Drivetrain Clunk

lightfoot · Post by **lightfoot** » Mon May 12, 2014 6:51 am

When I let the clutch out on my 68 F100 240 six I hear a clunk just before full engagement. If it’s happening in 2nd and 3rd I don’t notice it, but it’s pretty noticeable in 1st. It doesn’t happen when the clutch plates initially engage and the truck starts rolling, but as I said, it’s just before full engagement. Any idea what’s causing this??? I’ve only had the truck a few weeks, and just started driving it over the weekend so I can’t say how long this has been going on. When I test drove it I only put on about 4 miles and didn’t notice it.

Madman · Post by **Madman** » Mon May 12, 2014 7:36 am

Is it coming from inside the clutch? Adjacent to it? Or can you not quite tell? If you have the truck shut off and cycle the clucth, does it still make the noise, or some noise?

ultraranger · Post by **ultraranger** » Mon May 12, 2014 8:57 am

It would probably be a good idea to check the U-joints.

tac · Post by **tac** » Mon May 12, 2014 9:52 am

Yep, check U-joints.

lightfoot · Post by **lightfoot** » Mon May 12, 2014 10:00 am

ultraranger wrote:It would probably be a good idea to check the U-joints.

What would I be looking for? I did try rotating the crankshaft back and forth by hand and there was some movement like, 10 or 15 degrees but I don't know it that's normal or not.

lightfoot · Post by **lightfoot** » Mon May 12, 2014 10:01 am

Madman wrote:Is it coming from inside the clutch? Adjacent to it? Or can you not quite tell? If you have the truck shut off and cycle the clucth, does it still make the noise, or some noise?

Seems to be comming from behind the cab, not the clutch. No noise it the truck is off.

chad67 · Post by **chad67** » Mon May 12, 2014 11:50 am

Climb under the truck with a large screwdriver (I say this now because if you're my age, climbing back out is an ordeal). First, try to turn the drive shaft by hand, you should have very little play. Then try to pry the driveshaft in every direction with the screwdriver at both u-joints. Any more than very minor movement is not good. If one is bad, change them both.

Madman · Post by **Madman** » Mon May 12, 2014 8:24 pm

Yep, I 'd have to agree since it isn't in the clutch area, start with the u-joints....

Mancar1 · Post by **Mancar1** » Mon May 12, 2014 11:04 pm

I also would replace U joints. 99% chance that is what it is.

BobbyFord · Post by **BobbyFord** » Tue May 13, 2014 7:24 am

It could also be the diff unloading and rotating after braking. A lot of the time this is due to worn spring/shackle bushings.

lightfoot · Post by **lightfoot** » Tue May 13, 2014 7:34 am

OK, I tried twisting the u-joints and nothing moved other than the drive shaft itself. I can move it maybe 10 degrees back and forth but don’t see any movement at the yoke.
Is this normal wear of the trans spline and rear axle gears?
Could the clunk still be caused by the u-joints?

ultraranger · Post by **ultraranger** » Tue May 13, 2014 10:44 am

A few months back, my truck would sometimes clank when putting the transmission in gear and the truck developed a very bad shake at around 35 mph. I slid up under the truck and grabbed the drive shaft, at the rear U-joint, and could move the drive shaft up and down. I moved to the front of the drive shaft and couldn't move that end.

I took the shaft out to replace both joints. Even though I couldn't move the front of the shaft, when it was installed, the needle bearings on that end were just as bad as the rear u-joint.

This may or may not be your problem but u-joints are cheap and if you have never changed them there's no telling how long they have been on there or their condition.

lightfoot · Post by **lightfoot** » Tue May 13, 2014 11:09 am

ultraranger wrote:A few months back, my truck would sometimes clank when putting the transmission in gear and the truck developed a very bad shake at around 35 mph. I slid up under the truck and grabbed the drive shaft, at the rear U-joint, and could move the drive shaft up and down. I moved to the front of the drive shaft and couldn't move that end.

I took the shaft out to replace both joints. Even though I couldn't move the front of the shaft, when it was installed, the needle bearings on that end were just as bad as the rear u-joint.

This may or may not be your problem but u-joint are cheap and if you have never changed them there's no telling how long they have been on there or their condition.

I didn't try moving the drive shaft up and down, thanks for that tip. You're probably right about replacing the u-joints too so I'll be ording them soon.

lightfoot · Post by **lightfoot** » Tue May 13, 2014 11:11 am

BobbyFord wrote:It could also be the diff unloading and rotating after braking. A lot of the time this is due to worn spring/shackle bushings.

Not sure what you mean by the differential unloading and rotating after braking. Can you give me more details? Thanks,

BobbyFord · Post by **BobbyFord** » Tue May 13, 2014 7:06 pm

lightfoot wrote:...Not sure what you mean by the differential unloading and rotating after braking. Can you give me more details? Thanks,

Under braking the rear axle housing tries to rotate, pinion downward. If there is slop in the leaf spring bushings, it will allow further rotation than normal. After coming to a stop, with your foot on the brake the axle cannot rotate back to it's normal position. When you let off the brake and begin to accelerate the axle housing rotates back the other way.
If you or a friend has a GoPro, you can mount it under the vehicle and observe the driveline under acceleration and braking.

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