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May 10, 2017
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Charge Pump Failures on CTLs and SSLs

In this Shop Talk Blog post, our focus is going to be on charge pump failures on compact track loaders and skid steer loaders.  We’ll be focusing on a CAT 246 skid steer hydraulic motor and brake assembly that was sent in to us for diagnosis.  We’ll also talk about how to tell if you are having the same problem, and what to do when you encounter similar symptoms.

Background

A while back we were sent a hydraulic motor and brake assembly as a core.  The customer reported a weak motor, which can actually be caused by several different things and not all of them involve the hydraulic motor itself.  Upon investigation, we discovered that the final drive was not at fault … it was the charge pump.

final-drive-travel-motor-damaged-brake-discs-weak-charge-pumpForensic Analysis of the Hydraulic Motor

After the cleaning the exterior, we disassembled the hydraulic motor and cleaned the parts.  Next we performed an in-depth failure analysis.  The first thing noted by the technician was the poor condition of the brake discs:  they were scratched, burned up, and broken.  Obviously this points to a brake problem, and not a motor problem.  The evidence indicated that the brake was not fully releasing, which caused the motor to seem weak.

The bad part about this diagnosis is that the serious damage that resulted could have been avoided if the equipment owner had known what to look for once the weak motor symptoms started.

final-drive-travel-motor-weak-charge-pump-brake-problems-damaged-brake-housing

When the brakes failed to fully release, they were dragging while the machine was running.  This lead to major damage to the splines on the brake housing that was beyond any reasonable hopes of repair.

In the images you can see scarring and entire chunks of metal missing.  That metal has to go somewhere in the hydraulic system, so you can imagine the resulting secondary damage to the hydraulic drive motor.

What the customer thought was a weak final drive turned out to be an issue with the brake not releasing properly.  As this problem went on unaddressed, it caused severe damage to the brake discs and the brake housing, as well as secondary damage to the motor. So, what causes the brake to not release properly?

Well, normally these types of failures are caused by either leaking brake seals (which result in insufficient fluid to achieve a brake release) or a weak charge pump (insufficient pressure to release the brakes).  The charge pump is responsible for providing the pressure needed to release the brakes and help prevent freewheeling. In this case, our forensic inspection indicated it was a weak charge pump.

What To Look For …

cat-248b-skid-steer-wheel-loader

Here are the symptoms associated with a weak charge pump:

  • Resistance noticed on the controls
  • High pitched squealing coming from the drive motors
  • The machine will no longer move

By the time the machine refuses to move forward or backwards, the damage is likely irreparable and the final drive will need to be replaced.  Here’s another bad thing about charge pump failures: you can replace the final drives with new or re-manufactured drives only to have the same thing happen again.  That’s why it is important to eliminate the other possibilities before you send in your final drive or travel motor.

Checking the Pump Pressure

final-drive-track-drive-track-motor-failureIf you are experiencing the symptoms described above (resistance to controls, squealing motors, or a machine that will not move), then take the time to check the charge pump/gear pump pressure before you assume it is a final drive failure — and before it includes a final drive failure.

If you do not already own a service manual for your machine, obtain one. In the section dealing with the gear pump/charge pump (possibly under hydraulic charge pump pressure) you will find the information you need on how to check the pressure. The manual will tell you what the pressure should be on your particular machine.  Keep in mind that every machine is going to have different pressure readings, so you need to have the service manual for your particular machine for reference.

Conclusion

If your final drive motor seems weak, don’t forget that it might be a problem with a weak charge pump.  Before you pull your final drive to send it in for repair or reman, take a little while to verify that your charge pump is working correctly by checking the pressure coming out of the charge/gear pump.  This simple step could save you thousands of dollars!


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