Step # 3 Booster Pump Motor Does Not Run

STEP # 3: BOOSTER PUMP MOTOR DOES NOT RUN
Confirm that the 30 amp brake system fuse is not defective, otherwise replace. Next, connect the pressure switch and pump motor electrical connectors and conduct the following electrical circuit checks:

A. Pressure Switch (Ohmmeter Test)

1. De-pressurize system by pumping brake pedal at least ten times with ignition switch off.
2. Using suitable ohmmeter, all terminals should have continuity with each other. If not, switch must be replaced.

B. Booster Motor (Ohmmeter Test)

1. Meter should show resistance between motor terminals "B" and "D".
2. Meter should indicate open circuit between motor terminals "K" and "C".
3. Replace motor and pump assembly if above conditions are not met.

C. Pressure Switch Connector and Harness (Volt-Ohmmeter Test)

1. Make sure parking brake is released.
2. With ignition on, using a suitable ohmmeter, pressure switch terminal "A" should show continity with motor terminal "D".
3. With ignition on, using a suitable voltmeter, pressure switch terminal "B" should be 12 volts relative to vehicle chassis ground.

4. If voltage at pressure switch terminal "B" is low or zero, disconnect electrical connector from combination valve. If voltage at "B" is now normal, combination valve wire harness or switch is at fault. If voltage at terminal "B" remains low or zero, red brake warning light circuit is defective.
5. If any of the above tests fail, wire harness and electrical connections must be replaced or repaired.

D. Pump Motor Connector and Harness (Volt-Ohmmeter Test)

1. With ignition on, using suitable ohmmeter, motor terminal "A" should show continuity with vehicle chassis ground.
2. With ignition on, using suitable voltmeter, motor terminal "B" should show 12 volts relative to chassis ground.
3. With ignition on, using suitable voltmeter, motor terminal "C" should show 12 volts relative to chassis ground.

Note:
This is the main electrical supply to the motor. If voltage is not present, check the 30 amp brake system fuse.

4. Recheck terminal "D" as per Step 3.C.1 above.
5. If any of the above tests fail, wire harness and electrical connections must be replaced or repaired.