* Low or Contaminated Transmission Fluid: This is the most common and often the easiest to fix. Low fluid leads to insufficient lubrication, causing friction and wear on the internal components. Contaminated fluid (dirty, burned, or with metal particles) exacerbates the problem. Regular fluid and filter changes are crucial for automatic transmission longevity.
* Worn Clutch Packs: The 3rd and 4th gear clutch packs are responsible for engaging those gears. Over time, these packs wear down due to friction and heat. This wear leads to slippage, inability to engage properly, or complete failure of those gears.
* Damaged or Worn Planetary Gearset: The planetary gearset is a complex system of gears that allows for different gear ratios. Damage to this system, often from low fluid or debris, can prevent 3rd and 4th gear from functioning correctly.
* Faulty Valve Body: The valve body controls the flow of transmission fluid to the different clutch packs and bands. A malfunctioning valve body can prevent the correct fluid pressure from reaching 3rd and 4th gear clutches, resulting in their failure to engage. This often manifests as inconsistent shifting or complete gear loss.
* Broken or Worn Shift Linkage: While less common for complete gear failure, problems with the shift linkage can prevent the transmission from selecting 3rd and 4th gear properly.
* Internal Hydraulic Problems: Leaks within the transmission's hydraulic system can deprive the gears of the necessary fluid pressure for proper engagement.
* Previous Damage or Abuse: Severe impacts, overheating, or sustained hard driving can cause internal damage that may not immediately manifest but will eventually lead to gear failure.
In short: Diagnosing the problem requires a professional mechanic to inspect the transmission. They will likely need to remove the transmission and disassemble it to determine the exact cause of the failure. Simply adding fluid might mask the problem temporarily but won't solve the underlying issue.
