Idler Pulley

Replace the idler pulley in the accessory drive belt system to eliminate noise or wobble and ensure proper belt tension.

Warnings

Tools required

Parts

• Idler pulley assembly × 1 — Use OEM specification or Gates 38006
• Idler pulley bracket bolt × 1 — Use OEM specification if damaged

Preparation

1. Park vehicle on level surface and engage parking brake
2. Allow engine to cool completely
3. Disconnect negative battery cable to prevent accidental fan engagement
4. Diagram or photograph accessory belt routing for reference

Procedure

1. 1

   Release belt tension
   Locate the belt tensioner or adjustable component (typically the alternator or power steering pump on this engine). Loosen the pivot bolt and adjustment bolt on the alternator to create slack in the belt. On 1970 AMC 390 V8 engines, the alternator typically serves as the tension adjustment point.
2. 2

   Remove accessory drive belt
   With tension released, slip the accessory drive belt off the pulleys. Remove the belt completely from the engine bay and inspect it for wear, cracks, or glazing. Set aside for reinstallation or replacement.
3. 3

   Locate idler pulley
   Identify the idler pulley assembly on the passenger side of the engine block. The idler pulley is typically mounted on a bracket bolted to the engine block or timing cover, positioned to maintain proper belt alignment and tension.
4. 4

   Remove idler pulley mounting bolt
   Using a 1/2 inch socket and breaker bar, remove the center bolt securing the idler pulley to its bracket. The bolt threads into the bracket or engine block. Hold the pulley steady while removing the bolt to prevent spinning.
5. 5

   Remove idler pulley
   Pull the idler pulley assembly straight off the mounting stud or bracket. Inspect the mounting bracket for cracks, damage, or wear. Check the mounting surface for debris or corrosion and clean if necessary.
6. 6

   Inspect and clean mounting area
   Clean the idler pulley mounting surface on the bracket and engine block using a wire brush or shop rag. Remove any rust, dirt, or old gasket material. Inspect the bracket mounting bolts for tightness if the bracket is separate from the engine.
7. 7

   Install new idler pulley
   Position the new idler pulley onto the mounting bracket, ensuring proper alignment with the pulley grooves facing outward. Verify the pulley rotates freely without binding or noise before securing.
8. 8

   Secure idler pulley bolt
   Thread the mounting bolt through the idler pulley and into the bracket by hand to prevent cross-threading. Tighten the bolt firmly using a socket wrench. Since no torque specification is provided, tighten securely but do not over-torque - approximately 25-30 ft-lbs is typical for this application. Verify the pulley still rotates freely after tightening.
9. 9

   Reinstall accessory drive belt
   Route the accessory drive belt over all pulleys following the diagram or photo taken during disassembly. Ensure the belt is properly seated in all pulley grooves and not twisted. On the 390 V8, typical routing is: crankshaft, water pump, alternator, idler pulley, and power steering pump.
10. 10

    Adjust belt tension
    Using the alternator as the tensioning point, pry the alternator away from the engine to create proper belt tension. The belt should deflect approximately 1/2 inch under moderate thumb pressure at the longest span between pulleys. Tighten the alternator adjustment bolt while maintaining tension, then tighten the pivot bolt.
11. 11

    Reconnect battery and test
    Reconnect the negative battery cable. Start the engine and observe the belt and idler pulley operation. Listen for any unusual noises such as squealing, grinding, or rattling. Check that the belt tracks properly on all pulleys without slipping off to one side.

Reassembly

1. Verify all mounting bolts are properly tightened and the idler pulley rotates freely
2. Confirm accessory belt is properly routed and tensioned to specification
3. Double-check that no tools or parts were left in the engine bay

Verification

• Start engine and let idle for 2-3 minutes while observing belt operation
• Listen for any abnormal noises from the idler pulley or belt system
• Verify belt maintains proper tension without slipping during engine operation
• Check that all accessories (alternator, power steering, water pump) operate normally
• Inspect belt alignment across all pulleys - belt should track centered on each pulley

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