* Low or Contaminated Transmission Fluid: This is the most frequent cause. Low fluid level prevents proper lubrication and hydraulic pressure needed for shifting. Contaminated fluid (dark, burnt smelling) indicates internal wear and friction material breakdown, further hindering shifting. Check the fluid level (with the engine running and at operating temperature) using the dipstick – it should be between the "add" and "full" marks. If it's low, add the correct type of transmission fluid (consult your owner's manual). If it's dirty, a transmission fluid and filter change is necessary.
* Transmission Solenoids or Valves: These electrically controlled components regulate fluid flow within the transmission to control gear shifting. A faulty solenoid or valve can prevent the transmission from shifting correctly. This requires professional diagnosis and likely replacement of the faulty component(s).
* Torque Converter: The torque converter is a fluid coupling between the engine and transmission. A faulty torque converter can prevent the transmission from engaging or shifting properly. This is usually a more expensive repair, often requiring replacement of the entire unit.
* Transmission Control Module (TCM): The TCM is the computer that controls the transmission. A malfunctioning TCM can lead to incorrect shifting or no shifting at all. Diagnosis requires specialized equipment to scan for error codes.
* Shift Linkage (if applicable): While less common in fully electronic automatic transmissions, a cable or linkage problem *could* theoretically affect shifting (though this is less likely in a '98 Cavalier).
* Internal Transmission Problems: Worn clutch packs, broken bands, or other internal damage within the transmission itself can prevent proper shifting. This is usually the most expensive repair and might require a rebuild or replacement of the transmission.
* Speed Sensor: An inaccurate speed sensor reading can confuse the TCM, leading to shifting problems.
Troubleshooting Steps:
1. Check the transmission fluid: This is the first and easiest thing to check.
2. Check for error codes: Use an OBD-II scanner to check for any diagnostic trouble codes (DTCs) related to the transmission. These codes can provide valuable clues.
3. Listen for unusual noises: Grinding, whining, or other unusual noises from the transmission indicate internal problems.
Important Note: It's generally not recommended to attempt major transmission repairs yourself unless you have extensive experience. Improper diagnosis or repair can lead to further damage and significant cost. If the transmission isn't shifting properly, take your Cavalier to a qualified mechanic or transmission specialist for diagnosis and repair.
