I've been posting off and on here whenever i get pissed off enough about my
truck to see if anyone can help. It's a 78 1/2 ton 4x4. At one precise
speed, approximately 33-35 mph, the thing starts to buck up and down. It's
not a side to side motion, it's honestly what you would call a 'gallop'.
It's been doing this for over a year and i've done everything i can think of
to stop it. the transmission was rebuilt about a year ago, had no effect,
the axles have been swapped with new gears put in, new calipers, pads,
rotors turned, i've had 2 sets of wheels and 3 sets of tires all mixed and
matched on there at some point in time with no effect at all. I had one end
of the driveshaft replaced cause it was showing a little wear. I have no
damn clue what is causing this vibration. I have asked every gearhead and
mechanic i know and no one has any ideas beyond a tire being out of round or
rotors needing to be turned (which i have already tried). Any input or
advice would be greatly appreciated.
assuming it is an automatic, i am wondering if it is trying to shift gears.
i have a little 4 cyl s10 and if i get in the right speed/load it will
start to buck as it trys to shift up and down. i see this on the tach and a
little gas takes it over the edge and it stops. also, maybe a u-joint...
How about removing the rear drive shaft and running in 4wd this would
eliminate or confirm a rear shaft problem you will need a dummy yoke to stop
oil leaks, how about running it on a lift at 35 mph, see if a helper can
spot any drive train movement or excess runout.
I was able to try it in neutral today, it did the exact same thing. An odd
note, it gets worse the warmer the truck is. I live in northeast AL and
today it's about 55 degrees and it didn't start until about 5-10 min of
driving it. yesterday, it was about 65-70 degrees and it was worse the
first time i drove it. I tried it in 4wd and 2wd back when i took out the
rear driveshaft and drove it, it did the same thing, same vibration at the
same speed. Gear wise, it's an automatic, and if you accelerate hard
through that speed, it's not as noticeable,but it's still there.
My old '78 blazer had similar issues, but the gallop was at higher
speeds ~50mpg. (6" lift with 35" tires) Mine turned out to be the
mechanical fan and/or clutch assembly. I removed the fan, went with
elecrtic and the gallop went away. It's a long shot but I am sure
you're willing to try anything at this point.
I don't think the rear line is bad, but would it cause that vibration at one
speed? I also forgot to mention that for about 6 months the brakes seemed
awful, like they wouldn't stop the truck hardly. Come to find out, the
booster was bad. But what seemed odd was that during the booster not
working, the gallop seemed to not bee there. I don't know if it actually
stopped or if i was just holding my breath hoping i would be able to stop
and wasn't paying attention for the gallop.
suspension's factory rear leafs with 2 in. blocks, and replacement front
springs for 2 inches of lift all around. It has the dual shocks on the
front end, i replaced 2 of them a year or so ago, the other fronts seemed
ok, and i replaced the rear shocks about 2 months ago. All the bushings are
polyurethane and we greased them well before putting the lift on, which has
been on there about 2 1/2 years.
My 84 K5 Blazer did the same thing as yours. I though it was the calipers
and asked a friend, he said it may be but might also be the brake lines. I
replaced my front calipers and then the lines. The bucking went away with
the replacement. My bucking would have shaken your teeth out of your head
dentures or not.
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