Owner's Problem Description
The owner of a 1993 Chrysler LeBaron convertible reports an issue with the speedometer, which consistently reads 12 miles per hour slower than actual speed. The owner discovered this discrepancy using a GPS, noting that when the vehicle is driving at 30 mph, the speedometer indicates 42 mph. The owner has already replaced the speedometer sensor, but the issue persists and seeks further advice on potential causes and repairs.
Professional Technical Analysis
The 1993 Chrysler LeBaron utilizes a mechanical speedometer driven by a vehicle speed sensor (VSS) located on the transmission. Given the reported discrepancy of 12 mph, a thorough analysis is warranted. The vehicle's speedometer is affected by several factors, including tire size, which must be verified to ensure it matches factory specifications (typically 205/70R14 for this model). Incorrect tire sizing can lead to significant speedometer inaccuracies. Additionally, the transmission computer, particularly if it's a 4-speed unit, may require recalibration using a Chrysler DRB3 scanner to ensure that the correct tire size is programmed. If a new speed sensor was installed, it should be verified that it is functioning properly and within specifications (output voltage should be around 5V AC while driving). The vehicle's mileage and wear patterns should also be considered, especially if the car has been modified or if there are issues with the wiring harness. Given these connections, any electrical issues can cascade and affect the VSS operation. A thorough inspection of the VSS wiring, connector integrity, and potential faults in the PCM (Powertrain Control Module) should be performed, as these components are interrelated. Current vehicle condition assessment indicates the need for immediate attention to ensure proper functioning and safety while driving.
Possible Causes
Most common causes (ordered by frequency):
- Tire Size Discrepancy: The speedometer accuracy can be significantly affected by incorrect tire size. The stock tire size for a 1993 Chrysler LeBaron is typically 205/70R14. If the tires are larger or smaller than this specification, the speedometer reading will be inaccurate. Using a GPS for comparison can help confirm this issue, as the speedometer will read lower than actual speed when larger tires are fitted. - Speedometer reads consistently lower than GPS speed, confirmed tire size is incorrect.
- Faulty Speedometer Sensor: The replacement speedometer sensor may be defective or incorrectly installed. The speed sensor output should be measured; it typically provides a voltage signal of about 5V AC while the vehicle is in motion. If the signal is weak or fluctuating, it may not be sending accurate information to the speedometer, resulting in incorrect readings. - New speedometer sensor installed, but inconsistent output voltage observed during testing.
- Transmission Control Module (TCM) Calibration: The TCM may require recalibration to account for any changes in tire size or modifications to the transmission. If the tire size is programmed incorrectly, the speedometer will not function properly. A Chrysler DRB3 scanner is necessary for this calibration, and it may take about 5 minutes to complete. - Previous programming history unknown, possible need for TCM reprogramming.
- Wiring and Connector Issues: Poor electrical connections or damaged wiring between the speed sensor and the TCM can also lead to inaccurate speedometer readings. A thorough inspection of the wiring harness and connectors is essential to ensure there are no breaks, shorts, or corrosion that could impact signal transmission. - Visual inspection reveals signs of wear or corrosion on connectors, intermittent connection issues reported.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Verify Tire Size: Start by confirming that the tires installed on the vehicle are indeed the correct size (205/70R14). Use a tire size gauge to measure the tire diameter and ensure compliance with factory specifications. If the tires are incorrect, replacing them with the correct size may resolve the speedometer issue.
- STEP 2 - Test the Speedometer Sensor: Using a multimeter, check the output voltage of the speedometer sensor while the vehicle is in motion. The expected output should be approximately 5V AC. If the voltage is significantly lower or fluctuating, the sensor may be faulty and require replacement.
- STEP 3 - Inspect Wiring and Connectors: Conduct a thorough visual inspection of the wiring harness and connectors associated with the speed sensor and TCM. Look for any signs of wear, corrosion, or disconnections that could affect signal integrity. Repair or replace any damaged components as necessary.
- STEP 4 - Reprogram TCM if Necessary: If the tire size is confirmed to be correct and the sensor is functioning properly, use a Chrysler DRB3 scanner to check if the TCM needs recalibration. Follow the manufacturer's guidelines to reprogram the TCM if required, which typically takes about 5 minutes to complete.
