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Home >> GMC >> 1991 >> Value Van 7.4 N >> Repair and Diagnosis >> External Pages >> Different car >> Section 8 (Engine Controls - Tests W/Codes - Diesel) >> Self-Diagnostic System >> Entering Or Exiting Diagnostic Mode

Entering Or Exiting Diagnostic Mode

WARNING: This page does not describe the selected car, but rather 22 other vehicles, including the 1992 GMC Yukon, 1992 GMC Vandura Special, 1992 GMC Vandura, 1992 GMC Value Van, and 1992 GMC Rally Camper Special. However, it is still accessible from the selected car via links, so may be relevant.
  1. With key on and engine off, connect a jumper wire between ALDL terminal "B" (diagnostic terminal) and terminal "A" (ground). See Fig 1. The Diesel Electronic Control (DEC) system will enter diagnostic mode.
  2. In this mode, ECM will display Code 12 by flashing the SERVICE ENGINE SOON light once, followed by a short pause, then 2 flashes in quick succession.
  3. Code 12 will be displayed 3 times. If no other codes are stored, Code 12 will continue to flash until diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper from ALDL connector.
Fig 1: Identifying ALDL Connector Terminals
G120880Courtesy of GENERAL MOTORS CORP.
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