Before I got my Blazer I used my 92 GMC Vandura 2500 as my main gig rig. It's a beautiful TrailWagons conversion and I've hauled my gear all over the Northwest in it. I'm getting ready to head to Montana for a gig this week and I'm having a terrible time tracking down a vibration issue. The vibration comes on suddenly, above 35mph. It lasts until I back off the throttle and then it goes away immediately. It most often happens around 55mph but has occurred at various speeds all the way from 35 to 70mph. It's a violent up-and-down shaking that vibrates everything in the van.

I've had the van since 2003. The vibration used to be a minor issue but over the years it has gotten worse. My first thought was U-joints in the driveshaft. I pulled the shaft and checked the joints. They seemed fine. I took them to my local shop for a second opinion and they also thought they were fine. I had them replaced with brand new ones just to eliminate any possibility of problems with the old joints. The vibration was just the same as always.

I took the van over to the shop and had them check the balance on all four wheels. They were all balanced. I also had them put the van on their lift and run it to check for any vibration in the drive train. They didn't find anything. I brought it home and checked to see if any front end components were loose. Everything is nice and tight. I replaced the lower ball joints in 2009.

I'm pretty confident that the vibration comes from the drive train somewhere because when it happens it immediately goes away if I let off the throttle. It's not the kind of regular, periodic vibration from a bad tire or bent wheel. In the morning I'm going to put it up on jack stands and run it and see if I can find any vibration in the driveline or rear end. When it was on the lift at the shop the suspension was unloaded and hanging freely. I'm also going to pull the inspection cover on the transmission and see if there's anything unusual about the flex plate and torque converter.

Any other ideas? What else should I check? There are a lot of brilliant people here on the Blazer forum.

 

driveshaft out of balance or bad center bearing if its a 2 piece driveshaft.

usually you can tell the difference between a driveshaft vibration and a tire vibe. the driveshaft vibration is way faster. it is possible that its something inside the transmission. locked up fan clutch maybe?

 

It's a one-piece driveshaft so no center bearing.

I ran out of time working on the window motor today. It took me all day just to get the regulator and motor out of the driver's side door. The motor came loose from the regulator and I had to take it out to tighten the bolts and put in a new one. If you've never tried to take a window regulator out of a G van then consider yourself lucky.

Because of spending the whole day on the window motor I didn't get back to diagnosing the driveline. I'll get back to work on it when I return from Montana. In the meantime I'm going to jam all the gear into the Blazer and drive it over the mountains.

Thanks for all the help!

 

I got back to work on the van this week. I took a friend out for a ride to help diagnose the vibration. He suggested putting the transmission into neutral as soon as the vibration started. When I put the transmission in neutral the vibration immediately went away. This leads me to suspect torque converter shudder. I'm going to take it in for a full transmission flush and filter change and see if that clears up the problem. I've got more travel gigs coming up later in the month and through October and I really want to get the van back to top shape.

 

I got the van back from the transmission flush and filter change this morning. The shudder is gone. The shifts also feel firmer. The moral of the story is make sure you change transmission fluid regularly, even if you only drive 5000 miles a year.