* Transmission Fluid Viscosity: When cold, the transmission fluid thickens significantly. This increased viscosity makes it harder for the hydraulic system to operate correctly, especially the valve body which controls gear shifting. The problem resolves once the fluid warms up and thins out. This is often a symptom of using the incorrect weight of transmission fluid or fluid that has degraded over time.
* Worn or Damaged Valve Body: The valve body contains the solenoids and valves that control fluid flow to the various clutch packs within the transmission. Wear or damage to these components, particularly seals and passages, can exacerbate the effects of cold, thick fluid. The problem is more pronounced when cold because the already sluggish flow is further hampered.
* Low Transmission Fluid: Low fluid levels will make the problem worse. Even a small amount low can cause issues, especially when cold, as there is less fluid to lubricate and pressurize the components.
* Transmission Filter Clogging: A clogged transmission filter restricts fluid flow, compounding the issue of increased viscosity when cold.
* Internal Transmission Wear: More serious internal wear within the transmission (e.g., worn clutch packs, damaged planetary gears) can contribute to the issue, but is less likely to only manifest when cold. Usually more significant issues would be present even when warm.
* Torque Converter Issues: In some instances, problems with the torque converter's internal components (e.g., clutch slippage) could contribute to the sluggish shifting, particularly in the lower gears, but this is less directly linked to the 3-4 shift specifically.
What to do:
It's crucial to have a qualified mechanic diagnose the problem. Simply adding fluid without knowing the underlying cause could worsen the situation. They'll likely start by checking the fluid level, assessing its condition (color, smell), and inspecting the transmission for any leaks. Further diagnostic tools might be needed to determine the exact cause. Ignoring the problem can lead to significant transmission damage.
