Help with diagnosis

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Oct 6, 2004
Longmont, CO
I have a 1976 FJ55, stock. 2F, 4 on the floor. typical

So yesterday leave the house for work. Back out of the drive way. I put it in first and let out the clutch and step on the accelorator and hear a single hard knock come from aproximetly the right hand side of the transmission and I feel it in the shifter knob. I dont have another vehicle so i have to press on as i am late for work :mad:. As I drive I notice it but only when i change gears so im thinking transmission issues. On the way I decide to play with it a bit. and I let off the gas and step back on it. Clunk. it happens again. So I put it in neutral and coast. and step on teh gas....nothing. hmmm I was also thinking engine mounts but they are all in tact.

Any ideas?

Lets think about this. Right hand side of the transmission is the output to the rear axle, because axles have the differential on the right hand side. It does not do it when the clutch is in, so it is not your engine. It happens when you change gears or step on the gas while in gear. This narrows it down to occurring between your transmission and wheels.

When you let off the gas or depress the clutch, it allows the driveline to freespool at rate of speed you are at, instead of being regulated by the engine. Slack is allowed, and when you change gears the engine speeds up the driveline, taking the slack out as it rotates the gears, driveshaft, and axles. That is when you experience the knock you hear/feel. When you let off the gas in gear, the slack comes into play, allowing the free play to go to the other side. It's like having a sloppy steering setup, you go from pulling one direction to pulling the other. It's the same principle. When you hammer on the gas again, it eats up the slack and gets it going the other way again.

If your transmission has issues, you would know and it would wreck itself. The problems would be during the entire drive, not just on get-up-and-go times. If it were your axle differential, that too would begin to destroy itself and cause problems throughout your journey. This leaves the thing between the two, the driveshaft.

The only variation there are the U-joints, which can and do wear out. When they wear out, slack is created, which is noticeable on acceleration when slack is taken out of the driveline. Vibrations will be created, but may not be noticeable at first. Vibrations on a 55 are harder to notice than a 40. I'd suggest choking the wheels, taking the parking brake off (unless it is the parking brake on the rear brakes, not the transfer case), putting the transmission in neutral, and rotating the driveshaft to check for play. Also move it up and down & side to side. If there is play in it, you found your problem. It sounds like it is the U-joint up by the transfercase output. Good luck on it!

I was also thinking engine mounts but they are all in tact.

What did you do to verify the structural integrity of your front and rear engine mounts?
Another possibility is loose U-bolts allowing the axle to "rock" on the springs as you transfer on/off the power. Or possibly a loose pinion nut (although that would also cause oil leakage)

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