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Reference Manual


Chapter 10: Emissions Equipment: ECM ALDL


Located under the radio and to the left of the ash tray, is the ALDL (Assembly Line Diagnostic Link). Sometimes also refered to as the ALCL (Assembly Line Communications Link).

ALDL connector
The ALDL connector

The ALDL provides an easily accessible link to the ECM. This link can be used to read stored trouble codes, observe ECM and sensor operation or to put the ECM in a different mode for troubleshooting or service. Trouble codes can be retrieved as simply as by shorting an ALDL terminal to ground and counting flashes of the SES lamp. Or with the use of a Scan Tool, which reads the serial data output by the ECM, will allow not only the retrieval of stored trouble codes, but also the provide ability to observe the values each information sensor is sending to the ECM and the desired state of each ECM controlled device.

ALDL connector terminals
The ALDL terminals

Position Function

A Ground
B Diagnostic Terminal
C AIR Switching Solenoid
D SES Lamp (Serial Data)
E No Connection
F TCC Solenoid
G No Connection
H EGR Control Solenoid
I No Connection
J No Connection
K No Connection
L No Connection
M No Connection

Terminal Functions:

Ground (A) Connects to the Instrument Panel Ground
Diagnostic Terminal (B) Four modes of operation are available by use of the Diagnostic Terminal. Each mode is selected by resistance to ground of the Diagnostic Terminal:
  • Normal or Open mode (20K ohm or greater resistance):
  • This is the normal mode of operation. In this mode no data will be transmited on the serial data (SES) line.
  • Diagnostic Mode (Diagnostic Terminal grounded):
  • With the ignition "ON", engine "OFF":
  • Code 12 will be flashed by the SES light, followed by any stored trouble codes.
  • With the ignition "ON", engine "ON" (running):
  • This is the mode used to set ignition base timing.
  • ALDL mode or Special mode (10K ohm resistance):
    In this mode, all information incorporated into a specific engine and ECM is obtainable. However, in this mode the system operating characteristics are modified as follows:
  • Closed loop timers are bypassed
  • EST (spark retard, if used) is disabled
  • IAC (if used) will be fully extended
  • On some engines, canister purge will be enabled
  • P/N restrict functions will be disabled.
  •  
    Note: any information available in this mode will only be obtainable by the use of a scan tool.
  • AIR Switching Solenoid (C) This terminal is connected to the Air Switching Solenoid Control Driver.
    This terminal will be grounded when the Air Switching Solenoid control circuit has been grounded (activated) by the ECM when the proper operating conditions have been meet. This terminal will also be grounded when the key is "ON" and the engine is "OFF".
    SES Lamp (D) This terminal is connected to the SES Lamp Driver.
    This terminal is also used for serial data when a Scan Tool is connected to the ALDL. This terminal will also be grounded when the key is "ON" and the engine is "OFF".
    TCC Solenoid (F) This terminal is connected to the TCC Solenoid Control Driver.
    This terminal will be grounded when the TCC Solenoid control circuit has been grounded (activated) by the ECM. This terminal will also be grounded when the key is "ON" and the engine is "OFF".
    EGR Solenoid (H) This terminal is connected to the EGR Solenoid Control Driver.
    This terminal will be grounded when the EGR Solenoid control circuit has been grounded (activated) by the ECM

    Serial Data:

    Serial Data from the ECM is transmited using the SES driver circuit. When placed into either ALDL or Backup mode, the ECM strobes the SES driver to output data at a rate of 160 baud or 160 bits per second. Every data packet is 20 bytes (25 bytes for '88), or 160 bits, and when overhead is included, takes about 1.25 seconds to transmit.

    Serial Data will contain the following:

  • A Calibrator ID
  • The last value obtained from each information sensor
  • The state of each device switched by the ECM
  • A list of trouble codes stored in memory

  • Table of Contents Back Top of Chapter

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