COVER-BRUSH 5A5897 - Caterpillar

Pressure Switch Tests
Test Procedure for 355-3148 Pressure Switch
Pressure switches are used to protect the compressor. The high pressure and low-pressure switches have three possible switch positions which are shown below:OPEN - System pressure is low.CLOSED - System pressure is correct.OPEN - System pressure is high.The two OPEN positions of the pressure switch protect the air conditioning system. The OPEN position prevents the compressor from actuating when potentially harmful pressures exist in the system. The CLOSED position is the normal operating switch position. The following table contains the logic for the pressure switches:
Table 2
Pressure     Range     Contact Position (Pressure Rise)     Contact Position (Falling Pressure)    
High Position 3     Above
2800 kPa (406 psi)     OPEN     OPEN    
Hysteresis Band (1)    
1750 kPa (254 psi) to
2800 kPa (406 psi)     CLOSED     OPEN    
Middle (Position 2)    
275 kPa (40 psi) to
1750 kPa (254 psi)     CLOSED     CLOSED    
Hysteresis Band (1)    
170 kPa (25.0 psi) to
275 kPa (40 psi)     OPEN     CLOSED    
Low Position 1    
0 kPa (0 psi) to
170 kPa (25.0 psi)     OPEN     OPEN    
( 1 ) Pressure must pass through the hysteresis band to provide the correct conditions for the switch contact.
Illustration 6 g00651406
Use the following procedures to test the pressure switch:
Disconnect the pressure switch from the machine harness.
Connect the 257-9140 Digital Multimeter to the two connector pins. Set the multimeter to the OHMS test mode.
Measure the resistance of the pressure switch.
If the system pressure is between 275 kPa (40 psi) and 1750 kPa (254 psi), the multimeter will display a value that is less than 1 ohm.
If the resistance that is displayed is a value of OL (high resistance), and the system pressure is within acceptable limits, replace the pressure switch.
When the pressure switch fails, test the arc suppressor for proper operation.
If the pressure switch is removed, make sure that the wires on the new pressure switch are tie-wrapped. Securing the pressure switch prevents damage that may be caused by vibration.Note: The multimeter may display OL if the contacts on the pressure switch are dirty. The contacts on the pressure switch can become dirty if the arc suppressor is no longer functioning.Note: The multimeter will display OL if the pressure switch is not threaded into the air conditioning system.Test Procedures for Arc Suppressors:
189-3158 Arc Suppressor
Illustration 7 g00651378
189-3158 Arc Suppressor
Place the multimeter in the DIODE test mode.
Place the red probe in the red terminal (positive terminal) of the multimeter.
Place the black probe in the black terminal (negative terminal) of the multimeter.
Place the red probe on Pin 2 of the connector on the arc suppressor. Place the black probe on Pin 1 of the connector.
The multimeter should display a value between 0.5 VDC and 1.0 VDC.
Place the red probe on Pin 1 of the connector on the arc suppressor. Place the black probe on Pin 2 of the connector on the arc suppressor.
The multimeter should display OL.
If the above conditions do not exist, the component