Mercedes Idler Pulley Replacement Instructions

A common problem with Mercedes-Benz cars, is the idler pulley (also known as Guide Pulley or Sheave Pulley) failing as early as 100,000 miles. Changing the idler pulley on a Mercedes-Benz is a relatively easy job for most DIYers and required less than one hour of your time. In this article, you will find step by step instructions and a video on how to change Mercedes idler pulley.

Average cost to change Mercedes-Benz idler pulley yourself is under $50 USD.

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A bad idler pulley can cause the serpentine belt to come off. Once the belt comes off you will experience multiple problems if you continue to operate the vehicle.

Power steering is difficult to turn
Alternator stops working and no longer charges the battery
Battery warning light on the dashboard
Air Conditioner stops working
Engine Overheats
Engine stalls once the battery is drained

At the early stage indicating that the idler pulley is failing, you will only hear a squeaky noise coming from the front of the engine. Inspect the idler pulley if you hear noise from the engine. The noise can be described as whining. Remove the serpentine belt and check the idler pulley for play or noise by spinning it by hand. You should replace a bad idler pulley immediately, to avoid future problems caused by complete idler pulley failure.

What you will need

Parts

Tools

Torx Socket Set
or
Female Torx

Note that the older models Mercedes-Benz engines require a Torx Bit Socket. This is common on M112 and M113 engines from 1997 to 2007. Newer Mercedes-Benz idler pulleys (on M272 and M273 and newer models) require female Torx socket. Always verify by inspecting the bolt on the center of your idler pulley.

Instructions

Park your Mercedes-Benz on an area where you can work. If the idler pulley failed while you were driving it is possible to change it on the road. You need to make sure you are working in a safe area.
Allow the engine to cool down for at least 30 minutes.
Pull the hood release under the dashboard on the driver's side.
Locate the idler pulley at the front of the engine.
Remove the serpentine belt. Use a 17mm socket and ratchet on the tensioner nut to unload the tensioner.
Use a pin to lock the tensioner in the pre-loaded position. You can use a nail or a 5mm Allen wrench as a pin. For more help follow this guide on how to remove Mercedes-Benz serpentine belt.
Slide the serpentine belt off the idler pulley. It is not necessary to remove the serpentine belt completely unless you are replacing it.
Use a flathead screwdriver to remove the plastic cap on the center of the idler pulley. Do NOT discard the cap because it needs to be reinstalled.
Use a T-50 torx to remove the idler pulley bolt. The bolt is long and requires patience to remove. Some models require female Torx socket.
Install the new idler pulley and tighten the bolt to Mercedes-Benz specifications.
Install the dust cap.
Turn the idler pulley by hand to make sure it is spinning freely.
Reinstall the serpentine belt. If the belt is not long enough to slide over the tensioner pulley, make sure it sits properly in all the various pulleys.
Use your 17mm socket and ratchet to move the tensioner upward in order to remove the locking pin.
Slowly load the tensioner.

Torque Specifications

Typical Mercedes-Benz idler pulley bolt torque:

Newton-meters 35 Nm
Foot-pounds: 25.81 ft-lb

There is a lot of confusion on many Mercedes-Benz online communities regarding the recommended torque for the idler pulley. We strongly recommended that you verify the correct torque specs by calling your Mercedes-Benz dealer because these values can vary.

Notes

It is recommended (not required) to change the tensioner at the same time you change the idler pulley.
If your vehicle has a viscous fan, it may need to be removed to access the idler pulley. Common on pre-2000 models.