Pinpoint Test U: The Brake Vacuum Pump Runs Continuously
Anti-Lock Control
Pinpoint Tests
Pinpoint Test U: The Brake Vacuum Pump Runs Continuously
Refer to Wiring Diagram Set 42, Vehicle Dynamic Systems for schematic and connector information. Diagrams By Number
Normal Operation
On vehicles equipped with a 3.5L engine, an electrically operated pump is used to supply additional vacuum to the brake booster. The ABS module controls the pump through a relay located in the Battery Junction Box (BJB). When activated, the relay supplies voltage to the brake vacuum pump. The ABS module uses a sensor mounted on the brake booster to monitor brake booster vacuum. The ABS module activates the pump when the vacuum in the brake booster is below 48.23 kPa (12 in-Hg) and deactivates the pump once the vacuum in the brake booster reaches a minimum of 68.32 kPa (17 in-Hg).
If the sensor on the brake booster suffers an internal failure, the vacuum pump may operate continuously (or it may fail to operate) and one or more DTCs is et in the ABS module. If a large vacuum leak exists in the brake booster or vacuum hoses the brake vacuum pump runs continuously. Areas of high elevation can also cause the brake vacuum pump to run continuously. Altitude affects vacuum readings, subtract 4.0193 kPa (1 in-Hg) from the reading for every 304.8 m (1,000 ft) of elevation above sea level.
- DTC P258A (Brake Booster Vacuum Pump Relay Circuit Failure) - This DTC sets in continuous memory if the ABS module detects a short to ground in the relay control circuit.
- DTC P258B (Brake Booster Vacuum Pump Plausibility Failure) - This DTC sets in continuous memory if the ABS module detects that the desired vacuum level is not reached in a specified time in the brake booster when the brake vacuum pump is commanded on. This could be caused by one or more leaks in the vacuum system or an internal failure of the ABS module.
This pinpoint test is intended to diagnose the following:
- Wiring, terminals or connectors
- Brake booster vacuum sensor
- Brake vacuum pump relay
- Brake vacuum pump
- ABS module
PINPOINT TEST U: THE BRAKE VACUUM PUMP RUNS CONTINUOUSLY
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U1 CHECK THE DTCs FROM THE ABS MODULE SELF-TEST
- Review the DTCs retrieved and recorded during Inspection and Verification.
- Are DTCs C109D, C109E and C109F present?
Yes
DIAGNOSE any vacuum sensor DTCs first. GO to Pinpoint Test I. Pinpoint Test I: DTCs C109D, C109E and C109F
No
GO to U2.
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U2 CHECK THE BRAKE BOOSTER AND VACUUM HOSES FOR LEAKS
- Verify that all the vacuum hoses are connected to their ports.
- Inspect the following items for leaks:
- brake booster body
- brake booster-to-master cylinder seal
- brake booster-to-bulkhead seal
- brake booster vacuum sensor seal
- brake booster-to-vacuum pump hose
- Are all hoses connected and are all items free of any leaks?
Yes
GO to U3.
No
CONNECT and/or INSTALL new components as necessary. CLEAR the DTCs. REPEAT the self-test.
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U3 CHECK THE BRAKE BOOSTER VACUUM LEVEL (BBVACLVL) PID
- Enter the following diagnostic mode on the scan tool: DataLogger - ABS Module.
- Start the engine.
- Monitor the BBVACLVL PID while pressing and releasing the brake pedal.
- Does the PID display change when the pedal is pressed?
Yes
GO to U4.
No
GO to Pinpoint Test I. Pinpoint Test I: DTCs C109D, C109E and C109F
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U4 CHECK THE BRAKE VACUUM PUMP RELAY
- Ignition OFF.
- Disconnect: Brake Vacuum Pump Relay.
- Install a known good relay in place of the brake vacuum pump relay.
- Ignition ON.
- Does the brake vacuum pump run continuously?
Yes
REMOVE the known good relay. GO to U5.
No
REMOVE the known good relay. INSTALL a new brake vacuum pump relay. CLEAR the DTCs. REPEAT the self-test.
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U5 CHECK THE RELAY CONTROL CIRCUIT FOR A SHORT TO GROUND
- Disconnect: ABS Module C135.
- Measure the resistance between the brake vacuum pump relay circuit CCB30 (GY/VT), harness side and ground.
- Is the resistance greater than 10,000 ohms?
Yes
GO to U6.
No
REPAIR circuit CCB30 (GY/VT). CLEAR the DTCs. REPEAT the self-test.
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U6 CHECK THE VACUUM IN THE BRAKE BOOSTER
- Ignition OFF.
- Connect: ABS Module C135.
- Ignition ON.
- Enter the following diagnostic mode on the scan tool: DataLogger - ABS Module.
- NOTE: Subtract 4.0193 kPa (1 in-Hg) from the reading for every 304.8 m (1,000 ft.) of elevation above sea level.
- Monitor the BBVACLVL PID.
- Is the brake vacuum reading higher than 68.32 kPa (17 in-Hg)?
Yes
GO to U7.
No
The elevation in your area is causing the brake vacuum pump to run continuously. This is a normal condition.
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U7 CHECK FOR CORRECT ABS MODULE OPERATION
- Check ABS module C135 for:
- corrosion
- damaged pins
- pushed-out pins
- Connect: ABS Module C135.
- Connect: Brake Vacuum Pump Relay.
- Make sure each connectors seat correctly, then operate the system and verify the concern is still present.
- Is the concern still present?
Yes
INSTALL a new ABS module. REFER to Anti-Lock Brake System (ABS) Module Service and Repair. TEST the system for normal operation.
No
The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector. CLEAR the DTCs. REPEAT the self-test.
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