* Governor Problems: The governor controls the shift points based on vehicle speed. A malfunctioning governor (either mechanically stuck or electrically faulty, depending on the transmission type) could prevent the upshift to third gear. The car might be "thinking" it's going slower than it actually is.
* Transmission Valve Body Issues: The valve body contains a network of valves and passages that direct the hydraulic fluid to control the clutch packs and bands within the transmission. Worn, sticking, or damaged valves can cause incorrect shift patterns. A common problem is a malfunctioning 2-3 shift valve.
* Shift Solenoid Problems: Electronic transmissions use solenoids to control the valve body. A faulty 2-3 shift solenoid would prevent the shift from occurring. This is a common point of failure in older automatics.
* Low Transmission Fluid: Insufficient transmission fluid prevents proper hydraulic pressure, leading to delayed or missed shifts. Check the fluid level (when the engine is running and warmed up) and its condition (color and smell). Burnt-smelling fluid is a strong indication of a problem.
* Transmission Linkage (if applicable): Some older automatics use a mechanical linkage to connect the transmission to the shift lever. This linkage could be bent, broken, or improperly adjusted, causing incorrect shifting. This is less likely on a 1991 model but worth considering.
* Throttle Position Sensor (TPS) or other sensors: The transmission's computer relies on input from various sensors (TPS, speed sensor, etc.) to determine the appropriate shift points. A faulty sensor providing incorrect information can lead to improper shifting.
Troubleshooting Steps:
1. Check the transmission fluid: This is the easiest and cheapest thing to check first.
2. Have the transmission fluid and filter changed: Even if the level seems okay, old fluid can cause problems.
3. Obtain a professional diagnosis: A qualified mechanic with experience in older automatic transmissions should be able to diagnose the specific cause. They'll likely use a scanner to check for transmission codes and test the sensors and solenoids.
4. Consider a rebuild or replacement: If internal transmission components are damaged, a rebuild or, in extreme cases, replacement might be necessary.
It's crucial to address this issue promptly. Continuing to drive with a transmission problem could cause further damage and lead to costly repairs.
