Chart C-2D Rear Vacuum Brake (RVB) Check

Chart C-2D "Rear Vacuum Brake (RVB) Check:

Wiring Diagram - Rear Vacuum Brake (RVB):

CHART C-2D

REAR VACUUM BRAKE (RVB) CHECK


Circuit Description:

The rear vacuum brake is mounted on the carburetor and is controlled by a rear vacuum brake solenoid. When the solenoid is energized, no vacuum is applied to the RVB allowing the plunger to extend (rich). When the solenoid is de-energized, manifold vacuum will be applied to the RVB and pull the plunger in (lean). The RVB solenoid is controlled by the ECM. Battery voltage is supplied by ignition and the ground is completed by the ECM to energize the solenoid. RVB solenoid is energized for about 30 seconds if the engine coolant temperature is below a calibrated value (approximately 20 degrees C or 65 F). This improves cold driveability by keeping the RVB from applying during initial start-up. Poor cold driveability would result if the RVB is applied during initial start-up.


Test Description: Numbers below refer to circled numbers on the diagnostic chart.

1. This step checks for normal RVB operation. With the engine at normal operating temperature, the RVB plunger should retract and pull the choke valve open.

2. This step simulates a cold engine by disconnecting the coolant temperature sensor. After the sensor is disconnected, the ignition must be cycled each time you wish to get the 30 seconds of RVB action.

3. Checks to see if retracted RVB from step one was due to no electrical signal to energize the RVB solenoid, or to a faulty solenoid. The solenoid should be energized with the ignition "ON" and engine stopped. The test light should be "ON" in this step, if the electrical circuit is OK.


4. This step separates an electrical short to ground from a vacuum problem. By de-energizing the solenoid, the RVB should normally retract.

5. If the test light goes "OFF," the ECM quad-driver within the ECM is faulty and the ECM will have to be replaced.