ECM INOP CAUSED BY- A/C CLUTCH RELAY

VEHICLES AFFECTED: 1986 MODELS WITH 3.0L OR 3.8L ENGINES (VIN CODES B,3 OR L)

Some 1986 models may experience an inoperative ECM that is the result of an intermittent short in the A/C clutch relay. If this should occur, some or all of the following conditions may exist:

1. Check engine light on steady.

2. A/C compressor inoperative (clutch will not engage).

3. A code 12 will not be displayed when grounding the ALDL diagnostic lead.

4. A code 34 can be read by means of a scan tool only.

5. EGR valve has full vacuum signal at idle.

Normally, the dagnostics shown on page 6E3-A2-23 or 6E3- A3-23) of the 1986 Chassis Service Manual will result in replacing the ECM and checking all solenoids and relays controlled by the ECM. Some A/C relays will check out satisfactorily when cold but will short directly to the ECM when hot. To prevent reoccurence to an inoperative ECM, replace the A/C clutch relay.

CHART A - 2 WON'T FLASH CODE 12 "SERVICE ENGINE SOON" LIGHT ON STEADY 3.0L "N & H" SERIES FUEL INJECTION (PORT)

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

With the diagnosdc terminal grounded, the fight should flash a Code 12, fllowed by any trouble code(s) stored in memory.

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

1. 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.

2. This step will check for an open diagnostic circuit 451.

3. At this point the "service engine soon" 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.

4. 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 to one driver can damage any other driver in the set. Solenoid 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.

CHART A - 2 WON'T FLASH CODE 12 SERVICE ENGINE SOON" LIGHT ON STEADY 3.8L "A,C & H" SERIES FUEL INJECTION (PORT)

There should always be a steady "SERVICE ENGINE SOON" light when the igntion 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 diagnoode circuit 451.

1. 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.

2. This step will check for an open diagnostic circuit 451.

3. At this point the "SERVICE ENGINE SOON" light wiring is okay. The problem is a faulty ECM or PROM. The ECM is OK if a Code 51 in 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. Solenoid 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.

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.