Rear Blower Runs At Only One Speed

NOTE: After making any repairs, perform BODY VERIFICATION TEST under VERIFICATION TESTS.

Turn the ignition off. Disconnect the Rear Blower Motor Power Module C1 harness connector. See Figure. Disconnect the Automatic Temperature Control Module C2 harness connector. See Figure. Measure the resistance between ground and the Rear Blower Motor Control circuit. If the resistance is below 10K ohms, repair the Rear Blower Motor Control circuit for a short to ground. With the DRBIII®, reset the ATC after repair/replacement is complete. If the resistance is above 10K ohms, go to the next step.

NOTE: The Rear Blower Motor Power Module provides a 10-volt signal to the ATC Module over the Rear Blower Motor Control circuit. The ATC Module provides a variable duty cycle ground to the 10-volt signal based on input from the Rear Blower switch.

NOTE: When the Rear Blower switch is set to LO speed, the ATC Module provides a short duty cycle (less time grounding the signal voltage).

NOTE: As higher blower speeds are requested, the ATC Module increases the duty cycle (more time grounding the signal voltage).

NOTE: When the Rear Blower switch reaches HI speed, the duty cycle increases to where the signal pattern is almost a flat line (with brief voltage spikes).

Turn the ignition off. Reconnect all previously disconnected components. Connect the DRBIII® X10 Scope Probe (CH7061) to Channel 1 on the DRBIII®. Select the following from the DRBIII® menu: PEP Module Tools; Lab Scope; Live Data; and Lab Scope. Press F2 and then press Enter. Using the left/right arrow keys, set the time to 10ms/Div. Press F2. Using the up/down arrow keys, set the voltage range to +20v. Press Enter to move the cursor to Probe. Using the up/down arrow keys, set the Probe to X10. Press F2. Start the engine. Press the ATC Power switch on. Set the Rear System switch on the ATC module to the Rear Control position. Using the X10 Scope Probe, back probe the Rear Blower Motor Control circuit in the Automatic Temperature Control Module C2 harness connector. Set the blower switch on the Rear ATC Control panel to LO speed then slowly move it to HI speed while observing the DRBIII® display. The voltage on the scope pattern should cycle from approximately 2 volts (duty cycle on) to 10 volts (duty cycle off) for all blower speeds. The duty cycle pattern should change smoothly as the Rear Blower switch is turned from LO to HI. The duty cycle on time should change from approximately 8 ms at LO speed, to 15 ms at medium speed, and to 25 ms at HI speed. If the DRBIII® displays the voltage range and duty cycle as described above, replace the Rear Blower Motor Power Module. See appropriate AUTOMATIC A/C-HEATER SYSTEMS article in HVAC. With the DRBIII®, reset the ATC after repair/replacement is complete. If the DRBIII® does not display the voltage range and duty cycle as described above, replace the Automatic Temperature Control Module. See A/C-HEATER CONTROL PANEL under REMOVAL & INSTALLATION. With the DRBIII®, reset the ATC after repair/replacement is complete.