INFO. ON PCM ERRORS DISPLAYED W/TECH 1 CONNECTED. Chevrolet Chevy Suburban

REVISION: 10/25/94

THIS BULLETIN IS BEING REVISED TO INCLUDE THE 1995 MODELS. PLEASE DISCARD BULLETIN 377146 (GROUP REFERENCE 7 - TRANSMISSION). ----------------------------------------------------------------------

SUBJECT: INFORMATION ON PCM ERROR DISPLAYED WITH TECH 1 CONNECTED

MODELS: 1994-1995 CHEVROLET AND GMC TRUCK C/K & P TRUCKS AND G VAN WITH 6.5L DIESEL ENGINE (VINS F, P, Y - RPOS L65, L49, L57) AND AUTOMATIC TRANSMISSION

TRANSMISSION APPLICATIONS: 1994-1995 HYDRA-MATIC 4L80E (MT1) AND 4L60E (M30)

TRANSMISSION MODELS: LAP, LBP, LVP, LHP, LLP, LUP, LYP, CMD, CUD, KPD, LFP, LSD, CTP, ALP

SOME 1994 AND 1995 HYDRA-MATIC 4L80E (MT1) AND 4L60E (M30) AUTOMATIC TRANSMISSIONS MAY EXPERIENCE A PCM ERROR WHEN ROAD TESTING A VEHICLE WITH A TECH 1 CONNECTED. THE PCM SOFTWARE HAS AN ERROR THAT MISSES SPEED PULSES FROM THE SPEED SENSORS WHEN A SERIAL DATA DEVICE (TECH 1, CAMS, T-50, ETC.), IS CONNECTED.

WHEN THE TRANSMISSION IS IN THIRD OR FOURTH GEAR WITH THE TORQUE CONVERTER CLUTCH ENGAGED, THE SCAN TOOL TCC SLIP PARAMETER READING WILL TOGGLE BETWEEN THE ACTUAL VALUE AND A VALUE GREATER THAN THE ACTUAL.

THE TRANSMISSION INPUT AND OUTPUT SPEEDS WILL ALSO TOGGLE BETWEEN THE ACTUAL VALUE AND A LOWER VALUE.

THE VEHICLE SPEED PARAMETER WILL TOGGLE BETWEEN THE ACTUAL SPEEDOMETER READING AND A VALUE LOWER THAN THE SPEEDOMETER.

THE SOFTWARE WILL BE UPDATED FOR THE 1996 MODEL YEAR. THERE WILL NOT BE AN UPDATE TO THE 1994 AND 1995 PCM SOFTWARE.

SERVICE INFORMATION EXAMPLES: THE SCAN TOOL DATA FOR TORQUE CONVERTER CLUTCH SLIP WILL TOGGLE BETWEEN THE ACTUAL VALUE AND A VALUE 30-50 RPM'S GREATER THAN THE ACTUAL VALUE. THE DESIRED TORQUE CONVERTER CLUTCH SLIP SHOULD BE BETWEEN ZERO (0) AND FIVE (5) RPM'S. WHEN A SCAN TOOL IS CONNECTED TO A VEHICLE, THE TCC SLIP PARAMETER MIGHT EXHIBIT THE FOLLOWING READINGS: TCC SLIP 2 RPM, JUMPS TO 32 RPM, RETURNS TO 2 RPM, JUMPS TO 36 RPM, RETURNS TO 2 RPM, JUMPS TO 41 RPM, RETURNS TO 2 RPM, ETC. THE VALUE THAT THE TCC SLIP WILL JUMP CONSTANTLY VARIES.

IF THERE IS A SLIPPING CONDITION WITH THE TORQUE CONVERTER CLUTCH, IT MIGHT SHOW ON THE SCAN TOOL AS; TCC SLIP 50 RPM, JUMPS TO 100 RPM, RETURNS TO 50 RPM, JUMPS TO 82 RPM, RETURNS TO 50 RPM, JUMPS TO 97 RPM, RETURNS TO 50 RPM, ETC. THE VALUE THAT THE TCC SLIP WILL JUMP CONSTANTLY VARIES.

THE VEHICLE SPEED PARAMETER ON THE SCAN TOOL WILL TOGGLE BETWEEN ACTUAL VEHICLE SPEED AND THREE (3) TO EIGHT (8) MPH SLOWER THAN THE SPEED DISPLAYED ON THE SPEEDOMETER.

EXAMPLE: SPEEDOMETER READING 50 MPH; SCAN TOOL - VEHICLE SPEED 45 MPH, RETURNS TO 50 MPH, JUMPS TO 47 MPH, RETURNS TO 50 MPH, JUMPS TO 43 MPH, ETC. THE VALUE THAT THE MPH WILL JUMP CONSTANTLY VARIES. THE SCAN TOOL DATA WILL CONTINUE TO JUMP WHILE THE SPEEDOMETER READING WILL REMAIN STEADY.

THE TRANSMISSION OUTPUT SPEED PARAMETER WILL TOGGLE IN THE SAME MANNER AS THE VEHICLE SPEED PARAMETER, HOWEVER, THE RPM WILL TOGGLE IN THE 50-100 RPM RANGE.

THE VEHICLE SPEED AND THE TRANSMISSION INPUT AND OUTPUT SPEED PARAMETERS ARE NOT AFFECTED BY THE TORQUE CONVERTER CLUTCH OPERATION.

FIGURES: 0 ATTACHMENTS: 0

GENERAL MOTORS BULLETINS ARE INTENDED FOR USE BY PROFESSIONAL TECHNICIANS, NOT A "DO-IT-YOURSELFER." THEY ARE WRITTEN TO INFORM THOSE TECHNICIANS OF CONDITIONS OCCURRING 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 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.