* Governor: The governor is a speed-sensing device that tells the transmission when to shift. If the governor is malfunctioning (clogged, broken, or out of adjustment), it won't signal the transmission to upshift at the correct speed. This is a very common cause of this symptom.
* Transmission Control Solenoids: These electrically controlled valves direct fluid within the transmission to engage different clutches and bands, allowing for gear changes. A faulty solenoid (or wiring to a solenoid) could prevent the transmission from shifting properly. There are several solenoids in a TH400 (likely transmission) and one or more could be the problem.
* Valve Body: The valve body houses the internal hydraulic system that controls the transmission's shifting. Worn or damaged valve body components (like valves, springs, or passages) could disrupt the proper flow of hydraulic fluid, preventing shifts. This often involves internal wear and requires a transmission overhaul.
* Low Transmission Fluid: Low fluid level will cause shifting problems. Check the transmission fluid level *while the engine is running* and warm. The dipstick should indicate a correct level within the appropriate range. Low fluid can severely damage the transmission.
* Transmission Fluid Condition: Old, dirty, or burnt transmission fluid can prevent proper operation of the hydraulic system. The fluid should be a rich red color. If it's brown or black and smells burnt, it needs to be replaced and the transmission may require a more thorough service or rebuild.
* Vacuum Modulator (if applicable): Some automatic transmissions utilize a vacuum modulator to help control shift points based on engine load. A faulty vacuum modulator could interfere with the shifting.
Troubleshooting Steps (in order of ease):
1. Check the transmission fluid: This is the easiest and should always be the first step. Check the level and condition. If low, add the correct type of automatic transmission fluid (ATF) for your transmission (consult your owner's manual).
2. Check for trouble codes: If your truck has a diagnostic system (check the owner's manual), see if there are any codes stored in the computer that might indicate a problem with the transmission.
3. Inspect the governor: This is more involved and may require partial transmission removal.
4. Inspect the vacuum modulator (if applicable): Check for leaks or damage.
5. Have a professional diagnose it: If you are not comfortable working on the transmission yourself, it's best to take it to a qualified transmission mechanic for diagnosis and repair. This is particularly true for the valve body which often requires a specialized shop and rebuild of the transmission.
Ignoring the problem will likely lead to more extensive damage to the transmission, resulting in a far more costly repair.
