M929 A2 Suddenly stopped dumping

[aventurateutonic]

We use a 1992 M929 A2 every day while working on a remote road project. The dumping mechanism has worked well for the last three years we have used the beast. Just last week, all of a sudden, the dump bed stopped lifting.

The PTO engages, the retention latch moves aside, but it will not lift. Today, the guys tried troubleshooting it, checking all the lines, checking control linkages, inspecting the pump, etc., but to no avail, there was nothing obviously wrong with anything.

Has anyone else experienced this mystery?

Thanks

 

[Mullaney]

We use a 1992 M929 A2 every day while working on a remote road project. The dumping mechanism has worked well for the last three years we have used the beast. Just last week, all of a sudden, the dump bed stopped lifting.

The PTO engages, the retention latch moves aside, but it will not lift. Today, the guys tried troubleshooting it, checking all the lines, checking control linkages, inspecting the pump, etc., but to no avail, there was nothing obviously wrong with anything.

Has anyone else experienced this mystery?

Thanks

.
I'm just fishing here - but having had it for three years - does the hydraulic pump "humm" like it always did in the past? Really silly question, but what about fluid? Is the fluid level okay? See any leaky spots under the truck (maybe covered in dirt dust)? I can't imagine that the hydraulic cylinder just quit all at once but the "packing" just may have finally died. Pump could have died too - but the sound (pitch) should have changed.

 

[simp5782]

Check pressure at the fittings at the cylinder to see what pressure you have. That will tell you if the pump is out or lacking pressure to lift it or you have a cylinder issue.

Some trucks have test parts on the hydraulic systems. Some 929s don't. Just depends

 

[aventurateutonic]

Check pressure at the fittings at the cylinder to see what pressure you have. That will tell you if the pump is out, lacks pressure to lift it, or if you have a cylinder issue.

Some trucks have test parts on the hydraulic systems. Some 929s don't. Just depends

Folks, thank you for the diagnosis guidance.

We have settled on replacing the pump.

The control valve works and redirects ( we can tell by locking and unlocking the dump body latch cylinder ). In addition, there are no leaks on or around each of the cylinders; they are dry. When we lift the dump body externally, with the excavator concurrent with opening the control valve, the body will stay up on its own until we reverse the control--the cylinders appear solid, with no leaks, and maintain pressure.

 

[Jbulach]

Hope you guessed correct it could be a sheared key in the pto shaft, stuck pressure relief valve, or broken spring.

 

[aventurateutonic]

Thanks. The PTO is turning the pump properly. As for the pressure relief valve, where is that located? Is that part of the pump itself?

-Rolf

 

[Jbulach]

I would assume in the directional control valve, the same place as my 925, even though your valve is different.
Under the plug I marked with the white X.

 

[aventurateutonic]

@Mullaney , thank you. We suspected the same after looking at some diagrams of the control valve. We are about to tear it apart to see. I did find a replacement cartridge for the pump, just in case, for $275, which is much less expensive than a whole unit.

-Rolf

 

[aventurateutonic]

@Mullaney, here is what we saw.









It appears that spring for the plunger is not broken, the plunger is not slides easily, and the spring with the 'plunger' is working also. No clear signs of what could be an issue.

We are now left to consider the pump the culprit, though it appears all the impeller and its vanes are moving fine. Should be vanes on the pump push outward through centrifugal force, or is there typically a spring mechanism to keep them extended?

Thanks

-Rolf

 

[Mullaney]

@Mullaney, here is what we saw.

View attachment 874960

View attachment 874961

View attachment 874962

View attachment 874963

It appears that spring for the plunger is not broken, the plunger is not slides easily, and the spring with the 'plunger' is working also. No clear signs of what could be an issue.

We are now left to consider the pump the culprit, though it appears all the impeller and its vanes are moving fine. Should be vanes on the pump push outward through centrifugal force, or is there typically a spring mechanism to keep them extended?

Thanks

-Rolf

.
Hi Rolf.

The vanes in the pump are like a knife blade. Sharp on one side at an angle and that angle is about the thickness of the vane. Spring definitely looked good and the fluid appeared clean with no chunks of rubber from a failing hose. There is no spring behind the vanes. centrifugal force pushes the vanes out to contact the outer wall iof the pump and move fluid.

