DIAGNOSTIC CHART REPLACEMENT REVISED CHART SHOULD BE USED

VEHICLES AFFECTED: 1985 'C' Models Equipped With 3.8L MFI Engine

The Revised Diagnostic Chart A-2, Won't Flash Code 12 ("Service Engine Soon" Light On Steady) should be used in place of those currently in Section 6E3 of the 1985 Chassis Service Manual. These charts will be updated for the Chassis Service Manual with the next revision.

CHART A - 21 WON'T FLASH CODE 12 (" SERVICE ENGINE SOON " LIGHT ON STEADY )

There should always be a steady "SERVICE ENGINE SOON" Light when the ignition is "on" and engine stopped. Battery ignition voltage is supplied to the light bulb. The Electronic Control Module (ECM) will turn the light on by grounding circuit 419 at the ECM.

With the diagnostic terminal grounded, the light should flash a Code 12, followed by any trouble code(s) stored in memory.

A steady light suggests a short to ground in the light control circuit 419, or an open in diagnostic circuit 451.

If the light goes off when the ECM connector is disconnected, then circuit 419 is not shorted to ground. Also, check the connector terminals physically for proper contact at this time.

This stop will check for an open diagnostic circuit 451.

At this point the "Check Engine" light wiring is okay. The problem is a faulty ECM or PROM. The ECM is OK if a Code 51 is stored when the PROM is removed. Replace the PROM.

Solenoids and relays are turned "ON" or "OFF" by the ECM, using internal electronic switches called "drivers". Each driver is part of a group of four, called "Quad-Drivers". Failure of one driver can damage any other driver in the set.

Solonoid and relay coil resistance must measure more than 20 ohms. Less resistance will cause early Failure of the ECM "driver". Before replacing ECM, be sure to check the coil resistance of all solenoids and relays controlled by the ECM. See ECM wiring diagram for the solenoids and relays and the coil terminal identification.

When checking TCC solenoid, be sure to raise drive wheels and run above 30 MPH to close third gear apply switch, if used.

5.The lamp driver located near the ECM in the I.P. harness is required on carbureted models but serves no purpose on this unit. Therefore if it is found to be faulty, discard rather than replace it.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.