Use this procedure to determine if the auxiliary brake circuit is operating properly. No diagnostic codes are provided for the auxiliary brake output. There may be a problem with the circuit if the operator notes that the vehicle's braking is poor.
Background Information
The Engine Control Module (ECM) has an output that can be configured to activate an auxiliary brake. The "Auxiliary Brake Output" must be programmed to "J1/P1:12" in order to enable the auxiliary brake output. The OEM provides the wiring from terminal P1-12 to the system's components.
Typically, the ECM output energizes a relay. The relay's contacts apply battery power to the other components in the system. A switch on the dashboard interrupts the signal from the relay. This allows the operator to deactivate the system, if necessary.
The ECM continuously monitors the vehicle's operating conditions. The ECM provides battery voltage to terminal P1-12 when all of the following conditions are met:
- Engine speed is above 800 rpm.
- The percent of accelerator pedal position is less than eight percent.
- The clutch pedal is released.
When the cruise control is ON, the auxiliary brake output is related to the position of the service brake pedal. The ECM response depends on the mode of operation of the cruise control:
Coast Mode - Depressing the service brake pedal disengages the cruise control. The auxiliary brake output is activated. The output is deactivated when the driver releases the service brake pedal.
Latch Mode - Depressing the service brake pedal disengages the cruise control. The auxiliary brake output is activated. The output remains active until the ECM detects a change in the conditions that are monitored. For example, the accelerator pedal is above seven percent.
Manual Mode - Depressing the service brake pedal has no effect on the auxiliary brake output.
If the "PTO Configuration" is programmed to "Cab Switches", "Remote Switches", or "Remote Throttle", the auxiliary brake output is disabled whenever the PTO On/Off circuit is ON.
If the vehicle is equipped with an Anti-Lock Brake System (ABS), the vehicle may have an additional relay in the circuit for the auxiliary brake. This allows the ABS system to disable the auxiliary brake, if necessary.
Engines that are equipped with engine retarders
The auxiliary brake output is disabled when the vehicle speed is below the value that is programmed for the "Engine Retarder Minimum Vehicle Speed" parameter. The "Engine Retarder Minimum VSL Type" parameter provides the following two options:
Hard Limit (default) - As the vehicle speed decreases, the auxiliary brake output is deactivated when the vehicle speed goes below the "Engine Retarder Minimum Vehicle Speed".
Soft Limit - As the vehicle speed decreases, the auxiliary brake output remains activated when the vehicle speed goes below the "Engine Retarder Minimum Vehicle Speed".
The "Engine Retarder Delay" parameter provides a programmable delay after all of the basic conditions that are required to turn on the auxiliary brake are met. Once the conditions are met the ECM will delay the activation of the auxiliary brake by the programmed additional time. The programmable range is 0 to 3.0 seconds in 0.1 second increments.
The following components can cause problems with the auxiliary brake:
- Wiring or an electrical connector
- A circuit component
- ECM
The most likely cause of a problem is the wiring or a connector. The least likely cause is a problem with the ECM.
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| Illustration 1 | g01255307 |
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Typical circuit for the auxiliary brake output | |
Test Step 1. Inspect the Electrical Connectors and the Wiring
- Turn the keyswitch to the OFF position.
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Illustration 2 g01186024 Location of the J1/P1 ECM connectors (typical left side engine view) - Thoroughly inspect J1/P1 ECM connector. Also, thoroughly inspect the firewall bulkhead connector. Refer to Troubleshooting, "Electrical Connectors - Inspect" for details.
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Illustration 3 g01255305 Terminal locations at the P1 ECM connector for the auxiliary brake
(P1-12) Output No. 3 - Perform a
45 N (10 lb) pull test on each of the wires in the ECM connector that are associated with the auxiliary brake circuit. - Check the allen head screw for each of the ECM connectors for the proper torque. Refer to Troubleshooting, "Electrical Connectors - Inspect" for the correct torque values.
- Check the circuit's wiring for abrasion and for pinch points.
Expected Result:
All connectors, pins and sockets are completely coupled and/or inserted and the harness and wiring are free of corrosion, of abrasion, and of pinch points.
Results:
- OK - The harness and connectors appear to be OK. Proceed to Test Step 2.
- Not OK - There is a problem with the connectors and/or wiring.
Repair: Repair the wiring or replace the wiring, if necessary. Repair the connectors or replace the connectors, if necessary. Ensure that all of the seals are in the proper place. Ensure that all of the connectors are connected properly. Verify that the problem is resolved.
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Test Step 2. Check the Parameter Settings
- Connect Caterpillar Electronic Technician (ET) to the service tool connector.
