Control Pressure Tests

CONTROL PRESSURE TESTS
There are two methods of performing the control pressure test. One is to perform the test using the engine vacuum. The second method is to use a remote vacuum source such as the one provided in a distributor tester or a hand operated vacuum pump.

Engine Vacuum Method

NOTE: When the vacuum diaphragm unit is operating properly and the downshift linkage is properly adjusted, all the transmission shifts (automatic and kickdown) should occur satisfactorily. If the shifts do not occur within satisfactory limits or the transmission slips during shift points, the following procedure is suggested to determine engine, transmission, linkage, vacuum diaphragm unit, or valve body problems.

1. Attach a tachometer to the engine and a vacuum gauge to the transmission vacuum line at the vacuum unit.

Control Pressure Connecting Point:

2. Attach a pressure gauge to the control pressure outlet at the transmission.

CAUTION: Pressure gauges effect the shift quality of the transmission. Care should be taken not to accelerate or decelerate rapidly. Possible transmission failure could result.

3. Firmly apply the parking brake and start the engine. If the vehicle is equipped with a vacuum release parking brake, the service brakes should be applied since the parking brake will release when the engine is started.
4. Adjust engine idle speed. If the idle speed cannot be brought within satisfactory limits by adjusting the idle screw, check the throttle and downshift linkage for a binding condition. If the linkage is satisfactory, check for vacuum leaks in the transmission diaphragm unit and its connecting tubes and hoses. Check all other vacuum operated units (such as power brake) for vacuum leaks.

Control Pressure Test Chart:

5. Refer to the Control Pressure Test Chart, for diagnostic guides of what components are inoperative when the control pressure test are not within specifications.

Vacuum Pump Method
1. Disconnect and plug vacuum line at vacuum diaphragm unit.
2. Attach vacuum pump to vacuum diaphragm. Apply parking and service brakes.
3. Start engine.
4. Set vacuum at 15 inches, then record control pressure in all selector positions.
5. Run engine up to 1,000 RPM, then reduce vacuum to 1 inch.
6. Again record control pressure in all selector positions.
7. Refer to the Control Pressure Test Chart, for diagnostic guides and control pressure test specifications.