My F-150 '04 has a 5.4L Triton. I believe this is a pretty accurate diagram of the serpentine belt. I want to have a better understanding of what's under then hood. I'm pretty familiar with the basics of an internal combustion engine, but not as much with some of the more automotive parts.

enter image description here

Can someone give a brief explanation of the "Idler Pulleys", and why do I have 3 of them? Also, what is the "A/C Clutch"? Lastly, why does it say "Generator", when I know I have an alternator?


  • You did post the picture in there, but it looks as though you killed the code which allowed it to display. I added it back in, though you're right ... it's the same exact picture @BillOer put in his answer. Dec 4, 2015 at 15:03

1 Answer 1


Idler Pulleys are generally used to either provide constant tension or make the belt wrap around more of the next pulley, which equals more contact area around that next pulley. An idler pulley, by definition, does not drive a device.

The A/C clutch is used to engage and disengage the A/C compressor so as to provide a more-or-less constant temperature inside the passenger cabin. It's much less complicated than trying to engage or disengage that particular pulley. Another way to look at is that with the A/C clutch disengaged, that pulley is essentially another idler pulley.

An alternator is a type of generator, one that specifically provides AC current. But to describe an alternator as a generator is not inaccurate. To put it another way, an alternator is a sub-type of a generator.

Here's a diagram I found. It may not be exactly the same as your set up, but it should give you the basic idea.enter image description here

  • I thought I posted my picture with the question, but that is exactly my picture. Thanks, this helps!
    – Blake.W
    Dec 4, 2015 at 7:32
  • WRT what an idler pulley does. The tensioner pulley provides the tension throughout the system. The idler pulleys allows (as you said) the belt to wrap around a different pulley better. It can be used in place of an accessory which is not used in a vehicle, like the A/C compressor. Engineers would put an idler pulley in there to take its place without having to redesign the entire system. It can also be used to prevent the belt from flapping at certain locations. An alternator produces AC current, but then rectifies it to become DC current which the vehicle can use. Good answer overall +1. Dec 4, 2015 at 15:13
  • Another interesting think to note is that the belt's layout causes it to wrap at least 1/2 way around the high-torque pulleys which illustrates one of Bills points about the idlers.
    – JPhi1618
    Dec 4, 2015 at 18:44
  • @Paulster2, when I wrote about an idler puller providing tension, I was thinking of older engines where said pulley was attached to a plate with a slotted screw hole where the mechanic slightly loosened the bolt, tensioned the pulley, and then tightened the bolt.
    – BillDOe
    Dec 6, 2015 at 0:16
  • Hopefully you read the last sentence of what I said ... just pointing some things out for others for clarification purposes. Dec 6, 2015 at 1:02

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