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There's not a speed limiter in the cruise, right?

Discussion in '6.9L IH & 7.3L IDI Diesels' started by Detroit80, Oct 24, 2020.

  1. Detroit80

    Detroit80 Full Access Member

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    1990 F350 CCLB 4X4, 7.3 w Banks turbo, 5 Speed trans.

    I have owned the truck several years now, cruise control never worked since the day I bought it, and finally got it working a few days ago after cleaning the steering wheel switches.

    Took it for a test drive around the neighborhood, set it at 30mph, and used the set/accel to easily pull up to 45mph. Resume and coast also worked as expected. I figured all was well, until...

    ...road trip yesterday....I could set it at 65mph, but it slowly lose speed, about 1mph every mile or so. If I tapped, or held, the set/accel button, it would lose speed. I could feel the servo holding the pedal down when I set the speed, but it immediately started losing speed. So I said screw it, until the way home when there wasn't any traffic around and I could play with it more.

    I found it would hold 55mph fine, and even accelerate up small hills..but only to 55. If I set it at 65mph, it would again slowly loose speed until it got to 55, then hold steady.

    I'm ASSUMING it's a weak vacuum signal, or a tiny vacuum leak somewhere. But just wanted to make sure I wasn't going to be chasing a "problem" that's actually some kind of odd design intention, lol.
     
  2. The_Josh_Bear

    The_Josh_Bear Full Access Member

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    Nope it should hold 65 just fine all day long when working right. I haven't used mine in about 10 years but the last time I did it held 65 as long as the hill wasn't too much for the old girl.
     
  3. Detroit80

    Detroit80 Full Access Member

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    Engine power definitely isn't an issue here. I was running behind on the way there, said to hell with MPGs, and was cruising at 80-85mph the whole way after giving up on using cruise control..didn't even begin to slow down on the hills, and still had some left in it. On the way back, I reeled it back in to 65ish, and had tons of passing power.

    So it's definitely something in the cruise system. I need to track down a massive fuel leak in the Chubby truck today so I can get it sold, so I probably won't be able to dig deeper in to the Ford until next weekend. I still need to order up a new set of injectors, and figure I'll just replace _all_ the vacuum hoses/lines while I'm in there at the same time, as well as check the vacuum signal from the pump (though brake booster is working fine), and make sure the cruise servo can fully pull the cable in.
     

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