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HOMESERVICE MANUALSCHEVROLET1987HI-CUBE 6.2 J, 400/M40REPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 19 (ENGINE CONTROLS - BASIC TESTING - GASOLINE)DIAGNOSTIC CIRCUIT CHECKA-2, MIL ALWAYS ON OR WON'T FLASH CODE 12
1987 Chevrolet Hi-Cube 6.2 J, 400/M40

A-2, Mil Always On Or Won't Flash Code 12

1987 Chevrolet Hi-Cube 6.2 J, 400/M40SECTION A-2, Mil Always On Or Won't Flash Code 12
WARNING: This page does not describe the selected car, but rather 10 other vehicles, including the 1994 GMC Yukon, 1994 GMC Suburban, 1994 GMC Pickup, 1994 GMC Cab & Chassis, and 1994 GMC C3500 HD. However, it is still accessible from the selected car via links, so may be relevant.
  1. Turn ignition on, engine off. If MIL does not illuminate, see A-1, MALFUNCTION INDICATOR LIGHT (MIL) INOPERATIVE . If MIL is on, ground DLC test terminals "A" and "B". Code 12 should flash. If Code 12 does not flash, go to step 2). If Code 12 now flashes, problem is no serial data. Check serial data circuit for open or short to ground, or short to voltage between ECM and DLC terminal. If circuits are okay, replace faulty ECM or PROM (MEM-CAL).
  2. Turn ignition off. Disconnect ECM connectors. Turn ignition on. If MIL is on, check for short to ground in MIL driver circuit between MIL and ECM driver terminal. See Figure-Figure . Also, see WIRING DIAGRAMS  at the end of this article for terminal identification.
  3. If light is off with ECM connectors disconnected, turn ignition off. Reconnect ECM connectors. Turn ignition on with engine off and diagnostic terminal not grounded. Using a test light connected to ground, backprobe ECM terminal circuit to DLC terminal "B". See Figure-Figure . If MIL does not flash Code 12, go to step 4). If MIL flashes Code 12, check for open in DLC test terminal "B" circuit to ECM. If circuit is okay, check for open in DLC terminal "A" circuit to ECM and ECM ground. See Figure-Figure .
  4. Check PROM (MEM-CAL) for proper installation. If installation is okay, replace ECM using original PROM (MEM-CAL). Recheck for Code 12. If Code 12 is not present, replace PROM (MEM-CAL). If Code 12 is present, system is okay.
Fig 1: Diagnostic Circuit Check & Work Flow (Gasoline)
G93C76572
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