- Turn the keyswitch to the ON position.
- Use Cat ET to verify that the following parameters are programmed correctly:
- "Auxiliary Brake"
- "Engine Retarder Minimum Vehicle Speed" (if applicable)
- Turn the keyswitch to the OFF position.
Expected Result:
The parameters are programmed correctly.
Results:
- OK - The parameters are programmed correctly. Proceed to Test Step 3.
- Not OK - At least one parameter is programmed incorrectly.
Repair: Program the parameter to the correct value. Road test the vehicle. Continue with this procedure, if necessary.
STOP
Test Step 3. Check the Status Parameters
- Access the status screen on Cat ET for the accelerator pedal position.
- Turn the keyswitch to the ON position.
- While the status screen is being monitored, depress the accelerator pedal several times and release the accelerator pedal several times.
- Access the status screen on Cat ET for the clutch pedal position switch.
- While the status screen is being monitored, depress the clutch pedal several times and release the clutch pedal several times.
- Turn the keyswitch to the OFF position.
Expected Result:
The status parameters indicate that the circuits are operating correctly.
Note: The clutch pedal position switch is ON when the clutch pedal is depressed. The clutch pedal position switch is OFF when the clutch pedal is released.
Results:
- OK - The status parameters indicate that the circuits are operating correctly. Proceed to Test Step 4.
- Not OK - There is a problem with the accelerator pedal position.
Repair: Refer to Troubleshooting, "Sensor Signal (PWM) - Test".
STOP
- Not OK - There is a problem with the clutch pedal position.
Repair: Refer to Troubleshooting, "Speed Control (Auxiliary) - Test".
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Test Step 4. Use Cat ET to Check the Relay and Solenoids
- Turn the keyswitch to the ON position.
- Click on the following menus in order to access the "Auxiliary Brake Special Test" on Cat ET:
- "Diagnostics"
- "Diagnostic Tests"
- "Auxiliary Brake Special Test"
- Set the dash switches to the "High Braking Level" for the auxiliary brake.
- While you listen for the relay and solenoids to click, cycle the "Auxiliary Brake Special Test" from the ON to OFF and back to ON.
Note: You may need to be near the engine in order to hear the click of the relay and solenoids.
- Turn the keyswitch to the OFF position.
Expected Result:
There is an audible click from the relay and the solenoids.
Results:
- OK - There is an audible click from the relay and the solenoids. The ECM is operating correctly. The circuit is operating correctly.
Repair: If the auxiliary brake is not operating correctly, there is a problem with a component in the circuit. Send the vehicle to the OEM dealer for repair.
STOP
- Not OK - There is no audible click from the relay or the solenoids. Proceed to Test Step 5.
Test Step 5. Use Cat ET to Check the ECM
- Disconnect the J1/P1 ECM connectors.
- Install a 208-0059 Adapter Cable As (70-PIN BREAKOUT) at the J1/P1 ECM connector.
- Connect a voltage test lamp between terminals 65 (−Battery) and 12 (Output 3) at the breakout T.
Note: A multimeter cannot be used in place of the voltage test lamp when the ECM outputs are being tested.
- Turn the keyswitch to the ON position.
- Access the "Auxiliary Brake Special Test" on Cat ET.
- While you monitor the voltage test lamp, cycle the "Special Test" from ON position to OFF and back to ON.
- Stop the "Special Test".
- Turn the keyswitch to the OFF position. Restore the wiring to the original configuration.
Expected Result:
The voltage test lamp turns on when the test is active. The voltage test lamp turns off when the test is not active.
Results:
- OK - The voltage test lamp turns on when the test is active. The voltage test lamp turns off when the test is not active.
Repair: The ECM is operating correctly. The problem is in the components that are provided by the OEM. Inspect the vehicle wiring and then repair the vehicle wiring. Send the vehicle to the OEM dealer for repair, if necessary.
STOP
- Not OK - The voltage test lamp remained off while the test was active.
Repair: There is probably a short circuit in the wiring between the ECM and the relay. However, it is easier to test the ECM first. Perform the following procedure:
- Temporarily connect a test ECM.
- Use the "Auxiliary Brake Output Special Test" on Cat ET in order to test the ECM.
If the problem is resolved with the test ECM, install the suspect ECM. If the problem returns with the suspect ECM, replace the ECM. Verify that the problem is resolved.
If the problem is not resolved with the test ECM, install the original ECM. There is a problem in the wiring between the ECM and the relay.
Send the vehicle to the OEM dealer for repair, if necessary.
STOP