input shaft. To be used as an engine speed power output shaft.
The Cessna pump is maybe running off of that shaft and it's hydraulic output is has nothing to do with vehicle movement.
That pump might share a reservoir of the same fluid as the hydro however.
When you refilled the hydro, did you bleed the system?
On my Sundstrand 15U equipped mower you carefully/slightly disconnect the fluid line going to the cooler inside the rad.
Until you rust proof everything surrounding the open connection.

If it is driven down stream of a slipping clutch, it seems like the hydraulics would also be diminished.

 

[Butch(OH)]

Yes there is a small Cessna pump coming off the gearbox. I believe it operates the other hydraulic functions, cylinders etc. Or does it also have something to do with propulsion? The hydraulic cylinders have plenty of power.

The old filter isn't bad at all. The oil appeared normal

How close did you look at the fluid? After running it to stir the oil get some fluid from the tank under a strong light and look for sparkles, also check the filter you removed closely, if you see any metal at all then your in for spending some money

 

[kencombs]

Thats it but mine is green.

I've had the Machine 14 years. It was barely able to climb onto my gooseneck trailer a few years ago. But since the engine switch, the power is way down. Full throttle gas 172 ford engine.

It is an actual clutch on the flywheel. I didn't take the pressure plate off because I was preoccupied with trying to get the engine running and installed. It just didn't cross my mind. The pressure plate and throw out bearing looked great.

The clutch pedal is usually only used when starting the engine. I don't use the clutch at all because the cable is shot. I guess the clutch helps when you starting it in cold weather and the hydraulic oil is thick. I live in Arizona. Not a problem usually.
View attachment 67723
Above is the hydrostatic pump
View attachment 67724

Above is the hydrostatic motor.

Can I attach a hydraulic gauge somewhere? If so, where?

Can I block off fluid from the pump to the hydrostatic and see if it stalls the gas engine? That would eliminate the clutch and pump. I just don't know it that will damage anything.

As a general rule, never block the output of a hydraulic pump. I've seen drive tangs broken, pump cases split, all sorts of carnage as a result of that.

I'm curious as to the clutch arrangement. I've never seen a flywheel, clutch and pressure plate that could be removed and reinstalled without separating them. And in doing so one would see the condition of all the pieces.

There is probably a port on the pump output attach a gauge, but as mentioned above you really need the factory manual.

 

[Woodtractor1]

As a general rule, never block the output of a hydraulic pump. I've seen drive tangs broken, pump cases split, all sorts of carnage as a result of that.

I'm curious as to the clutch arrangement. I've never seen a flywheel, clutch and pressure plate that could be removed and reinstalled without separating them. And in doing so one would see the condition of all the pieces.

There is probably a port on the pump output attach a gauge, but as mentioned above you really need the factory manual.

Where would the gauge go?

 

[Woodtractor1]

As a general rule, never block the output of a hydraulic pump. I've seen drive tangs broken, pump cases split, all sorts of carnage as a result of that.

I'm curious as to the clutch arrangement. I've never seen a flywheel, clutch and pressure plate that could be removed and reinstalled without separating them. And in doing so one would see the condition of all the pieces.

There is probably a port on the pump output attach a gauge, but as mentioned above you really need the factory manual.

I didn't remove the clutch or pressure plate. I bought a used engine that came with clutch and pressure plate. I didn't take the pressure plate or clutch off because I didn't have an alignment tool when I would go to put it back on. They can be separated. Ford 172 gas engine

 

[Woodtractor1]

How close did you look at the fluid? After running it to stir the oil get some fluid from the tank under a strong light and look for sparkles, also check the filter you removed closely, if you see any metal at all then your in for spending some money

I didn't see any metal shavings in the filter. It was pretty clean.

 

[Woodtractor1]

An operators manual or the service manual? It describes it as 210 pages.

Those manuals are for a different model skid steer. Mine is an omc/mustang 770.
I have a parts manual and an operators manual

 

[kencombs]

I didn't remove the clutch or pressure plate. I bought a used engine that came with clutch and pressure plate. I didn't take the pressure plate or clutch off because I didn't have an alignment tool when I would go to put it back on. They can be separated. Ford 172 gas engine

Ah, that explains the clutch question for me. One thing that may account for the weaker performance of the new engine is the governor setting. Is it running at the same RPM as the old one? Really common for engines to be set at different speeds for different uses.

You really need the pump tech manual to locate the correct test port. You could gather fittings to put a tee in the line but if it's all hard line that is difficult.

 

[Woodtractor1]

The old engine was diesel and the tachometer didn't work. I can hook up a tachometer at some point but it really sounded like the rpms were pretty high. Do you know typical rpm range for Ford gas 172?

This is a frustrating one. So many possibilities and factors

 

[used red MN]

The engine revs high and does not bog or stall when I am trying to go over a small bump or incline, but there's just no power.

Not sure if this shows to others or not.
…”The engine revs high and does not bog or stall when I am trying to go over a small bump or incline, but there's just no power.” This tells me you are not using all the engine power that is available. Fact 2.. you didn’t pull the clutch off the used engine to check its condition tells the story to me. You’re going to have to get back to the clutch to check it out. The only saving grace you may have is that the the clutch free play is set incorrectly. Or the bad actuating cable may possibly be holding the clutch partially disengaged. Disconnect the cable end from the lever it operates down by the clutch. If it requires force to remove the pin that operates the lever and it springs to a different location away from the cable then that was likely your problem. That was holding the clutch partially released. All my terminology is based on a general idea of what the clutch cable layout is like in most machinery so equipped. Between non turbocharged gas and diesel engines of the same size the gas engine will make the most horsepower.

 

[Brendon-KS]

Having a mechanical clutch in front of a hydrostat isn't common and will make diagnosing the problem harder since there are more factors in play.

One quick and easy test for the motor is to check the case leakage. There will be a hose coming off the motor housing that leads to the cooler and return filter and this is the case drain. Take this hose from the motor and direct it into a bucket (just plug off the connection it was attached to on the other end.) When the machine is in neutral you should have a case drain flow of around 3 gpm maximum, ideally quite a bit less than that. If you have more flow than that it would suggest a worn-out rotating group that is allowing too much oil to escape around the pistons.

You can't check the pump this way because the pump leakage is combined with the oil going over the charge relief valve and comes out the case drain port all together.

I found a video of a Sundstrand pump undergoing tests on youtube. Does the picture below look like the back of your pump? If so the tee fittings coming out towards you in the picture are the gauge ports, the ones right close to the system ports. You really need two gauges capable of taking full system pressure to diagnose a hydrostatic pump. When the hydro is in neutral both ports should show charge pressure which is generally in the 200 to 400 psi ballpark, depending on how the hydro is configured for that application. When the hydro is stroked in one direction or the other the high pressure side of the loop will increase while the low pressure side will remain at charge pressure. It is critical that the entire loop remains at charge pressure or higher to avoid destroying the pistons and swashplate. The peak pressure on the high side will depend on the relief valve setting but it will be at least 3000 psi if not higher.

One way to determine if it is the clutch slipping rather than a faulty hydrostat would be to plumb a flow meter into the outlet of your auxiliary function pump. If this flow drops when the machine stalls out you will know that it is the clutch slipping since this flow would not be impacted by anything the hydrostat is doing.