I'm totally out of ideas. 1996 Mazda B2300, 5 speed, 4 cyl., 125k miles I have a vibration in all speeds, all gears, diminishes considerably when under heavy acceleration. Replaced engine mounts, u-joints, center bearing, had driveshaft balanced, replaced clutch, pressure plate, throwout bearing, pilot bearing, replaced oil in transmission & differential. Ran truck through the gears with rear section of driveshaft removed & no vibration. Whle driving, step on clutch or put in neutral, vibration stops. Inside of transmission looks like new, no shavings, no visible wear on gears, bearing, synchros, or shift forks. Engine idles smoothly. Only other clue is a rattle in the shifter handle.

Any ideas or guesses??

 

You have probably done this, but the only thing that comes to mind would be the transmission mount. If it is worn our allowing the tranny to move freely, that might cause a vibration. Good luck.

 

Thanks jwor! Yeah, the trans mount is like brand new.

 

You mention the "Engine idles smoothly", but in neutral or with the clutch in, have you increased RPM's to see if the engine produces a vibration. Neighbor of mine with a mustang fought a vibration for the longest, ended up with a flywheel punched slightly off center. Either way this would rule out the engine if it revs smoothly.

 

Again, thanks! Actually, I have run the truck, in each gear, with the back half of the driveshaft disconnected. Regardless of how much throttle I use, it accelerates smoothly, with no vibration. Given that this condition really doesn't put a torque load on the engine & trans, I'm sort of leaning towards something funky going on with the engine when under a specific range of load. When actually driving on the road, if I accelerate with a heavy foot on the throttle, it is smoother than if I use lighter throttle. Puzzling the hell outta me! (Geesh, if I only had a chassis dynamometer!!)

 

Looks like you have tried everything. Only thing not in motion when you are testing is the rear end itself. Not sure what in the drive axle would cause a vibration though. I suppose it is a possibility though. Not sure how to test that safely without a car lift.

 

I hate to sound like a "been there, done that" kind of guy but I did run the truck with the rear axle on jack stands & it seems fine. I am guessing there simply isn't enough load on the drivetrain without running on the road? Frustrating, ain't it?

 

I can only imagine. I cannot think of anything else at the moment. If you get it figured out, post the solution. I would really like to see what is causing this issue.

I do have one thought. Working on larger trucks in the past. Bearings under no load would make little if any noise. Put a heavy load on the same bearing and it sounds like a jack hammer. You have replaced everything else from what you said. I am wondering about axle bearings and such at this point.

Again, good luck sir. This is a mystery.

 

Yeah, this is a puzzler!! Thanks for your efforts. If/when I figure it out, I will post the fix.

 

Check for a dent in the driveshaft. A dent will throw a driveshaft out of balance. Also, whatever caused the dent may have been the driveshaft a bit. Have you checked the universal joints? A worn out u-joint can cause a vibration under load, but sometimes they will "center" themselves when everything is just free-wheeling without a load.