| Subject: | Revised PCM Controlled Air Conditioning (A/C) Circuit Diagnosis and Revised
Information Sensors/Switches Description |
| Models: | 2000 Chevrolet Impala |
This bulletin is being issued to revise the Circuit Description of the PCM Controlled
Air Conditioning (A/C) Circuit Diagnosis and the PCM Controlled Air Conditioning Description
of the Information Sensors/Switches Description in the Engine Controls sub-section
of the Service Manual. Please replace the current information in the Service Manual
with the following information.
The following information has been updated within SI. If you are using a paper
version of this Service Manual, please make a reference to this bulletin on the affected
page.
Circuit Description (from PCM Controlled Air Conditioning (A/C) Circuit Diagnosis)
When A/C is selected through the HVAC controller, a 12 volt
signal is supplied to the body control module (BCM). The BCM in turn signals the powertrain
control module (PCM) through the Class 2 serial data circuit that A/C has been
requested. The PCM then determines if conditions suitable for A/C operation are present.
If the PCM determines that the engine operating conditions are within the acceptable
ranges, the PCM will enable the A/C relay.
The PCM provides a ground path for
the A/C relay coil. This action closes that A/C compressor relay contacts and B+
voltage is supplied to the compressor clutch coil. The compressor clutch coil has
its own ground path.
The PCM will enable the A/C unless any of the following conditions are met:
| • | The throttle position is more than 96 percent . |
| • | The A/C pressure is more than 472 psi (4.9 volts) or less
than 9 psi (0.1 volt) as determined by the A/C pressure sensor. |
| • | The ignition voltage is less than 10.0 volts. |
| • | The engine speed is more than 4700 RPM. |
| • | The engine coolant temperature (ECT) is more than 124°C (255°F). |
| • | The intake air temperature (IAT) is less than 5°C (41°F). |
| • | A Class 2 communication error occurs between the HVAC controller
and the PCM. |
A/C Request PCM Input (from Information Sensors/Switches Description)
The A/C request signal indicates to the PCM that an A/C mode is selected at
the A/C control head. The PCM uses this information to adjust the idle speed before
turning ON the A/C clutch. If this signal is not available to the PCM, the A/C compressor
will be inoperative.
