Procedure for Diagnosing the Correct Operation of the Service Brake System on Certain Telehandlers {4269} Caterpillar

Telehandler

TL1055D (S/N: ML51-UP; MNT1-UP)

TL1255D (S/N: ML71-UP; MYW1-UP)

TL642D (S/N: ML81-UP; MLG1-UP)

TL943D (S/N: MLD1-UP; MLJ1-UP)

Reference: Service Magazine, M0105143, "Brake Purging Procedure Is Now Available for Certain Telehandlers".

Illustration 1	g06072661
(1) 483-4027 Brake Pedal Gp

Features of the Brake Pedal Group

Tier 4i and previous Generation TL telehandlers used a braking valve that functioned as a powered brake valve combined with a master cylinder. In this system, with the presence of power from the hydraulic system, oil is fed to the brakes from the implement system hydraulics via a pressure regulating valve in the brake valve. In the absence of power from the hydraulic system, the displaced oil is fed from displacing the master cylinder piston. These systems have a short pedal stroke when in powered mode (because oil is sourced from the implement pump), and a long pedal stroke when in unpowered mode (because oil is now coming from master cylinder).

Tier 4f TL telehandlers have a boosted step-bore type master cylinder braking valve, with function-specific valves internally which regulate the brake operation. The master cylinder consists of two major sections: the hydraulic boost section and brake system (master cylinder) section, which is isolated from one another by seals, and do not exchange fluids. The brake system fluid, which is Automatic Transmission Fluid (ATF) - Mobil ATF D/M, is displaced by two different bore sizes within the master cylinder. The boost section is connected into the implement hydraulic system.

Illustration 2	g06088762
(2) Hydraulic boost section (3) Master cylinder section (A) Boost pressure supply (B) Boost drain return port (C) Brake reservoir port (D) Brake post (E) Boost piston (F) Internal valves (G) Boost pressure poppet (H) Secondary port (J) Primary port

The master cylinder is a hydraulic boosted type, which means that the force applied to the master cylinder pistons comes from a combination of two different methods: hydraulic pressure acting against a boost piston (when in power mode, such as with engine running), or by direct force on the brake pedal via the linkage, brake pushrod and boost pressure regulating valve stem.

Powered mode

Pressing the brake pedal first closes a drain path to the tank in the boost section's piston, and then opens the boost pressure poppet to begin applying hydraulic pressure to the boost piston. The boost piston in turn, being connected to the master cylinder pistons, pushes the master cylinder to begin displacing fluid to axle brakes. Once sufficient fluid is displaced to take up the brake disc gap, the pressure required to engage the brakes will begin to increase. As the pressure in the master cylinder rises, the force on the boost pressure poppet will also increase (as long as the pedal is continually depressed), which will modulate additional boost pressure into the boost piston to further engage the brakes.

Illustration 3	g06088763
Boost power modulation (F) Internal valves

This will be true until the full level of boost pressure at the inlet of the master cylinder booster is reached inside the boost piston. Any additional pedal force beyond this point will cause the poppet to fully finish its stroke and permit the boost pressure regulating valve stem to mechanically contact the boost piston. Now, further pedal force will add to the force applied by the boost pressure, but only at the rate of the applied force at the pedal multiplied by the pedal linkage geometry ratio.

Illustration 4	g06088871
Full boost pressure reached inside the boost piston. Boost regulating valve in contact with the boost piston (F) Internal valves

During the application of the vehicle brakes, when the boost pressure is modulated from "0" to max boost pressure, the required change in pedal effort is minimal because the hydraulic fluid pressure is doing most of the work. Once the boost pressure reaches maximum inside the boost piston, a defined force change at the pedal can be felt because the hydraulic pressure cannot add any additional force to squeeze and further squeeze correlates to the level of (human) effort on the pedal.

Unpowered mode

In unpowered mode, the principle is similar to powered mode, except the boost pressure available remains at "0", and the boost pressure poppet strokes fully first, allowing the boost pressure regulating valve stem to contact the boost piston directly and in turn, push the master cylinder directly. The amount of force on the axle brakes is determined by the force at the pedal multiplied by the pedal linkage geometry ratio.

Illustration 5	g06088766
Unpowered mode (F) Internal valves

The step-bore functions in the following manner. The secondary and primary bores displace fluid while the required pressure in the brake system is low, for example when closing the brake disc gap to make initial contact of the brake discs. After brake discs make contact, additional oil displaced from the master cylinder needs to be forced at a higher pressure to apply braking effort by further applying the brake discs. The internal valves direct oil from the secondary bore back to the reservoir, while maintaining displacement on the primary bore to the vehicle brakes. In this way, the master cylinder can fill the axle brakes with fluid with a shorter stroke, and also allow higher pressures to be generated with lower brake pedal effort by acting on only the primary bore after filling.

Factors that affect the final brake position :

• Actual brake disc gap. The axles are fitted with a self-adjusting mechanism to maintain a proper brake disc gap. However, there is some variation from one axle to the next.

