Here's why:
* Vehicle Speed Sensor (VSS) Function: The VSS is a crucial component. It measures the speed of the vehicle's wheels and sends that information to various systems, including:
* Transmission Control Module (TCM): The TCM uses the VSS signal to determine the appropriate gear for optimal shifting. Without a signal, the TCM defaults to a safe mode, often first gear.
* Speedometer: The speedometer directly uses the VSS signal to display vehicle speed.
* Failure Modes: A faulty VSS can either completely fail (no signal) or send an incorrect signal. Both scenarios would explain your symptoms. A complete failure results in a non-working speedometer and the transmission staying in first gear. An incorrect signal might also cause erratic shifting or other transmission issues.
* Other Possibilities (Less Likely): While less probable given both symptoms simultaneously, other potential causes include:
* Faulty TCM: Though less likely to cause *both* problems, a malfunctioning TCM could disrupt both speedometer function and gear shifting.
* Wiring Issues: Damaged or corroded wiring between the VSS, TCM, and speedometer could interrupt the signal.
* Speedometer Cable/Gear (if equipped): Older vehicles use a mechanical speedometer cable. If your LHS uses one, a broken cable would explain the speedometer issue, but is unlikely to affect the transmission. This is less likely on a '99 LHS, which likely uses electronic signals.
Troubleshooting Steps:
1. Check the VSS: Locate the Vehicle Speed Sensor (consult your repair manual for its location—it's usually near the transmission). Inspect it for physical damage, corrosion, or loose connections.
2. VSS Testing: A multimeter can be used to test the VSS's output voltage and resistance. A repair manual will provide the proper testing procedures. This is often the best place to start.
3. Wiring Inspection: Carefully examine the wiring harness connecting the VSS to the TCM and the speedometer for any damage, breaks, or corrosion.
4. TCM Diagnostics: If the VSS checks out, a professional scan tool capable of reading TCM codes is necessary to check for any transmission-related fault codes.
Recommendation: Because diagnosing this problem accurately requires electrical testing and possibly specialized tools, it's best to take your 1999 Chrysler LHS to a qualified mechanic for diagnosis and repair. Attempting repairs without the proper knowledge and tools could worsen the issue. The VSS is relatively inexpensive to replace, making it a good starting point for a mechanic.
