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Vacuum motors are found on many four-wheel drive vehicles. The vacuum motor uses a shift fork to share power between the front and rear axles.



What is a Vacuum Motor?

The vacuum motor is an important part of the linkage between the shifter on the transfer case and the front axle. The vacuum motor assembly allows you to connect and disconnect the front axle from the rest of the drivetrain without exiting your truck or SUV.

How does a Vacuum Motor Work?

Instead of using a mechanical linkage or a cable, the transfer case shifter and vacuum motor act on a shift fork using vacuum signals and a rubber diaphragm. When the vacuum motor receives the correct signal, the shift fork acts on a coupler that connects the front axle to the rest of the vehicle’s drivetrain.

How are Vacuum Motors Made?

The vacuum motor consists of a metal or plastic housing, a rubber diaphragm, a shift fork, and inlets for vacuum lines. It may also have a switch with an electrical connector to trigger the “4WD” light on the dash.

The housing is often pressed together, making the diaphragm inaccessible for replacement. If your vacuum motor has malfunctioned, you will need to purchase a factory or aftermarket assembly to replace it.

Why do Vacuum Motors Fail?

  • Rubber and plastic parts, like the vacuum motor’s internal diaphragm and the external vacuum lines, oxidize and break down with normal use.
  • If you suspect your vacuum motor no longer functions properly, check the vacuum lines first.
  • Vacuum motors may also fail due to physical damage caused by trail features, although this is not common given how most rear axles are designed.

What are the Symptoms of Vacuum Motor Failure?

  • The vehicle fails to activate the front axle when shifted into 4WD.
  • 4WD light does not illuminate when the vehicle is shifted into 4WD.

What are the Implications of Vacuum Motor Failure?

  • Off-road capabilities are severely reduced when the front axle cannot be activated.

Vacuum motors are a source of much consternation and debate among 4WD users and enthusiasts. Vacuum motors on some makes and models are notorious for wearing out prematurely, but the OEM and aftermarket replacement parts are far more cost effective and much easier to install than aftermarket linkage and cable kits.

If you have questions or concerns about vacuum motors or any of your vehicle’s components, come into your local Pep Boys where we can answer any question, help you find any part, or perform any vehicle service you might need.