My concern or maybe my question with a bad pump is that they generally don't suddenly stop working. Going bad slowly - where it took more engine speed to turn the pump longer to accomplish the task. Or the sound of the pump gradually becomes more of a whine than a whirring sound.

A gauge might cost a lot less than any other parts you could buy. Something that could read between 2500 and 3000 psi. The pump on the M923 should read at least 1600psi. Anything less is a problem. 1400psi suggests that the control valve should be replaced.

Having the ability to measure GPM isn't easy, but the tipper trucks are only 5gpm so we aren't looking for a lot of fluid - just steady pressure.

The Technical Manual TM 9-2320-272-24-2 (available on this site as 929 Series) on Page 4-15 (PDF Page 699) has the steps to walk through troubleshooting. Not trying to just say read the book, but this book does show you where to measure pressure. Knowing that first will tell us if the pump is good and the control valve is bad - or not. Without spending money.

Hope that helps?

 

[Jbulach]

Did you check the spring and for debris inside the piston?

 

[aventurateutonic]

.
Hi Rolf.

The vanes in the pump are like a knife blade. Sharp on one side at an angle and that angle is about the thickness of the vane. Spring definitely looked good and the fluid appeared clean with no chunks of rubber from a failing hose. There is no spring behind the vanes. centrifugal force pushes the vanes out to contact the outer wall iof the pump and move fluid.

My concern or maybe my question with a bad pump is that they generally don't suddenly stop working. Going bad slowly - where it took more engine speed to turn the pump longer to accomplish the task. Or the sound of the pump gradually becomes more of a whine than a whirring sound.

A gauge might cost a lot less than any other parts you could buy. Something that could read between 2500 and 3000 psi. The pump on the M923 should read at least 1600psi. Anything less is a problem. 1400psi suggests that the control valve should be replaced.

Having the ability to measure GPM isn't easy, but the tipper trucks are only 5gpm so we aren't looking for a lot of fluid - just steady pressure.

The Technical Manual TM 9-2320-272-24-2 (available on this site as 929 Series) on Page 4-15 (PDF Page 699) has the steps to walk through troubleshooting. Not trying to just say read the book, but this book does show you where to measure pressure. Knowing that first will tell us if the pump is good and the control valve is bad - or not. Without spending money.

Hope that helps?

@Mullaney, I'll pick up a hydraulic pressure gauge today ( good to have, regardless ). We'll check the pressure at the output of the pump to start and then make our way through the system ( basically a voltmeter for hydraulics ). Thank you for your suggestions and information.

 

[aventurateutonic]

Did you check the spring and for debris inside the piston?View attachment 874977

@Jbulach , thank you for the question. We did not take apart the piston itself. We will do that today along with taking pressure measurements.

Thank you

-Rolf

 

[aventurateutonic]

Folks, we resolved the problem. We did get a pressure gauge and started by testing the output of the pump. To our surprise, it read zero, which immediately knew could not be correct. So we started working backward from the tank. We topped off the hydraulic fluid tank and then worked out all the air from the tank to the pump. Surprisingly, after a moment of priming, we got pressure which held steady at 2500 PSI.

We then buttoned everything up, and it has been working fine ever since. It is hard to imagine that an airlock could cause such a problem, but it appears that it is all that it was.

Thanks for everyone's help.

-Rolf

 

[NY Tom]

Thanks for reporting back your findings.

 

[Mullaney]

Folks, we resolved the problem. We did get a pressure gauge and started by testing the output of the pump. To our surprise, it read zero, which immediately knew could not be correct. So we started working backward from the tank. We topped off the hydraulic fluid tank and then worked out all the air from the tank to the pump. Surprisingly, after a moment of priming, we got pressure which held steady at 2500 PSI.

We then buttoned everything up, and it has been working fine ever since. It is hard to imagine that an airlock could cause such a problem, but it appears that it is all that it was.

Thanks for everyone's help.

-Rolf

.
Wow! Well, that is good news for sure and a lot less expensive than it could have been.
Having a way to measure fluid pressure is a nice addition too.