* Valve Body Problems: The valve body is the "brain" of the transmission, controlling fluid flow to engage different gears. A malfunctioning valve body, due to worn parts, a broken valve, or a clogged passage, can prevent the transmission from shifting correctly into higher forward gears. This is a very common cause of this specific symptom.
* Low/Reverse Clutch Pack Issues: The clutch packs are responsible for engaging the gears. If the clutch packs for the higher forward gears are worn, burnt, or otherwise malfunctioning, they won't engage properly. However, the reverse clutch pack seems to be functioning correctly.
* Governor Problems: The governor regulates shift points based on engine speed. A faulty governor could prevent proper upshifts, though it's less likely to solely cause this specific failure mode.
* Shift Linkage Issues (Less Likely): While less probable given the symptom, a problem with the linkage connecting the shifter to the transmission could be a contributing factor, though this would likely manifest in multiple gear issues, not just a restriction to reverse and low gear.
* Fluid Problems (Contributing Factor): Low transmission fluid level, contaminated fluid, or the wrong type of fluid can exacerbate any of the above problems and prevent proper shifting.
In short: You need professional diagnosis. Trying to fix this yourself without specialized knowledge and tools is strongly discouraged. The problem is almost certainly internal to the transmission and requires a mechanic experienced with automatic transmissions to diagnose and repair. Simply adding fluid won't likely solve the problem; the underlying mechanical issue needs to be addressed. Ignoring the problem will likely lead to further, more costly damage to the transmission.
