85 Suburban vibration?

Unknown member · Oct 24, 2002

Hi guys. I own a 85 GMC Suburban 3/4 ton two wheel drive. It has the 400 transmission and a 454 motor. It has a oscilating vibration at about 67 m.p.h. on up. I feel it in the floor and in the seat. I can run it up to 70 m.p.h. and take it out of gear and it stops. When stopped I can put the truck in park and get the r.p.m.'s up to where they were at 70 m.p.h. and there is no vibration. I can put the truck in 1st or 2nd and get the r.p.m.'s up to where they were at 70 m.p.h. and there is no vibration. I have put new U-joints on it and it still has the vibration. I have looked at the drive shaft for a thrown weight and did not see anything. I bought the truck like this thinking it was the U-joints so I don't know how long it has had the vibration. Man if you could help me out here Id be very thankful!! Mark

1780 · Oct 24, 2002

I would start by lifting the rear of the sub put in gear and whatch sto see if the drive line has any hop in it if it does there is the prob

Guest · Oct 24, 2002

Mine does the same thing and it didn't start till after I replaced the 700R4/208 with the TH400/241. My dad's 2wd truck with a th400 did it too and my g/f's folks 1/2 ton sub with a 700 does it too.
Edit: same rear driveline too for both combos too

269 · Oct 24, 2002

My 88 Wagoneer had some slight vibration and some noise coming from the tranny/driveline. The noises and vibrations went away after i replaced the worn tranny mounts with new Energy Suspension mounts.

Guest · Oct 27, 2002

I have been trying to pay more attention to it and listen a little closer and it appears that the oscilating vibration quickens under accelleration. When Im ascending an incline such as an overpass the r.p.m.'s pick up to maintain a speed of 73 m.p.h.. When the r.p.m.'s pick up the vibration quickens. Once I reach the crest of the incline the r.p.m.'s return to where they were and the vibration returns or slows to where it was. What do you guys think?
Thanks, Mark

643 · Oct 28, 2002

1 piece drive shaft or two pice driveshaft???

Look into improper balancing off all things which rotate according to vehicle speed not RPM - i.e. bent rims, bad tires, driveshafts, bent axles, etc.

Guest · Oct 28, 2002

on my vibe it only started after putting the tranny, t-case and crossmember in otherwise it is the same 1 piece driveline, rims, tres, everything. It is possible in my case that tranny mounts being bad since I did use the crossmemeber/ mounts that I got form the junkyard with the tranny combo

85 Suburban vibration?

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