Here's why:
* Transmission Shifting: The TCM relies on the VSS to determine vehicle speed and engine RPM to select the appropriate gear. Without this input, the TCM may default to a safe mode (often a limp mode in first gear or neutral) or simply fail to shift at all.
* Speedometer/Odometer: The speedometer and odometer directly use the signal from the VSS to display speed and mileage. No signal = no reading.
Possible Causes (beyond the VSS itself):
* VSS Wiring: A broken, corroded, or poorly connected wire to the VSS can interrupt the signal.
* VSS Connector: The connector at the VSS can be dirty or damaged.
* TCM: In rare cases, a faulty TCM might also be the culprit, but this is less likely if *only* the transmission and speedometer are affected.
What to do:
1. Check the VSS: Locate the Vehicle Speed Sensor (usually on the transmission case) and inspect it for any obvious damage. It's a fairly common component and relatively inexpensive to replace.
2. Check the Wiring: Carefully inspect the wiring harness leading to the VSS for any breaks, cuts, or corrosion.
3. Check the Connector: Disconnect the VSS connector and inspect it for corrosion or damage. Clean the connector with electrical contact cleaner if necessary.
4. Professional Diagnosis: If you're not comfortable working on your vehicle, take it to a mechanic for diagnosis. They have the tools and expertise to properly diagnose the issue and ensure the correct repair.
It's *highly recommended* to not drive the vehicle until the problem is resolved, as driving with a malfunctioning transmission can cause further damage.
