Instruments - Inoperative, Erratic Speedometer/Odometer
89chevy12Number: 89-148-8C
Section: 8C
Date April, 1989:
Subject: INOPERATIVE OR ERRATIC SPEEDOMETER/ODOMETER
Model and Year: 1987-89 L CAR
TO: ALL CHEVROLET DEALERS
Some 1987-89 L cars have exhibited a condition in which the speedometer/odometer is either inoperative or erratic. This may be caused by a variety of different conditions in the total speedometer circuit and a complete understanding of this entire system is needed to properly repair the vehicle.
SYSTEM DESCRIPTION
The signal for the speedometer/odometer comes from the VEHICLE SPEED SENSOR (VSS) mounted on the transmission. The VSS creates an AC signal that is sent to the electronic control module (ECM) through two wires (purple and yellow). The ECM processes this VSS signal and creates a new signal based on data stored in the ECM's MEM-CAL (PROM) such as the axle ratio, engine, transmission, etc. The new ECM speed signal is sent to various devices that have to know the vehicle speed to operate properly. These devices include the instrument cluster, speedometer, cruise control module (V6 only), and safety belt retractor release solenoid timer (1989 only).
A more detailed system description, along with additional schematics and tests may be found in Section 8A-33 of the L car Service Manual.
DIAGNOSTIC TIPS
- EXAMINE THE WIRES COMING FROM THE VEHICLE SPEED SENSOR (VSS) MOUNTED ON THE TRANSMISSION. IF THESE TWO WIRES ARE CHAFED OR BROKEN, AN INTERMITTENT OR INOPERATIVE SPEEDOMETER/ODOMETER COULD RESULT. REPAIR THE WIRES USING THE PROCEDURE SHOWN IN SECTION 8A OF THE SERVICE MANUAL.
- THE ATTACHED CHART LISTS THE MOST RECENT MEM-CAL (PROM) PART NUMBERS FOR THE VARIOUS 1987-89 BUILD COMBINATIONS. THIS MAY BE USED AS A REFERENCE FOR CUSTOMER COMMENTS OF INACCURATE OR ERRATIC SPEEDOMETER/ODOMETER. OPERATION.