OT: 1990 F150 vibration at 65-70 mph under load
 

Totally stock '90 F150. It vibes pretty good between 65 and 70 mph, then, above 70 mph, it's gone. I turned off the AOD, but same vibration. If I let off the throttle (between 65 and 70 mph) the vibration goes away. So, it only vibes when under throttle and between 65 and 70 mph.

Tires are almost new. Shocks are old.

I can only think of 3 things:

Unbalanced driveshaft
U-joints
Something in the rear



Tough to tell where the vibes are starting from, but I do NOT feel it in the steering at all.

Any ideas?

thanks,

Mark

my first guess would be driveshaft unbalanced or joints also does that truck have the long shaft with the carrier bearing they were famous for vibrating when they started to go

I'd bet u-joints, unless you have a whine which would indicate a rear diff problem.

Ditto on the carrier bearing if it's a 2WD with 2 piece driveshaft.

It's a 4x4.

What's that rubber bellows on the driveshaft? I think it was located at the front end of the driveshaft, near the front u-joint.

That seals the slip joint-that is another possibility for a vibe. Is the truck lifted ??

might be worth taking the shaft out and checking it over. i use A1 driveshaft they are in baltimore great guys not much more than doing it yourself and the will check balance as well. if you find a bad joint put in a good one i wont use anything but dana spicer i learned the hard way!!

I had an 89 F-350,4x4 with the same problem. I had the tires balanced, drive shaft balanced and it still vibed. It was the tranny tailshaft bushing. Check it out.

Kurt.

What's a tailshaft bushing? This is my first 4x4.

The truck is bone stock.

I had a simular problem. There would be a vibration under load and only at certain speeds. Replacing the U-joints solved the problem.

Don't forget that you have a handful of U-joints in the front axel of that thing too!