How to Replace the Tensioner on a 1987 Mercedes 300E

The Mercedes E-Class was introduced in 1986. The 1987 Mercedes 300E was equipped with a 3.0-liter V-6 engine, capable of producing up to 188-horsepower. The belt tensioner on the 1987 300E is located directly on the front of the engine. The tensioner is responsible for keeping the drive belt tension on the belts, necessary for proper operation of the engine pulleys. Replacing the tensioner on the 1987 300E should take no longer than one hour, even if you have never done this type of work before.

Tools Used: 3/8-inch drive ratchet and socket, with 3-inch extension, New belt tensioner, Pen and paper

Replace Tensioner

Raise the hood of the 300E. Visually inspect and locate the "Belt Routing Diagram" beneath the hood of the car. The belt routing diagram is a visual aide to assist in placing the belt back onto the engine. If the vehicle you are working on does not have a diagram, then you'll need to draw one. Draw a box on a piece of paper representing the front of the engine. Draw circles on the paper, representing the various pulleys. Draw a thick line between the pulleys to represent the drive belt routing.

Visually inspect and locate the tensioner on the front of the engine. The tensioner assembly is a "teardrop" shaped metal object, with a pulley at the pointed end of the teardrop. Remove the tension from the serpentine belt by depressing the tensioner pulley, using a 38-inch drive ratchet and socket. Depress the tensioner assembly with one hand, and remove the belt from the tensioner with your other hand. Keep as much of the belt in place on the engine as possible.

Remove the tensioner assembly mounting bolt from the center base of the tensioner, using a 3/8-inch drive ratchet and socket. Turn the bolt counterclockwise until it it removed from the tensioner. Pull the tensioner assembly off of the engine by hand. Install the new tensioner assembly onto the front of the engine, and start the mounting bolt by hand. Tighten the tensioner mounting bolt using the 3/8-inch drive ratchet and socket.

Reinstall the belt onto the engine, using a "Belt Routing Diagram" for aide or the diagram you drew earlier. Route the belt so that the tensioner is the last part you have to place the belt onto. Depress the new tensioner assembly by pushing it down, using a 3/8-inch drive ratchet and socket on the pulley. Loop the belt over the tensioner pulley, and release the tensioner slowly.

Visually inspect your work, to ensure that you have placed the belt in the center of each pulley. If the belt needs adjusted depress the tensioner assembly halfway, and make the proper adjustments. Use a 3/8-inch drive ratchet and socket to depress the tensioner.

Tips & Warnings

When changing the tensioner on the 300E, it is recommended that you inspect the belt for possible damage. Look for any chunks missing from the belt, or exposed layers of material that holds the belt together. Turn the belt inside out to inspect the actual ribs of the belt. Bend the belt towards your person, and look for any visibly noticeable cracks in the belt. If there is more than one crack in the belt within a one inch span, then the belt is bad and needs to be replaced. The 1987 300E uses an 887K6 sized belt.

Never place your fingers between the belt and any pulley, while you have the tensioner depressed. The tesioner can slip from your grasp causing the belt to snap tight instantaneously. A belt tensioner can exert up to 100-pounds-per-square-inch of force on the belt, when released suddenly. Failure to adhere to this warning could result in severe injury or loss of limb.

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