So, my Bronco ( 1990 EB, 5.8 Lightning Motor, E4OD, BW1356 Manual ) has been running pretty smoothly, though after replacing the water pump I made the mistake of saying "Finally, no more leaks... " in front of it, and about 2 hours later my steering gear started peeing on the driveway, but that's another story. I was driving on the freeway the other day, and after merging on to a side highway, where the speed limits drops to 45, then back to 55, I noticed a sudden vibration. It starts right around 40-45 and stays fairly steady. I tried coasting in Neutral and the vibration was unchanged. My first thought was U-Joints, but when I checked the driveline it was solid. I, foolishly perhaps, had my wife plant her foot firmly on the brake and goose the throttle a couple times, as well as cycle it through the gears while I looked underneath, checking for parts that may be have loosened up after the tranny swap, but everything looked tight, although I did notice I lost a single bellhousing bolt on the driver's side, the remaining bolts appeared tight. I did *not* cycle the overdrive on and off to see if that makes a difference, however. What would a broken tranny mount look like ? I reused the one that was on the tranny I bought becuse I spoke to the previous owner of the donor vehicle and the tranny mount was replaced when they had the tranny rebuilt earlier that year.

Any thoughts are appreciated.

Thanks !!!

 

Given your explanation, I'd check the double-cardon u-joint at the front of the rear drive shaft. These can begin to go south and not show any real play in the driveline. What happens is the centering ball and spring begin to weaken and don't hold the drive shaft in perfect alignment. And since there are basically TWO u-joint spiders in the space of about 4" the slight misalignment translates into vibrations. You will typically notice the vibration more when the driveline is under no load (coasting) or when it is slowing (under-driving) the truck.

The amount of play will not seem significant. I finally pinpointed this issue in my own by removing the rear drive shaft and running in 4HI. (Basically front-wheel-drive). This eliminated the vibrations... because there was no rear driveline.

 

Well, I suppose I will have to get off my *** and rebuild and install my front driveshaft. it fell by the wayside ( figuratively speaking ) after putting the new tranny in since it needed new u-joints, I've had other stuff eating up my spare change.

Thanks for the reply !!!

 

I might add, when you checked the driveshaft, did you have it in newtral? If it's in park and the truck is on the slightest hill, the shaft will appear tight. Of course, have someone hold the brakes or block the wheels.

 

Good point ! It was on level ground, in park, with the brake set, but I can't say there was no pressure on the driveline.

Thanks for the tip !!!