Intermittent Vibration - I'm Stumped

[Minuit]

For reference, this is the exact same vibration I posted about a year and a half ago. It has not gotten any better or worse since then. Since then, the following driveline-related work has been done to no change in the vibration.

 

- Tires repeatedly balanced, once using a Hunter road-force balancer equipped shop

- Driveshaft U-Joints replaced by machine shop

- Tires rotated after 5,000 miles by me. They appear to be wearing fine and they do not have any visible defects.

 

Necessary information: '91 4.0, 2WD, shortbed, D35 with multiple bad bearings, no lift. Cooper Discoverer A/T3 tires with roughly 5,500 miles. Tires were rotated by me about a month ago.

 

I have been fighting an intermittent driveline vibration in my truck for almost 2 years now. This vibration occurs at any speed above about 60 mph and does not depend on throttle position or gear (no change when coasting in neutral). The vibration is completely intermittent and seemingly random (although turning does seem to make it either get better or worse) - I can hold a speed for 10 minutes and have it change multiple times between jarring and perfectly smooth without doing anything on my part. The vibration occurs in the entire cab and does not seem to come from one side or single area of the truck. It is felt both in the seat and steering wheel. I have watched the truck drive on the highway and had others do the same - from the outside nothing looks to be wrong from any angle. People have told me it could be anything from a bad transmission mount to a bent wheel. I suspect driveshaft balance but I have no evidence to back this up.

 

I'm tired of diagnosing and throwing parts and work at this problem. This is one of the last issues this truck has. It needs to go away. It's keeping me from enjoying driving my truck.

[opsled]

Pull the drive shaft and have it spun by a reputable shop. They can cause your symptoms even if they look good. The only way to eliminate it as the culprit is to have it checked.

 

opsled

[HOrnbrod]

^^ Agree with this. Also while your drive shaft is out, check the slip yoke bushing inside the AW4 tail extension. I had one that was sloppy in my 2WD years ago, and the bushing was egged out. I replaced the bushing and the slip yoke because of the pitting on the shaft, and it got rid of the random vibrations I was having. 

 

http://comancheclub.com/topic/35049-vibration-issues/?hl=tail+piece&do=findComment&comment=351589

[KC0GFG]

I concur, check the drive shaft balance. visually inspect the shaft to see if there is any evidence of a balancing weight that's fallen off.

 

but since your under there give a good look at the under carriage, give the drive shaft a good wiggle see if any thing seems loose

[Minuit]

Driveshaft was done. 2wd Comanche driveshafts have a layer of rubber inside them, and this eventually wears out and causes an imbalance. A new driveshaft fixed the problem. While it was out, I checked the bushing at the tailhousing and it seemed fine, but I did discover my transmission mount is worn out.

 

[fiatslug87]

Shiny, thanks for posting your solution.

[zomeizter]

Driveshaft was done. 2wd Comanche driveshafts have a layer of rubber inside them, and this eventually wears out and causes an imbalance. A new driveshaft fixed the problem. While it was out, I checked the bushing at the tailhousing and it seemed fine, but I did discover my transmission mount is worn out.

 

My 4WD 'manche has that same drive shaft with the rubber, I had it balanced recently because mine too had a similar vibration issue as in your case. This problem was dramatically reduced with the balance job but not 100% eradicated, guess I'm due a new drive shaft...

[HOrnbrod]

I had a new solid drive 3" shaft made several years ago due to the same problem. I brought it to the local shop for balancing, but the guy said it probably wouldn't last as the rubber sheath was deteriorating. So I ordered a new one from Tom Wood and no more vibes.