C12 Engine Squeal when downshifting

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by golfnut34, Oct 30, 2019.

  1. golfnut34

    golfnut34 Bobtail Member

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    Oct 25, 2019
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    I have a C12 in a 2003 Freightliner FLD that has developed a high pitch squeal when increasing RPMs to downshift. I've also noticed a fan belt squeal a few times when starting the truck. I put a ratchet on the belt tensioner a few days ago and quickly checked the AC compressor, alternator, and idler bearings and didn't notice anything out of the ordinary which lead me to believe that the fan clutch is the problem? I've suspected a fan clutch problem for a while now as I don't think the fan ever actually shuts off. Engine temps remain low until I put it under load but then it climbs to 200 and stays there. It never gets over 200. How does one diagnose this? Should the fan spin freely with the engine off? What will it cost to repair if that's the problem? Its a farm semi and doesn't see many miles/year. Mainly at harvest time.
     
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  3. BoxCarKidd

    BoxCarKidd Road Train Member

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    How does the fan hub spin with the belt loose?
    Belt squeal on start up is often a slipping Alt belt. Grooves worn down on the alt pulley is common. A new belt may fix it for a short time. Then the pulley wears the belt and it can slip on other things it drives as well. Start there.
     
    spsauerland Thanks this.
  4. RustyBolt

    RustyBolt Road Train Member

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    If it's a higher pitch than a "belt squeal", it's one of your pulley bearings. Isn't really anything in a fan clutch that will make that type of squeal.

    I just went through the same troubles with my C15. Took a digital thermometer to mine once it got bad enough that I could make it stay squealing for a few minutes. Alternator pulley went up about 40 degrees. All others held close to their previous temps. I changed it out with a new one. And .... Still squealing. New idlers and tensioners. Much better. But at higher RPMs, I still have a bit of squeal. AC compressor will be next once it gets bad enough to worry about.
     
  5. golfnut34

    golfnut34 Bobtail Member

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    Oct 25, 2019
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    I went and checked the fan again with the belt loose and it moves freely both ways. I'll probably be taking the truck to a shop in the next week or so to have it looked at.
     
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