U1500 - Control Assembly, Auto HVAC

Schematic Reference HVAC Air Delivery/Temperature Control Schematics

Steps 1-8:

Steps 9-10:

CIRCUIT DESCRIPTION
The front HVAC (ECC) module communicates with the rear (AUX) HVAC control module via the Keyboard Data Display (KDD). This communication link is only used by the two HVAC control modules with Electronic Climate Control (ECC).

CONDITIONS FOR RUNNING THE DTC
The ignition is turned ON.

CONDITIONS FOR SETTING THE DTC
- There is a loss of communications between the front HVAC (ECC) module and the rear (AUX) HVAC control module. These circuits are 2819 (KDD) data and 2820 (KDD) clock data.
- The above conditions are present for 3 seconds.

ACTION TAKEN WHEN THE DTC SETS
The automatic portion of the heater and A/C system will be disabled.

The checksum of each KDD message received from the rear HVAC module will be calculated and checked against the checksum received as part of that message. If the checksums do not match, a checksum error will be considered to have occurred and DTC 1500 will be set as a current code.

CONDITIONS FOR CLEARING THE DTC
- The DTC will become history if the heater and A/C programmer no longer detects a failure.
- The history DTC will clear after 50 fault free ignition cycles.
- The DTC can be cleared with a scan tool.

IMPORTANT
As a result of the conditions which set this DTC, the rear system will lose all automatic control. When this DTC is set, the rear system blower can be controlled manually by the fan speed control knob on the Rear Aux Control Module (head).

TEST DESCRIPTION
The number(s) below refer to the step number(s) on the diagnostic table.
2. This step verifies that the KDD data circuit is functioning.
3. This test verifies that the KDD clock data circuit is operating at the correct frequency.