* Broken or seized crankshaft pulley: This is the most common cause. The pulley itself might be broken internally (often the rubber damper section) or the pulley might be seized to the crankshaft due to corrosion or rust. This prevents the pulley from rotating even though the crankshaft is turning.
* Broken crankshaft pulley bolt: The bolt that secures the pulley to the crankshaft could be sheared or stripped. This would prevent power transfer.
* Seized crankshaft: While less likely if accessories spin freely, the crankshaft itself could be seized due to a severe lack of lubrication, a spun bearing, or other internal engine damage. This is a much more serious problem.
* Interference: Something might be physically preventing the pulley from turning. This is unlikely but possible, perhaps a foreign object wedged against it.
* Timing belt issue (if applicable): While you mention the accessories turn freely, a severely broken or slipped timing belt *could* also cause this, but typically you'd also have a non-running engine. This is less probable given your description.
Troubleshooting Steps:
1. Visual Inspection: Carefully inspect the crankshaft pulley. Look for any obvious cracks, damage, or signs of seizing. Check the bolt that secures it – is it intact and tight?
2. Try to turn the pulley by hand: With the engine OFF, try turning the crankshaft pulley. If it's difficult or impossible to turn by hand, that strongly suggests a problem with the pulley or crankshaft.
3. Listen for unusual noises: When you try to start the engine, listen carefully for any unusual grinding or knocking sounds which could indicate a serious internal engine problem.
4. Check the serpentine belt: While you say the accessories turn freely by hand, verify the serpentine belt isn't loose, broken, or otherwise interfering with the pulley. This is a less likely issue in your situation.
Important Note: If you are not mechanically inclined, it's best to have a qualified mechanic diagnose and repair this issue. A seized crankshaft or other internal engine damage can be expensive to fix. Attempting repairs yourself without proper knowledge and tools could cause further damage.
