* Low Transmission Fluid: This is the simplest and cheapest thing to check first. Low fluid can cause all sorts of shifting problems, including failure to engage higher gears. Check the fluid level with the engine running and at operating temperature (consult your owner's manual for the correct procedure). If low, top it off with the correct type of automatic transmission fluid (ATF) for your vehicle. If the fluid is dark, burnt, or smells burnt, it's likely time for a complete fluid and filter change, or even a more serious transmission repair.
* Faulty Governor: The governor regulates shift points based on vehicle speed. A malfunctioning governor might prevent the transmission from shifting into fourth gear at the appropriate speed. This usually requires professional diagnosis and repair.
* Shifting Linkage or Cables (if applicable): Older vehicles might use cables to control shifting. These cables can become stretched, broken, or misadjusted, preventing proper shifting.
* Internal Transmission Problems: This is the most likely and most serious cause. Several internal components can fail and prevent a fourth-gear engagement:
* Worn-out clutch packs: The clutch packs responsible for engaging fourth gear might be worn down or damaged.
* Broken or damaged bands: Similar to clutch packs, these are crucial for proper gear engagement.
* Problems with the valve body: The valve body controls the hydraulic pressure that shifts the gears. A malfunctioning valve body can prevent fourth gear from engaging.
* Low fluid pressure: Various internal problems can lead to low fluid pressure, preventing gears from engaging correctly. This often necessitates a complete transmission overhaul or replacement.
* Solenoid Problems (if equipped): Some automatic transmissions use solenoids to control shifts electrically. A faulty solenoid could prevent fourth gear.
* Transmission Computer Issues (if equipped): Some 1984 models might have a simple transmission control unit (though this was less common back then). A faulty computer or related wiring could prevent correct shifting. This is less likely than mechanical problems.
Troubleshooting Steps:
1. Check the transmission fluid: As mentioned above, this is the first and easiest step.
2. Check for any error codes: If your vehicle has a diagnostic system (check your owner's manual), it might display codes indicating a transmission problem.
3. Listen for unusual noises: Grinding, whining, or other unusual sounds coming from the transmission indicate internal problems.
4. Test drive: Try driving the vehicle and observe when (or if) it shifts. Note the speed and conditions when the issue occurs.
5. Consult a mechanic: If the problem persists after checking the fluid, a professional mechanic experienced with older automatic transmissions is needed to diagnose and repair the issue. They have the tools and expertise to diagnose the specific problem.
Given the age of the vehicle, it's more likely to be a mechanical problem within the transmission itself. Don't attempt major repairs yourself unless you have significant experience working on automatic transmissions. The cost of repair might exceed the vehicle's value depending on the extent of the damage.
