Somewhere in the drivetrain...
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Joined: Apr 2006
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From: Kansas City, KS
Somewhere in the drivetrain...
I'm stumped. 79 F150 4x4 351M auto. Just developed a shifting problem. Shifts from 1 to 2 normally, but if shifting into 3rd (with my foot on it) the truck bucks like it's fighting itself. Feels exactly like a carrier bearing failing, the whole truck rocks...but I don't have a carrier bearing. U-Joints look good, no play. Transfer case has oil as do both differentials. Where do I look next?
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Trans fluid looks ok. Modulator was OK at last check, and it still shifts fine under small load. No issue in reverse. If I take the freeway to or from work, I've got a hill to climb while getting on the ramp/getting up to speed. It shifts into 3rd at about 35-40 mph, and bucks like it's losing a piece of something. Took back roads to work yesterday and today, no issue. ????
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Possible I suppose. The vacuum line to the modulator didn't look great, but was still providing vacuum at last check. Can't cost more than a couple of bucks to replace...I'll do that this weekend. Thanks.
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From: Kansas City, KS
Originally Posted by hunter_firefighter
Let me know if that fixes it. 
Nope.
Sorry for the delay. Didn't get to the parts store until yesterday. So this morning I got brave and got on the freeway again. Same thing. Right at 45 or so it shifted into 3rd HARD and the whole truck shook.
Think I'll stick to the backroads for a while.
Any more ideas?
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