INTERMITTENT CONNECTION AT ECM PCM/TCM SENSORS OR COMPONENTS

Subject: INTERMITTENT CONNECTION AT ECM/PCM/TCM SENSORS OR COMPONENTS

Model and Year: ALL 1991 LIGHT DUTY TRUCKS

TO: ALL CHEVROLET DEALERS

Some 1991 Light duty trucks equipped with Electronic Control Modules (ECM) or Powertrain Control Modules (PCM) or Transmission Control Modules (TCM) may experience a condition, in which the female terminals (150 series Metri-pack) in certain sensor or component connectors do not make a solid connection to the male terminals in the opposing connectors. This can result in an intermittent connection. The 1991 Truck Service Manual directs a check of connections anytime an intermittent condition is found. The following chart lists the component and circuit numbers that may experience this condition.

COMPONENT CIRCUIT DESCRIPTION ------------------------ ------- ----------------- Intake Air Temperature 472 IAT signal (IAT) 2.5L only 469 5 volt return (ground) Coolant Temperature Sensor 410 CTS signal (CTS) 452 5 volt return (ground) Throttle Position Sensor 417 TPS signal (TPS) 452 5 volt return (ground) Electronic Spark Timing Module 453 EST Reference low (EST) 424 EST Bypass 430 EST Reference high 423 EST Control

Transmission Vehicle Speed Sensor (All Auto. & Manual Transmission) 821 Speed Signal high 822 Speed Signal low Transmission Input Speed Sensor (4L80E only) 1230 Input Speed Signal low 1231 Input Speed Signal high Transmission Output Speed Sensor (4L80E with Four Wheel Drive only) 1232 Output Speed Sensor high 1233 Output Speed Sensor low

If an intermittent condition is found, the following test procedure should be performed.

Insert a new mating male terminal, P/N 12047581 or 12045773, into the female terminal and check for a slight drag when the terminals are separated. Loose terminals without drag should be replaced.

If the amount of drag is questionable, remove the terminal from the connector and inspect the terminal for the condition shown in FIGURE 1. Replace any terminals found defective with terminal P/N 12047680.

General directions of terminal replacement can be found in the appropriate 1991 Truck Shop Manual in the Fuel and Emissions Manual.

IMPORTANT: Do not attempt to repair a defective terminal.

All of the above terminal part numbers are contained in J-38125 or are available from GMSPO.

Parts are currently available from GMSPO.

WARRANTY INFORMATION

For vehicles repaired under warranty use:

Labor Operation Number: T7096

Labor Time: This is a ST (Straight Time) operation which requires time documentation as detailed in the claims processing manual. Zone approval is required beyond .5 hours.

NOTE: Labor Operation is coded to base vehicle coverage in the warranty system.

Trouble Code: 92

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.