BUS +/- Signals Open Or No Response From A/C-Heater Control Module

Possible Causes:

Attempt to communicate with CTM.
Replace Fuse No. 25.
A/C-Heater Control Module shorted to ground.
Fused Ignition Switch Output circuit shorted to ground.
Junction Block shorted to ground.
Ground circuit(s) open.
Fused Ignition Switch Output circuit open.
Open PCI Bus circuit.
A/C-Heater Control Module.

CAUTION: Always turn ignition off before disconnecting or connecting any control module connector. Failure to do so could result in damage to control module.

NOTE: Ensure vehicle being tested has a fully charged battery. If it does not, false symptoms may occur.

NOTE: For component location, see appropriate illustration under COMPONENT LOCATIONS . For terminal identification, see appropriate illustration under CONNECTOR IDENTIFICATION . For circuit and wire color identification, see WIRING DIAGRAMS . For component removal and installation procedures, see appropriate component under REMOVAL & INSTALLATION .

Testing:

Turn ignition off. Connect DRBIII(R) to DLC. Turn ignition on. Using DRBIII(R), select BODY, then BODY COMPUTER. Is the DRBIII(R) able to identify or communicate with the CTM? If yes, go to next step. If no, see PCI BUS COMMUNICATION FAILURE .
Turn ignition off. Remove and inspect Fuse No. 25 (10-amp) from Instrument Panel Junction Block. Is the fuse open? If yes, go to next step. If no, go to step 6 .
Replace Instrument Panel Junction Block Fuse No. 25. Turn ignition on. Remove and inspect Fuse No. 25. Is the fuse open? If yes, go to next step. If no, check circuit between Instrument Panel Junction Block C11 harness connector terminal No. 5 and A/C-Heater Control Module C2 harness connector terminal No. 7 for an intermittent short to ground. Repair as necessary. Perform BODY VERIFICATION TEST under VERIFICATION TESTS.
Turn ignition off. Replace Instrument Panel Junction Block Fuse No. 25. Disconnect A/C-Heater Control Module harness connectors. Turn ignition on. Remove and inspect fuse No. 25. Is fuse open? If yes, go to next step. If no, replace A/C-Heater Control Module. Perform BODY VERIFICATION TEST under VERIFICATION TESTS.
Turn ignition off. Disconnect Instrument Panel Junction Block C11 harness connector. Measure resistance between ground and A/C-Heater Control Module C2 harness connector terminal No. 7. Is resistance less than 5 ohms? If yes, repair Fused Ignition Switch Output circuit for a short to ground. Perform BODY VERIFICATION TEST under VERIFICATION TESTS. If no, replace Instrument Panel Junction Block. Perform BODY VERIFICATION TEST under VERIFICATION TESTS.
Reinstall Fuse No. 25. Disconnect A/C-Heater Control Module harness connectors. Using a 12-volt test light connected to 12 volts, probe A/C-Heater Control Module C2 harness connector terminal No. 1 and A/C-Heater Control Module C3 harness connector terminal No. 4. Does test light illuminate at both circuits? If yes, go to next step. If no, repair open in the Ground circuit(s). Perform BODY VERIFICATION TEST under VERIFICATION TESTS.

NOTE: Ensure Fuse No. 25 is installed in the Instrument Panel Junction Block.

Turn ignition on. Using 12-volt test light connected to ground, probe A/C-Heater Control Module C2 Harness connector terminal No. 7. Does test light illuminate? If yes, go to next step. If no, repair open in Fused Ignition Switch Output circuit. Perform BODY VERIFICATION TEST under VERIFICATION TESTS.

NOTE: Ensure there is PCI Bus communication with other modules on the vehicle before proceeding. If not, see PCI BUS COMMUNICATION FAILURE .

Connect Scope Input Cable (CH7058) to channel one connector on DRBIII(R). Connect Cable-To-Probe Adapter (CH7062) and Black and Red test probes to scope input cable. Using the DRBIII(R), select PEP MODULE TOOLS, LAB SCOPE, LIVE DATA and 12 VOLT SQUARE WAVE. Press F2 for SCOPE. Press F2 and use DOWN arrow to set voltage scale to 20 volts. Set probe to x10. Press F2 again when complete. Connect Black test probe to ground. Connect Red test probe to A/C-Heater Control Module C1 harness connector terminal No. 2. Turn ignition on. Observe DRBIII(R) lab scope. Does voltage toggle from zero to approximately 7.5 volts? See Fig 1 -Fig 3 . If yes, go to next step. If no, repair open in PCI Bus circuit. Perform BODY VERIFICATION TEST under VERIFICATION TESTS.
If there are no possible causes remaining, replace A/C-Heater Control Module. Perform BODY VERIFICATION TEST under VERIFICATION TESTS.

Fig 1: Identifying PCI Bus Short To Ground Or Open Circuit Waveform

Courtesy of DAIMLERCHRYSLER CORPORATION

Fig 2: Identifying PCI Bus Short To 12 Volts Waveform

Courtesy of DAIMLERCHRYSLER CORPORATION

Fig 3: Identifying Typical PCI Bus Waveform

Courtesy of DAIMLERCHRYSLER CORPORATION