Cummins ISX11.9 CM2250 – Service Manual 019-262   Air Conditioning Pressure Switch Circuit

Resistance Check

TOC

 CAUTION 

The leads must fit tightly in the connector without expanding the pins in the connector otherwise the connector will be damaged.

Disconnect the original equipment manufacturer (OEM) harness connector from the electronic control module (ECM). Insert one of the test leads into the switch return of the OEM harness connector and connect the alligator clip to the multimeter probe. Insert the other lead into the air conditioning pressure switch signal pin of the harness connector and connect the alligator clip to the other multimeter probe.

When the system head pressure is low, the multimeter
must show a closed circuit (10 ohms or less). If the circuit is
not closed, inspect the switch return and the air conditioning pressure switch signal wire for an open circuit, provided that the switch has been previously checked. Refer to the OEM troubleshooting and repair manual for repair procedures. If the resistance is within the specification, the switch return and the air conditioning pressure switch wire
must be checked for a short circuit to ground, a short circuit from pin-to-pin, and a short circuit to an external voltage source.

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Check for Short Circuit to Ground

TOC

To isolate the air conditioning switch circuit when checking for an electrical short, disconnect the OEM harness from the ECM.

Adjust the multimeter to measure resistance. When the system head pressure is low, insert a test lead into the air conditioning pressure switch signal pin of the OEM harness connector and connect it to a multimeter probe. Touch the other multimeter probe to the engine block ground. Measure the resistance.

The multimeter
must show an open circuit (100k ohms or more). If the circuit is
not open, there is a short circuit to ground in the air conditioning switch circuit, provided that the switch has been previously checked. Repair or replace the wire connected to the air conditioning pressure switch signal pin in the OEM harness. Refer to Procedure
019-071.

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Check for Short Circuit from Pin to Pin

TOC

Check for a short circuit from pin-to-pin. Isolate the air conditioning circuit by removing the OEM harness from the ECM. Insert the lead into the air conditioning pressure switch signal pin. Connect the alligator clip to the multimeter. With the other lead inserted into the switch return pin, measure the resistance. The multimeter
must show an open circuit (100k ohms or more).

Allow the head pressure to drop and remove the lead from the air conditioning pressure switch signal pin and check all other pins. When the system head pressure is low, measure the resistance. The multimeter
must show an open circuit (100k ohms or more). If the circuit is
not open, there is a short circuit between the air conditioning pressure circuit and any pin that shows a closed circuit. Repair or replace the wires in the engine harness. Refer to Procedure
019-043. Repair or replace the wires in the OEM harness. Refer to Procedure
019-071.

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Check for Short Circuit to External Voltage Source

TOC

Turn the keyswitch to the ON position. When the system head pressure is low, adjust the multimeter to measure VDC. Insert a test lead into the air conditioning pressure switch signal pin of the OEM connector and attach it to a multimeter probe. Touch the other multimeter probe to the engine block ground. Measure the voltage. The voltage
must be 1.5 VDC or less. If the voltage is
not correct, there is an external voltage source connected to the circuit, or there is a short circuit between the air conditioning pressure switch circuit and a wire carrying power in the engine or OEM harness. Remove the voltage source, or repair or replace the wiring in the OEM harness. Refer to Procedure
019-071. Remove the voltage source or repair or replace the wires in the engine harness. Connect all components after completing the repair.

NOTE: If the air conditioning pressure switch circuit was approved in all of the previous tests, it is functioning properly.

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Last Modified:  30-May-2008