The transmission will shift to neutral, and then slam into gear. This happens more often when the van is on a steep decline, and when it is cold outside. It will sometimes fail to engage, and then only go into gear when the rpms reach 2500 or more. Do I need a new transmission?
I have seen this issue before on vehicles with your same drive train. First I would check the transmission fluid level. Do this by starting the engine, letting it warm up to operating temperature, and with the vehicle in PARK, open your hood and check the transmission fluid level and condition. Pull the dipstick for the transmission and give it a wipe, re-insert the dipstick fully and pull it out again -- this time noting the level. There should be engraved markings on the stick indicating proper fluid level. Once you've determined the level, wipe it off with a white rag. See if the color is dark and if it has a burnt plastic smell. The color should be bright red to pink with a heavy petroleum smell.
If the level is low, have an experienced mechanic inspect your vehicle for any transmission fluid leaks. The transmission cooler lines in your vehicle are notorious for leaking, as is the fluid pan. Look underneath your vehicle for any evidence of transmission fluid. Another thing I have seen often leaks is the transmission cooler, which is integrated into the A/C condenser that sits behind your vehicles grill in front of the radiator. If the cooler is leaking, you will see saturation on the fins of the A/C condenser.
With a low fluid level condition, the pump in the transmission cannot build enough line pressure to get the gears engaged. If the fluid smells burnt and has a dark colored texture, this indicates a transmission clutch failure. If your fluid level is normal both in level and smell, you may have a faulty torque converter. This is the part that transfers power from the engine to the transmission. Another possibility is that the transmission or transmission computer (TCM) has failed, which is less likely.
Have a certified mechanic, such as one from YourMechanic, inspect your transmission's gear slippage to eliminate any further complications with your vehicle.
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