Things to Check (with caveats):
* Obvious Transmission Fluid Leaks: Check under the truck for any signs of transmission fluid leaks. Look around the transmission pan, bell housing, and transmission cooler lines. Low fluid levels can cause shifting problems. *Note: A significant leak after an impact is likely, and requires professional attention.*
* Transmission Fluid Level and Condition: If you're comfortable doing so, check your transmission fluid level using the dipstick. It should be at the "full" mark when the engine is running. Also inspect the fluid's color and smell. Burnt smelling, dark brown, or black fluid indicates significant damage. *Do not attempt this if you are unfamiliar with the process; incorrect procedure could lead to injury*.
* Wiring Harness Damage: The rear-end collision could have damaged the wiring harness near the transmission or the transmission control module (TCM). Look for any broken wires, frayed insulation, or connectors that are loose or damaged, particularly around the rear of the vehicle. This is a difficult check to perform if you lack experience.
* Shift Linkage: While less likely to be the direct cause of third gear overdrive failure after a rear-end collision, it's possible that the impact somehow bent or damaged the shift linkage. This would result in an issue with gear selection.
What NOT to do:
* Don't continue driving the truck: Driving with a damaged transmission will likely worsen the problem and potentially cause irreparable damage.
* Don't try to repair the transmission yourself unless you're a qualified mechanic: Transmission repairs are complex and require specialized tools and knowledge.
What you SHOULD do:
* Get it towed to a reputable mechanic or transmission specialist: This is the best course of action. They can properly diagnose the problem, determine the extent of the damage, and provide an accurate repair estimate. Mention the accident in detail. The impact could have damaged internal transmission components not readily apparent externally.
* File a claim with your insurance company: Since the damage was caused by a collision, your insurance should cover the repairs, including the transmission issue (assuming you have collision coverage).
In short, don't try to troubleshoot this yourself. The combination of the accident and the transmission failure strongly suggests significant internal damage. Professional help is needed to diagnose and repair the issue safely and cost-effectively.