• Pedal Free Play. Excessive free play will increase the pedal travel by the amount of free play. See Adjusting Pedal Free Play.

• Status of the park brake. When the park brake is set, the brake effectively holds the front axle service brake piston in the applied state (no disc gap on front axle). The required displacement of the master cylinder is then only to take up the disc gap on the rear axle service brake.

• Boost Pressure. Boost pressure is fed via the Pressure Reducing Valve (PRV) in the priority valve (ICM). When the park brake is applied, and no other function is active, the boost pressure is equal to implement system standby pressure, which is lower than the setting of the PRV. When the park brake is released, the standby pressure rises above the setting of the PRV (via a Load Sense connection from the park brake release valve), providing maximum potential boost pressure. As pressure levels increase in the boost section of the master cylinder, less foot effort is required to achieve the same brake system pressure in the axles. Higher boost pressures and similar level of foot effort yields higher brake system pressure, and with the higher pressure comes a slight increase in volume due to brake disc and hose compliance.

These last two factors are especially apparent in the behavior of the system between non-powered and powered modes. With the engine off, the park brake is applied, and the pressure developed in the master cylinder is lower (less system compliance), both reducing the volume required and giving a short stroke and stiff pedal feel. With the engine running and park brake released, the full system volume is required, and with the aid of full boost pressure, the braking effort at the pedal for a given braking pressure is low. The brake system pressure behavior for each of these scenarios can be observed with the Analyzer tool in Diagnostics >> System >> Brk Pedal Prs.

Procedure to Check the Functionality of the System

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Do not operate or work on this product unless you have read and understood the instruction and warnings in the relevant Operation and Maintenance Manuals and relevant service literature. Failure to follow the instructions or heed the warnings could result in injury or death. Proper care is your responsibility.

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Personal injury or death can occur from not following the proper procedure or the recommended tooling. To prevent the possibility of injury or death, follow the established procedure using the recommended tooling.

1. Place the machine on a firm level surface free of obstacles.

2. Connect the analyzer tool.

3. Start the engine.

4. Using analyzer, navigate to Diagnostics >> System >> Brk Pedal Prs.

5. Release the park brake.

6. Pump the service brake several times slowly.

7. Wait for 10 seconds approximately (analyzer should read less than 103 kPa (15 psi).

8. Apply the service brakes with a single pedal press to full travel.

   Note: This is the point where maximum boost pressure is reached inside the boost section.

   There will be a defined force change and the analyzer should read 5378 to 5861 kPa (780 to 850 psi) for the TL943D, TL1055D, and the TL1255D. Or 6205 to 6550 kPa (900 to 950 psi) for the TL642D.

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   Illustration 6	g06072683
   (A) Measure the distance from the pedal to fire-wall (B) Free play with engine running is 0.0 to 3.0 mm (0.00 to 0.12 inch)

   Note: Remove the floor mat before measuring the distance from the pedal to fire-wall.

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   Illustration 7	g06072684

9. Have an assistant measure the distance from the back of the pedal arm to the cab fire wall. This distance should be greater than or equal to 38.1 mm (1.50 inch) for the TL943D, TL1055D, and the TL1255D. Or greater than or equal to 50.8 mm (2.00 inch) for the TL642D. Refer to Illustration 7.

10. If the pedal distance is less than 38.1 mm (1.50 inch) for the TL943D, TL1055D, and the TL1255D or less than 50.8 mm (2.00 inch) for the TL642D, check the pedal free play. If the free play is correct, then purge the brake system. If free play exists, the pedal may be adjusted to zero free play in the following steps :
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    Illustration 8	g06072702
    (C) Locknut (D) Push rod (E) Push rod gap (F) Stop ring (G) Piston

    Note: Push rod gap can be measured only with booster fed pressure.

    1. Piston (G) must be in the full back position, making contact with stop ring (F).

    2. Piston (G) is moved against the stop ring (F) by the booster pressure. When the booster is not fed, piston (G) cannot return completely against the stop ring (F). This occurrence could cause an incorrect gap measurement.

Free Play Adjustment

Note: Do not adjust when the engine is off. Over adjustment of the free play may result in brake drag.

1. Slide boot up to reveal locknut (C) and push rod (D).

2. Start the engine to supply boost pressure and ensure that the push rod is fully returned to the stop ring (F).

3. Loosen locknut (C) to 17.0 mm (0.67 inch). Swivel the push rod first clockwise to shorten it (to be sure it was not previously over-adjusted), and then counter-clockwise to lengthen it until it contains the piston (G). Use only the finger effort to adjust. To achieve the free play as shown in Illustration 6, the push rod gap (E) must be 0.0 to 0.5 mm (0.00 to 0.02 inch).

4. Hold the push rod (D) with a suitable tool and tighten the locknut (C). Reinstall the boot.

5. Ensure the proper service brake operation.

Brake Bleeding Procedure

Refer to Service Magazine, M0105143, "Brake Purging Procedure Is Now Available for Certain Telehandlers".