Allison Transmission Slams Into Gear | Cause and Solution

The Allison transmission may slam into gear when you shift into drive or reverse if you feel heavy shaking or vibration.

You can also see the compartment shaking when you open the hood and shift gears.

It is a serious issue and needs to be diagnosed and solved quickly.

This article will benefit you if you have the Allison transmission slamming issue and want to find a solution.

In this article, I have explained the causes and solution to the Allison transmission slams into gear issue.

What Does It Mean Transmission Slams Into Gear?

The transmission slams into gear if the transmission or engine mounts are worn out.

The worn-out mount allows the transmission to move when changing between reverse, drive, neutral, or park at idle.

Causes and Solution to Allison Transmission Slams into Gear

04 factors that cause Allison’s transmission to slam into gear are transmission fluid level and quality, damaged or broken transmission or engine mounts, faulty sensors, and vacuum system.

Let’s have a detailed discussion.

01. Transmission Fluid

Transmission Fluid

The primary and most common reason for Allison transmission slams into gear is low or excessively high fluid levels and contaminated Allison transmission fluid.

So, when you feel the transmission slam into gear, you should check the transmission fluid first.

If the transmission fluid level is low, it causes difficulty in transmission shifting.

Check leaks if your transmission fluid level is low, as leaks are the leading cause of low transmission fluid levels.

Adding new transmission fluid will not solve your transmission slamming problem if you ignore leaks.

The other reason the transmission slams into gear is contaminated ATF (Allison Transmission Fluid).

Polluted transmission fluids’ frictional properties decrease and cause jerk, hard, or slam shifts.

Excessive black fluid or a burnt smell indicates contaminated transmission fluid.


Make sure there are no leaks first.

Change the old transmission fluid with premium quality transmission fluid.

02. Faulty Transmission or Engine Mounts

Faulty Transmission or Engine Mounts

Transmissions slam into gear if transmission or engine mounts wear out.

Mounts hold transmissions and engines tightly to the vehicle.

When a mount fails, it causes heavy jerks or slams when shifting gears.


Replacing the faulty mount is the most effective way to solve the problem.

Because damaged or broken mounts can cause damage to the engine as well as other components.

Replacing faulty mounts can cost around $300 to $500.

03. Faulty Sensor

Sometimes faulty sensors send incorrect data to the ECM and cause the transmission to slam into gear.

A faulty speed sensor sends higher or lower speed data to the ECM than the original.


You must take your vehicle to a repair shop and find out which sensor is causing the problem.

Then you can replace the faulty sensor.

04. Faulty Vacuum Modulator

A vacuum line connects the vacuum modulator and the intake manifold.

A vacuum modulator controls the transmission loads and how the transmission is reacted to.

Over time, vacuums are prone to leaking, becoming bent, and rupturing their diaphragms.

Symptoms of a failed vacuum modulator include:

  • White smoke
  • Incorrect shifting
  • Whistling sound
  • Rough idle etc.


Replace the modulator to solve the problem.

The replacement cost of the vacuum modulator is less than $120.

Final Words

Now you know why Allison transmissions slam into gear.

Once you identify the problem, you must take the necessary steps.

Visit a repair shop for a proper diagnosis and solution.

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