CODE 63 AND 64 ADDED TO SERVICE MANUAL DIAGNOSTICS

MODELS AFFECTED: 1988 REGALS

This bulletin adds Code 63 and 64 (MAP) Sensor Circuit diagnostics to Section "6E" of the 1988 Service Manual for all Regals with 2.8L LB6 (VIN W) engines.

CODE 63 MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR CIRCUIT (SIGNAL VOLTAGE HIGH - LOW VACUUM) 2.8L (VIN W) "W" SERIES (PORT)

Circuit Description:

The manifold absolute pressure sensor (MAP) responds to changes in manifold pressure (vacuum). The ECM recieves this information as a signal voltage that will vary from about 1/1.5 volts at idle (high vacuum) to 4/4.5 volts at wide open throttle (low vacuum) (See Figure 1).

Test Description: Numbers below refer to circled numbers on the diagnostic chart (See Figure 2).

1. Code 63 will set when:

* Engine Running * Manifold pressure greater than 75.3 kPa (A/C "OFF") 81.2 kPa (A/C "ON") * Throttle angle less than 2% * Conditions met for 2 seconds

Engine misfire or a low unstable idle may set Code 63.

2. With the MAP sensor disconnected, the ECM should see a low voltage if the ECM and wiring are OK.

Diagnostic Aids:

If idle is rough or unstable, refer to symptoms in Section "B" for items which can cause an unstable idle.

An open in CKT 452 or the connection will result in a Code 63.

Ignition "ON" engine "OFF," voltages should be within the values shown in the table on the chart. Also CHART C-ID can be used to test the MAP sensor.

Refer to "Intermittents" in Section "B".

CODE 64 MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR CIRCUIT (SIGNAL VOLTAGE LOW - HIGH VACUUM) 2.8L (VIN W) "W" SERIES (PORT)

Circuit Description:

The manifold absolute pressure sensor (MAP) responds to changes in manifold pressure (vacuum). The ECM recieves this information as a signal voltage that will vary from about 1/1.5 volts at idle (high vacuum) to 4/4.5 volts at wide open throttle (low vacuum) (See Figure 3).

Test Description: Numbers below refer to circled numbers on the diagnostic chart (See Figure 4).

1. Code 64 will set if:

* Engine rpm less than 600 * Manifold pressure reading less than 13 kPa * Conditions met for 1 second or * Engine rpm greater than 600 * Throttle angle over 20% * Manifold pressure less than 13 kPa * Conditions met for 1 second

2. This test to see if the sensor is at fault for the low voltage, or if there is an ECM or wiring problem.

3. This simulates a high signal voltage to check for an open in CKT 432. If the test light is bright during this test, CKT 432 is probably shorted to ground. If "Scan" reads over 4 volts at this test, CKT 416 can be checked by measuring the voltage at terminal "C" (should be 5 volts.)

Diagnostic Aids:

An intermittent open in CKTs 432 or 416 will result in a Code 64.

Ignition "ON" engine "OFF," voltages should be within the values shown in the table on the chart. Also CHART C-1 D can be used to test MAP sensor.

Refer to "Intermittents" in Section "B".

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.