Common Causes:
* Low Transmission Fluid: This is the *first* thing to check. Low fluid can cause numerous shifting problems, including complete failure to shift. Check the fluid level with the engine running and warmed up (following the procedure in your owner's manual). Also, inspect the fluid for color and smell – burnt fluid (dark brown or black, smelling burnt) indicates serious internal problems.
* Faulty Transmission Fluid: Even if the level is correct, old, contaminated fluid can cause shifting issues. A fluid change with a filter replacement is often a relatively inexpensive first step.
* Shift Linkage/Cable Problems (if applicable): Older cars might have shift cables or linkages. These can become worn, bent, or disconnected, preventing the transmission from receiving the correct shifting commands from the shifter. Inspection for damage or looseness is necessary.
* Solenoid Problems (Automatic Transmission): Automatic transmissions use solenoids to control fluid flow, directing shifts. A faulty solenoid can prevent proper shifting. Diagnosis usually requires specialized equipment.
* Valve Body Problems (Automatic Transmission): The valve body contains the internal hydraulic controls for shifting. Worn or damaged components within the valve body can cause various shifting issues. Repair or replacement is typically expensive.
* Torque Converter Problems (Automatic Transmission): The torque converter connects the engine to the transmission. Problems here can interfere with shifting. Diagnosis is complex.
* Transmission Control Module (TCM) Issues (Automatic Transmission): The TCM receives signals from various sensors and controls the transmission's operation. A faulty TCM can cause erratic shifting or complete failure to shift. Requires diagnostic scanning tools.
* Speed Sensor Problems: The vehicle speed sensor (VSS) provides crucial information to the TCM about vehicle speed; a faulty sensor can cause inaccurate shifting.
Less Common, but Still Possible Causes:
* Internal Transmission Problems: Worn clutches, bands, or planetary gears inside the transmission are significant problems requiring major repair or replacement.
* Electrical Problems: Issues with the wiring harness connecting the transmission to the TCM or other components.
Troubleshooting Steps:
1. Check the transmission fluid level and condition. This is the most crucial initial step.
2. Visually inspect the shift linkage or cables (if applicable). Look for damage, looseness, or disconnections.
3. Have the transmission scanned for diagnostic trouble codes (DTCs). A scan tool can identify specific problems with the TCM, sensors, or solenoids. An auto parts store might offer this service for free.
4. Consult a qualified mechanic. If you're not comfortable with automotive repair, a mechanic should diagnose the problem. They have the tools and expertise to properly diagnose and repair more complex issues.
Important Note: Attempting to diagnose and repair transmission problems yourself without the proper knowledge and tools can be risky and potentially expensive if you make things worse. It's generally advisable to seek professional help, especially with older vehicles like a 1991 Dodge Dynasty.
