Truck will not accelerate past 20 mph

Welcome to 2CarPros.

It sounds like you lost second gear. What I suggest is to first confirm the transmission fluid is clean and full. If it is, there are a few different things that can cause this. Things such a low transmission line pressure, a bad or worn clutch pack, or even a failed transmission control module (TCM).

Check the fluid first. Here are the directions on doing this. The attached two pictures attached correlate with these directions.

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Transmission Fluid Level Check

CAUTION: The vehicle should not be driven if the fluid level indicator shows the fluid below the DO NOT DRIVE mark or internal failure could result.

NOTE: If vehicle has been operated for an extended period of time at highway speeds, in city traffic, in hot weather or when pulling a trailer, the fluid needs to cool down to obtain an accurate reading.

NOTE: The fluid level reading on the indicator will differ from operating and ambient temperatures. The correct reading should be within the normal operating temperature range.

Under normal circumstances, the fluid level should be checked during normal maintenance. If the transmission starts to slip, shifts slowly or has signs of fluid leaking, the fluid level should be checked.

1. With the transmission in (P) PARK, the engine at idle, foot pressed on the brake, move the range selector lever through each gear and allow engagement of each gear. Place the range selector lever in the PARK position.
2. Wipe the fluid level indicator cap and remove the indicator.
3. Wipe the indicator with a clean cloth.

Picture 1

Picture 2

4. Install the indicator back in the filler tube until it is fully seated, then remove the indicator.

The fluid level should be within the normal operating temperature range.

High Fluid Level
A fluid level that is too high may cause the fluid to become aerated due to the churning action of the rotating internal parts. This will cause erratic control pressure, foaming, loss of fluid from the vent tube and Possible transmission malfunction and/or damage. If an overfill reading is indicated drain and refill the transmission.

Low Fluid Level
A low fluid level could result in poor transmission engagement, slipping, malfunction and/or damage. This could also indicate a leak in one of the transmission seals or gaskets.

Adding Fluid

CAUTION: The use of any type of transmission fluid other than specified could result in transmission malfunction and/or damage.

If fluid needs to be added, add fluid in 0.25L (1/2 pint) increments through the filler tube. Do not overfill the fluid. For fluid type, refer to the General Specifications chart.

Fluid Condition Check
1. Check the fluid level.
2. Observe the color and the odor. The color under normal circumstances should be dark reddish, not brown or black or have a burnt odor.
3. Hold the fluid level indicator over a white facial tissue and allow the fluid to drip onto the facial tissue and examine the stain.
4. If evidence of solid material is found, the transmission fluid pan should be removed for further inspection.
5. If the stain is a foamy pink color, this may indicate coolant in the transmission. The engine cooling system should also be inspected at this time.
6. If fluid contamination or transmission failure is confirmed by the sediment in the bottom of the fluid pan, the transmission must be disassembled and completely cleaned. This includes the torque converter, coolers and cooler lines.
7. Carry out diagnostic checks and adjustments; refer to Diagnosis By Symptom.

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Let me know what you find.

Take care,
Joe

You are welcome. Let me know what you find. Also, let me know if the check engine light is staying on while the engine is running.

Take care,
Joe