Diagnostic Instructions
DTC Descriptor
DTC P255F: Air Conditioning (A/C) Request Circuit Performance
Diagnostic Fault Information
Circuit
| Short to Ground
| Open/High Resistance
| Short to Voltage
| Signal Performance
|
A/C Request Circuit
| --
| --
| --
| P255F
|
Circuit Description
The HVAC control module controls the A/C request circuit by grounding the A/C request circuit via an internal solid state device called a driver. The primary function of the driver is to supply the ground for the component being controlled. The driver
has a fault line which is monitored by the engine control module (ECM). When the HVAC control module is commanding a component ON, the voltage of the a/c request circuit should be near 0 volts. When the HVAC control module is commanding the request circuit
to a component OFF, the voltage potential of the circuit should be near battery voltage. If the fault detection circuit senses a voltage other than what is expected, this DTC will set. This DTC is communicated from the HVAC control module to the ECM over the
pulse width modulated signal (PWM) on the A/C request circuit. There are three A/C request states the HVAC control module can communicate to the ECM. These states are: A/C Off, Normal A/C On, and Economy A/C On.
The ECM will monitor the request circuit for the following:
When the ECM detects any of the above malfunctions, this DTC is set and the affected driver is disabled.
Conditions for Running the DTC
| • | The ignition voltage is between 9-18 volts. |
| • | The engine speed is more than 80 RPM. |
| • | The HVAC control module driver transitions from ON to OFF or from OFF to ON. |
Conditions for Setting the DTC
The ECM detects an invalid duty cycle through the A/C request circuit. The following shows the invalid duty cycle ranges.
Any duty cycle falling between the ranges listed below will set DTC P255F.
| • | 0-10 percent Open or shorted to ground |
| • | 15-25 percent Economy A/C Off and Normal A/C Off |
| • | 35-45 percent Economy A/C On |
| • | 55-65 percent Normal A/C On |
| • | 75-85 percent HVAC Fault (Stuck button or faulty mode potentiometer) |
| | The DTC B1020 Auxiliary Electronic Control Unit (ECU) Performance can set also within this range. |
| • | 90-100 percent Short to Battery |
Action Taken When the DTC Sets
| • | The ECM will not illuminate the malfunction indicator lamp (MIL). |
| • | The ECM will store conditions which were present when the DTC set as Failure Records data only. This information will not be stored as Freeze Frame data. |
Conditions for Clearing the DTC
| • | A History DTC clears after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using a scan tool. |
Diagnostic Aids
Important: Be sure to verify that the ECM engine grounds are secure and clean.
If DTC P255F cannot be duplicated, reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to set occurs. This may assist in diagnosing the condition.
If the condition is not present, refer to
Testing for Intermittent Conditions and Poor Connections
.
Reference Information
Schematic Reference
HVAC Schematics
Connector End View
HVAC Connector End Views
Description and Operation
Electrical Information Reference
Scan Tool Reference
Circuit System Verification
Ignition ON, observe the ECM scan tool A/C request parameter while pressing the A/C request switch of the HVAC control head. The reading should change between Yes and No A/C request value.
Circuit System Testing
- Ignition OFF, disconnect the HVAC harness connector at the A/C switch.
- Test for less than 1.0 ohm of resistance between the HVAC control module ground circuit terminal C1 pin 12 and ground.
| ⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
- Ignition ON, verify the scan tool ECM A/C request parameter is No or open.
| ⇒ | If not the specified value, test the A/C request signal circuit terminal HVAC control module terminal C1 pin 8 for a short to ground. If the circuit tests normal, replace the HVAC control module. |
- Install a 3A fused jumper wire between the signal circuit terminal C1 pin 8 and the ground circuit terminal C1 pin 12. Verify the scan tool ECM A/C request switch parameter is YES or closed.
| ⇒ | If not the specified value, test the A/C request signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the HVAC control module. |
Repair Instructions
Perform the
Diagnostic Repair Verification
after completing the repair.
Control Module References
for HVAC replacement, setup, and programming
