Mike Donohue Posted June 18, 2014 Share Posted June 18, 2014 When in the big hole in 12th , the truck cruises fine , but if you let off the throttle , after about 5 seconds or so the engine rpm's drop then if you apply any throttle it starts picking up again almost like a centrifugal clutch , Happens with or without the jake on . No problems when in 13th. Any thoughts? Quote Link to comment Share on other sites More sharing options...
j_martell Posted June 18, 2014 Share Posted June 18, 2014 Clutch? (I got nothin') Weird it only happens in 12th.....clutch would be most of the top gears I suppose.....yep, I got NOTHIN.....DAMN 1 Quote Lack of planning on your part does not constitute an emergency on my part.... Link to comment Share on other sites More sharing options...
Mike Donohue Posted June 19, 2014 Author Share Posted June 19, 2014 That's what baffles me ! Clutch is great , could there be a air or electric solenoid/slave cylingder? Quote Link to comment Share on other sites More sharing options...
theakerstwo Posted June 19, 2014 Share Posted June 19, 2014 You don't have enough info here but if its a older 13 speed like a RTO then its sounds like a broken quill shaft and that's from a bad drive line virbration. Quote glenn akers Link to comment Share on other sites More sharing options...
bbigrig Posted June 20, 2014 Share Posted June 20, 2014 Tranny Aux box??? What seems weird is that from the sounds of your description it only happens in forgive the term "under direct" (Orange Fuller button in low position).But it should happen in every gear position with the button in that position in high range???? Something via high RPM"s in the trans is disconnecting your drive train from your power plant??? Can't say I've seen that one. Quote Link to comment Share on other sites More sharing options...
Mike Donohue Posted June 21, 2014 Author Share Posted June 21, 2014 Also has one other intermittent issue in reverse , sometimes while backing it will suddenly "jump" like theres a rock between the gears or something . 1st time having a Mack without a real Mack gearbox , sure does not seem as smooth or quiet as I've been used to. I suppose I should be planning on a rebuild of it . 2001 Vision with a 460, motor is fine just this gearbox had me on edge , not so wild on 3.70 rear gears , who spec's that in the last 2 decades for a road truck????? Quote Link to comment Share on other sites More sharing options...
bts-4120 Posted June 21, 2014 Share Posted June 21, 2014 I got same year and specs. other than a 427 3.70's are fine. If you plan on running it much over 70 it likes fuel. Sounds like you got some coming apart in the aux. box Quote Link to comment Share on other sites More sharing options...
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