SERVICE MANUAL UPDATE-SEC.6E2 REVISED CODE 24 CHART

SUBJECT: MANUAL UPDATE-SECTION 6E2, 6E3 - DRIVEABILITY AND EMISSIONS (REVISED CODE 24)

VEHICLES AFFECTED: 1989 CHEV "F" CARS W/5.0, 2.8, 5.7 ENGINE (VIN E,F,S,8)

The information in this bulletin serves to replace the Code 24 Diagnostic Charts and Facing Pages found in DRIVEABILITY AND EMISSIONS (SECTION 6E2 and 6E3) of the 1989 Chevrolet "F" Carline Service Manuals.

CODE 24 VEHICLE SPEED SENSOR (VSS) CIRCUIT "F" CARLINE

Circuit Description:

The ECM applies and monitors 12 volts on CKT 437. CKT 437 connects to the vehicle speed sensor which alternately grounds CKT 437 when the drive wheels are turning. This pulsing action takes place about 2000 times per mile and the ECM will calculate vehicle speed based on the time between "pulses".

A "Scan" reading should closely match with speedometer reading with drive wheels turning.

Disregard a Code 24 set when drive wheels are not turning.

Test Description: Numbers below refer to circled numbers on the diagnostic chart.

1. Code 24 will set if:

o CKT 437 voltage is constant o Engine speed is between 1400 and 3600 rpm o TPS is less than 2% throttle opening o Low load condition o Not in park or neutral o All conditions must be met for 4 seconds

These conditions are met during a road load deceleration.

2. A voltage of less than 1 volt, at the 15-way I/P connector indicates that the CKT 437 wire may be shorted to ground. Disconnect CKT 437 at the vehicle speed sensor buffer.

If voltage remains less than 10 volts, then CKT 437 wire is grounded or open. If 437 is not grounded or open, check for a faulty ECM connector or ECM.

Diagnostic Aids:

If "Scan" displays vehicle speed, check park/neutral switch CHART C-1A on vehicle with automatic transmission. If switch is OK, check for intermittent connections. An open or short to ground in CKT 437 will result in a Code 24. Refer to Section "8A" for complete wiring diagram.

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.