Check the tension and condition of the accessory drive belt at every maintenance.

The belt should be free from engine oil, scuffs, fraying, cracks and indentations.

A sign of insufficient belt tension and the need to replace it is a characteristic whistle when the engine is running.

If the belt tension is insufficient, the voltage at the generator terminals may decrease, and the circulation of coolant in the engine cooling system will deteriorate.

If you remove the belt not for replacement, then you need to mark one of the edges of the belt in order to maintain the direction of its movement during assembly.

Location of the drive belt of the units: 1 - coolant pump pulley; 2 - crankshaft pulley / vibration damper; 3 - fan pulley; 4 - tensioner pulley; 5 - intermediate pulley; 6 - generator pulley; 7 - compressor pulley

A worn out tensioner or a misaligned pulley can cause the belt to come off.

Cummins ISF3.8 Attachment Drive Belt Replacement

Use pulley alignment tool (#3163524) to check pulley installation.

If it is not available, you can use a ruler and an inclinometer.

Maximum pulley misalignment should not exceed 3°.

Repair, adjust or replace misaligned parts as needed.

Removing the accessory drive belt

The tensioner is spring-loaded and must be moved away from the belt.

Moving the tensioner in the wrong direction can damage it.

The tensioner is moved to the side so that the spring tongue is bent onto the mechanism body.

To loosen the belt, turn the tensioner to release the spring.

Cummins ISF3.8 Attachment Drive Belt Replacement

Caution: Excessive force when turning the tensioner in the direction of the weakening of the spring, or until it stops when the spring is already fully weakened, can lead to breakage of its lever.

Replacing the Cummins ISF3.8 Attachment Drive Belt

Turn the tensioner towards the spring tongue and remove the belt.


Check the drive belt for the following damage:

  • - cracks;
  • - salting;
  • - tears or cuts;
  • - curing;
  • - increased wear.

Replace the belt if damaged.

Check the intermediate and drive pulleys for wear or cracks.

Check the plastic pulleys for road grime deposits or particles of the material from which the pulley is made.

Remove dirt with a suitable tool and check for wear.

It is recommended to clean the intermediate pulleys, check their condition and reuse them rather than replace them.

Replacing the Cummins ISF3.8 Attachment Drive Belt

With the belt removed, check that the tensioner arm limiter touches the stopper on the spring housing.

If there is no contact, replace the tensioner.

With the tensioner removed, measure the gap between the tensioner spring and its lever to check for wear on the tensioner and bearing.

Tensioner: 1 - tensioner cover, 2 - tensioner arm, 3 - spring housing, 4 - pulley, 5 - gap

If the gap at any point exceeds 3mm, the tensioner is considered to be defective and should be replaced.

Usually, tensioners have an increased gap near the bottom th part of the spring housing, which causes the upper part of the housing to rub against the tension lever.

Be sure to replace the belt when replacing the tensioner.

Check the tensioner for signs of contact between the lever and the tensioner cover.

Signs of contact between two parts indicate a failure of the tubular axle bushing and the tensioner needs to be replaced.


Install the tensioner and tighten the bolt (if removed).

Tightening torque 43 Nm

Replacing the Cummins ISF3.8 Attachment Drive Belt

Install the belt on the engine according to the diagram that was drawn up at the time of its removal. Do not install the belt on the coolant pump pulley at this stage.

Move the tensioner toward the spring tongue and install the drive belt, placing it over the water pump pulley last.

Release the tensioner to tension the drive belt.

Check for proper belt position on the tensioner and accessory drive pulleys at the front of the engine.

After installing the belt, make sure that none of the tensioner arm stoppers touches the spring case stopper. If at least one stopper touches:

  • - Make sure the belt with the correct part number is installed.
  • - If the correct belt is installed, replace it.

After replacing the belt, if the tensioner arm stops are still touching the stop on the spring housing, replace the tensioner.

Cummins ISF3.8 Attachment Drive Belt Replacement

Check the position of the drive belt on the tensioner pulley.

The belt should be located in the middle of the pulley or close to the middle of it.

Incorrectly positioning the belt forward or backward can result in belt wear, belt slipping off the pulley, or uneven wear on the idler bushing.

Belt part number 3974030

Cummins ISF3.8 Attachment Drive Belt Replacement

Tensioner part number 5267127