System Diagnostics

Fault detection is through active and stored Diagnostic Trouble Codes (DTCs)

• DTCs are displayed by the DRBIII®.
• Active DTCs are those which currently exist in the system. The condition causing the fault must be repaired in order to clear this type of DTC.
• Stored DTCs are those which occurred in the system since the ATC Module received the last "clear diagnostic info" message.

The AC Cool down Test:

• Is actuated with the DRBIII®.
• Checks A/C system performance based on evaporator temperature sensor input.
• Forces the ATC to initiate the Cabin Heater Activation request when the vehicle is equipped with a Diesel Cabin Heater Assist.
  WARNING: DO NOT OPERATE THE DCHA IN AN ENCLOSED AREA SUCH AS A GARAGE THAT DOES NOT HAVE EXHAUST VENTILATION FACILITIES. ALWAYS VENT THE DCHA's EXHAUST WHEN OPERATING THE DCHA. REFER TO VENTING THE DCHA'S EXHAUST  UNDER CABIN HEATER IN THE GENERAL INFORMATION PORTION OF THIS SERVICE INFORMATION FOR PROPER EXHAUST VENTING INSTRUCTIONS. FAILURE TO FOLLOW THESE INSTRUCTIONS CAN RESULT IN PERSONAL INJURY OR DEATH.
  CAUTION: Do not activate the A/C Cool down Test with the engine off. Failure to follow these instructions can result in internal damage to the DCHA Heater Module.
  • The FCM monitors the PCI bus for the Cabin Heater Activation request. The request carries the status bit that the FCM requires to activate its' Cabin Heater Assist Control output. This output is a low side driver (coming from FCM pin 15) which supplies a ground signal to the Cabin Heater (pin 5). When the Cabin Heater receives this ground signal input, it interprets this as an activation signal.
  • The FCM low side driver is also capable of diagnostic sensing. The driver will sense an open circuit when the driver is off, and will sense a short to voltage when the driver is on. The FCM will set DTCs for both of these types of faults. For additional information, refer to CABIN HEATER  under General Information and Diagnostic Procedures in the service information.
• Will not run the air conditioning portion of this test if ambient temperature is below 12°C (53°F).
  • The forced Cabin Heater activation will occur even if the air conditioning portion of this test fails to initiate because AC Cool down test criteria was not met. When activated, the DCHA will operate for approximately seven minutes. Three minutes to run and four minutes to purge.
• Will pass the air conditioning portion of this test if the evaporator temperature drops 6.7°C (20°F) within two minutes of starting this test.
• Messages (air conditioning related only) display on the DRBIII® after running this test.
  • These messages will clear after paging back out of this test. Therefore, it is important to note all of the AC Cool down test messages before doing so.
  • All Cabin Heater-related DTCs display under Cabin Heater, Read DTCs.
• Will cause the DELAY and Snowflake VF segments on the ATC to flash for 162 seconds (RG with DCHA only). If the air conditioning portion of this test fails, the DELAY and Snowflake VF segments will continue to flash until the vehicle is driven more than three miles.