* Incorrect linkage adjustment: The shifting linkage connecting the shifter to the transmission could be improperly adjusted. This is the simplest and cheapest fix to investigate first. A misadjustment could prevent the transmission from selecting the correct gears.
* Shifting forks or rails: Despite a rebuild, the internal shift forks or rails within the transmission might be bent, damaged, or improperly installed during the rebuild. These components are crucial for selecting gears, and a slight misalignment can lead to this type of problem.
* Problems with the valve body: The valve body controls the hydraulic pressures that shift the gears. A faulty valve body, a mis-installed valve body, or incorrectly adjusted valves could be preventing proper gear selection. This is a more involved problem.
* Incorrectly installed or damaged synchronizers: Synchronizers are responsible for matching the rotational speeds of the gears before engagement. If they're damaged or installed incorrectly, the transmission might struggle to engage gears smoothly, or only select certain ones.
* Internal transmission damage (despite the rebuild): It's unfortunately possible that some internal damage occurred during or after the rebuild. A component might have been overlooked, damaged during reassembly, or a part may be defective.
* Low fluid (despite appearing full): While you say it's full, the fluid level might be low or the fluid itself might be the wrong type. Double-check the fluid level with the engine running and warmed up, and ensure it's the correct transmission fluid specified for your truck. An air bubble in the system is also a possibility.
* Governor issue: The governor controls shift points based on engine speed. A malfunctioning governor could cause the transmission to only shift to a specific gear.
Troubleshooting Steps:
1. Check the linkage: Carefully inspect the linkage connecting the shifter to the transmission. Adjust it if necessary according to the factory specifications or a reliable repair manual.
2. Check the transmission fluid level again: Do this with the engine running and at operating temperature. Ensure it's the correct type and quantity.
3. Consult a transmission specialist: If the linkage adjustment doesn't solve the problem, this is crucial. A professional with experience in transmission rebuilds can diagnose the problem far more effectively than trying to troubleshoot it yourself. They have the tools and expertise to identify internal issues. It may require a partial or full tear-down to find the cause.
Don't continue driving the truck in this condition, as it could cause further damage to the transmission. The fact that it's a fresh rebuild makes it more likely there's a procedural or part-related issue that needs professional attention. The cost of further damage will greatly outweigh the cost of proper diagnosis and repair.
