Diagnostic Guide for Bosch Hydraulic Brake Booster - keywords auxiliary backup fluid leaks low operation pressure pump

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

The following information is provided as a diagnostic aid when servicing and performing system checks on the Bosch Hydro-Max Brake Booster on medium duty trucks with hydraulic brakes RPO JE3.

CHECKING FOR FLUID LEAKS:

1. Thoroughly clean the entire brake booster and master cylinder with soap and water and dry them. Start the engine and apply the brakes with normal foot pedal force several times. Inspect for the location of the fluid leak.
2. Once the leak is located, determine if it is power steering fluid or brake fluid:

2.1.	Brake fluid will dissolve in water.

2.2.	Power steering fluid (normally red color) will float on water.

3. Follow appropriate service procedures to repair the fluid leak:

3.1.	If it is power steering fluid, check for leaks at connector fittings, brake booster, auxiliary backup pump or from the interface between the pump and the booster.

3.2.	If it is brake fluid, check for leaks at connector fittings.

DIAGNOSING BRAKE WARNING LIGHT AND ALARM ACTIVATION:

1. The brake master cylinder fluid indicator switch is on. Either the master cylinder reservoir is less than 1/2 full or the switch is malfunctioning.

1.1.	Fill the master cylinder reservoir with the specified brake fluid.

1.2.	If warning light and alarm remain on, remove the electrical connector from the fluid level indicator switch. If the light and alarm turn off, then the fluid level indicator switch is at fault.

2. During engine start up there was no voltage available to the auxiliary backup pump before the engine started. Check power to the backup pump.
3. If the ignition key is on and there is low flow or no flow of power steering fluid through the booster, then the flow switch is closed.
4. The auxiliary backup pump is not grounded.
5. Check for a malfunction at the Electronic Brake Control Module (ECBM)

DIAGNOSING AUXILIARY BACKUP PUMP CONCERNS

Notice: The backup pump should run only if the flow switch is closed and the brake pedal is applied indicating there is no power steering fluid flow through the brake booster ( the engine is not running, or there is a power steering pump malfunction, etc.).

1. Check for the backup pump does not run when the brake pedal is applied with the engine off.

Remove the electrical connector at the backup pump and test for 12 volts at the connector with the brake pedal applied. If there is no voltage, go to step #4.

2. Check for backup pump continues to run when the brake pedal is released.

This condition can occur if the relay is stuck in the closed position or the brake light switch and the flow switch are both closed. Check to see if the brake lights are on continuously.

3. Check for backup pump runs when engine is running and the brake pedal is applied.

This condition can occur when there is a loss of power steering fluid flow or if the flow switch is stuck closed. If braking and steering feel normal, then the flow switch is closed and removing the connector from the switch will stop the pump motor from running.

4. Check for no voltage at the backup pump connector when the brake pedal is applied and the engine is not running.

4.1.	There is no battery voltage to the power terminal of the relay. Remove the electrical connector from the relay and test for 12 volts at the power terminal. Check for a blown fuse.

4.2.	There is no voltage at the flow switch when the brake is applied. Remove the electrical connector from the flow switch and test for 12 volts with the brakes applied.

4.3.	There is no electrical continuity from the positive terminal of the flow switch to negative terminal of the switch. Check for continuity of the positive terminal to the brake booster housing as well as continuity of the negative terminal to the booster housing.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.