With Powertrain Control Module (PCM)

Control Module System:

PCM & VCM Diagram:

DESCRIPTION
The Powertrain Control Module (PCM) will be commonly referred to as a control module. The control module is designed to maintain exhaust emission levels to Federal or California standards, while providing good driveability and fuel efficiency. The control module monitors numerous engine and vehicle functions and controls the following operations:

- Fuel Control
- Ignition Control (IC)
- Knock Sensor (KS) System
- Exhaust Gas Recirculation (EGR)
- Automatic transmision shift functions

OPERATION
The control module supplies a buffered 5 or 12 volts of power to various sensors or switches. This is done through resistance in the control module which is so high in value that a test light will not light when connected to the circuit. In some cases, even an ordinary shop voltmeter will not give an accurate reading because its internal resistance is too low. Therefore, the use of a 10 megohm input impedance digital voltmeter J 39200 or equivalent is necessary to assure accurate voltage readings.

The input/output devices in the control module include analog to digital converters, signal buffers, counters and special drivers. The control module controls output circuits such as the injectors, EGR, A/C clutch relay, etc. by controlling the ground circuit through transistors or a device called a Quad-Driver Module (QDM) in the control module.

DIAGNOSTIC INFORMATION
The diagnostic trouble codes and functional checks in Computers and Control Systems are designed to locate a faulty circuit or component through logic based on the process of elimination. The charts are prepared with the requirement that the vehicle functioned correctly at the time of assembly or without multiple failures.

There is a continual self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures contained in this manual. The language for communicating the source of a malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the control module, a diagnostic trouble code is set and the Malfunction Indicator Lamp (MIL) Service Engine Soon may be illuminated.

MALFUNCTION INDICATOR LAMP (MIL)
The Malfunction Indicator Lamp (MIL) "Service Engine Soon" is in the instrument panel and has the following functions:

- It informs the driver that a fault has occurred, and the vehicle should be taken for service as soon as reasonably possible.
- It displays DTC stored by the control module, helping the technician diagnose system faults.
- It indicates "Open Loop" or "Closed Loop" operation.

As a bulb and system check, the lamp will come "ON" with the ignition "ON" and the engine not operating. when the engine is started, the lamp will go "OFF" If the light remains "ON," the self-diagnostic system has detected a problem. If the fault goes away, the light will go out in most cases after 1O seconds, but a DTC will remain stored in the control module memory.

When the lamp remains "ON" while the engine is operating, or when a malfunction is suspected due to a driveability or emissions problem, the "On-Board Diagnostic System Check MUST BE PERFORMED. Testing and Inspection

DATA LINK CONNECTOR SCAN TOOLS
The control module will communicate a variety of information through the Data Link Connector (DLC) terminal "9". This data is transmitted at a high frequency which requires a Tech 1 diagnostic (scan) tool for interpretation. There are several other scan tools available for displaying this same information.

The Tech 1 scan tool can be very useful in obtaining information which would be difficult or impossible to obtain with other equipment.

Tech 1 and scan tools do not make the use of diagnostic charts unnecessary, or indicate exactly where a problem is in a particular circuit. These charts incorporate diagnostic procedures using a scan tool where possible and most charts require the use of a Tech 1 scan tool when it is applicable.