andyman7931

This week I made my 600 mile trip this week pulling my 6500 lb trailer, but after driving the truck today without the trailer I noticed a weird noise and clunking sound whenever the transmission shifted up a gear when accelerating. What are the symptoms of a dead transmission mount? Is it easy enough to change in a parking lot with simple tools and the truck bottle jack?

I've been working my truck hard and with my shift kit while pulling the trailer, the 3-2 downshifts on the highway were HARD. the truck is a 351w with c6. The motor mounts have about 1000 miles on them, so I don't think they'd be bad unless a bad tranny mount caused them to fail. I should have replaced the tranny mount last week, but hindsight is 20/20

aliens8mycow

Are you sure it's not a u-joint? Usually that's the weak link, and the first to go (especially if you're getting a clunk when things engage). Unless it's really jammed in position, you can try turning the drive shaft while the truck is parked. Try both directions and see if it has some slack in it.

Is your truck 2 or 4 wheel drive?

andyman7931

the truck is 4wd. I checked the U joints last week, they weren't leaking grease and didn't have play in them, but they most likely have 150k miles on them. How hard are they to do? can I do it in the parking lot or should I find a shop. I'm away from home so I only have a small toolbox of tools.

Just call me Sean

iTrader: (1)

You can do anything in a parking lot with the proper tools. The tranny mount is easy if you have a proper jack. The u-joint is easy with a hammer or c-clamp, but you have to make sure the needle bearings don't fall over.

aliens8mycow

I'm assuming that you were towing in 2wd? More than likely it's the rear u-joint if that's what the problem is.

A simple check would be to put the truck in 4wd and see if it still clunks. The front u-joint is probably still good and will engage without letting the rear flop around.

If it still does the clunking in 4wd, it's probably not the u-joint.

wde3477

IMO, the hardest part of a u-joint changeout is getting the old caps out. C-Clamps and a BFH and 9/16" socket used as a punch usually do the trick. A little penetrating oil helps. Snap-ring pliers are also quite useful.

If you can set the parking brake, or park on a level surface with the tranny in neutral - gently roll the driveshaft back and forth by hand, and try to move the driveshaft yoke up-and-down. There should be absolutely no visible play between the yokes and u-joint cross-member.

Are you sure the 4WD is disengaged - meaning the front axle does not turn??? If by some chance the front end stays engaged - could be the axle/steering knuckle joints clacking instead of the driveshaft joints.

Another check requires a friend and a vigilent mind on safety. Watching the underside - have the friend go from neutral to drive and watch for unusual movement in the driveline. If you choose to do this - please be careful - don't want to hear about any members getting run-over or twisted up in the driveshaft.

Recommend to get the greasable u-joints for replacement. Also recommend that these are parts where you don't want to go for the cheapest.

andyman7931

Thanks all for the info. I'm probably going to have the afternoon off, so I'll crawl under the truck to see what I can find. I'll probably diagnose it myself and find a local shop to do the work for me since I'm on a business trip and working 70 hours per week. I don't want to take it to a shop without any idea of what is actually wrong so they can't take my money. I also only have a portable toolbox of tools, so I'd have to aquire a clamp and stuff to do the U joints myself. I'd tackle the transmission mount myself though.

I'm torn, I have a lot of $ in this old truck, but thinking that a newer truck may be more practical for my lifestyle now...

mitchalvin

If you are not careful you could bind the drive line and hide ujoints that are bad. I had this before in my F-250 until it blew out and trashed the drive line. I had just check it by crawling under and the ujoints felt tight. If I had left it in neutral I would have noticed the bad joint.