Specific clunk question

[SportZ2]

I have searched to try and find an answer to a specific clunk in the rearend and only found broad based answers. I'm hoping if I describe in detail when the clunk occurs someone might be able to help me pinpoint the problem area. here it goes.

 

I have a clunk whenever I put the car in reverse from Park at a stand still with the brake on (Auto tranny). N, D, 2,1 are all ok. I have no clunk when ever I manually shift from 1 to 2 to D no matter how hard I get on it. The clunk does occur though if I downshift from D to 2 to 1 and it is major. I'm guessing due to the fact that the car is moving and not standing still like when I go into reverse where the clunk is not near as loud.

 

I have replaced the m-bar busihings, but nothing else. Seems like everything seems to be in place. I have the V8 in the car now and never remember there being a clunk when I had the L6 turbo in it. I hope this is enough of a description for someone that might have had the same exact problem and knows exactly what to fix. If not, I'll just start replacing things one at a time.

 

Any help would be appreciated.

[innerware]

I am going to wager a guess. Since it does it when downshifting and going into reverse, you are putting load onto the driveshaft greater than normal and or reversing its direction. The sound can that you have sounds llike you have a loose (worn) universal joint in your driveshaft. Other reasons for a clunk in the rear are bad univerals at the halfshafts and diff mount. Although I don't think what you are describing sounds like that.

[datsunlover]

Another possibility is your tranny.. Could be some slop in the output shaft/driveshaft yoke and it 'kicks' when you go into reverse..

[SportZ2]

But downshifting does not change the direction of the driveshaft. The tranny is brand new (recently rebuilt) so I don't think thats the problem. If it was a worn out universial joint wouldn't the slop go both ways?

[HadesOmega]

So it doesn't clunk when you get on the throttle like if you blip it in gear?

[SportZ2]

No, it's perfectly fine unless going into reverse or downshifting from D to 2 to 1. I can upshift at 5 or 6k with no problems.

[jt1]

Lots of things can cause a clunk, and sometimes it's just a process of elimination, but since it seems to happen when the load on the R&P is reversed, I'm guessing 1) the diff has too much backlash or 2) the mustache bar mounts have some movement in them.

 

John

[mark]

I'm with JT on the Diff. Check the out put flanges for excess play. I bet you find alot of movement there.

[Dp351zcar]

My 71 did that and I jacked one rear tire off the ground and I turned one rear wheel and found my clunk was the connection between the inner and outer stub shafts.

Sorry on the correct names as my mind just went blank, the wife said dinner is ready and I am starved.

Anyway with my half shaft not moving I could move the wheel a little each way. The movement came between the companion flange splines and the stub shaft splines.

 

Don

[Guest el Jefe]

I had a similar problem with my '70 Chevelle. Every time I shifted from park to reverse or accelerated quickly I would hear a "clang". The yoke on my differential had a missing retainer (exatcly what is is called don't know or how it broke off) so the bearing cap came out. The yoke was a semi-circle with a little nub and brackets (completing the circle) to hold the cap from flying off from centripetal force. I don't know if you can see the little squarish nubs on the outside of where the u-joint sits or not, but it was gone. So, every time I accelerated quickly I would twist the u-joint causing the joint to hit against the yoke making a clunk and I never knew where it was coming from.

 

Well, I was showing off for a friend and hit a patch of sand on the road when we were going about 80 MPH, the tires slipped and strangely enough my front u-joint sheared off and not the rear u-joint. Luckily I had H-pipes just far enough back to keep the driveshaft from hitting the ground. My entire center console was hopping up and down and the under carriage was beat to hell. Not to mention it scared the ever living you know what out of my friend and me. I ended up using 4x4 u-joints after I replaced my driveshaft.

 

I don't know if this is something similar, but checking your drivetrain would be worth your time.

[Phantom]

Since you didn't mention what car this is in reference to I'm going to assume it's a 240Z based on your avatar. The description indicates that it's some form of lash. It's either in the differential or in the driveshaft universals or in the half-shaft universals. You can jack the wheels off the ground, rotate the half shafts and find it with a little patience.

[SportZ2]

An update to the specific clunk I was asking about.

 

I found out what it was.

 

The front diff mount was no good. The rubber was torn away from it's original spot and only holding on by a little bit. I found this out by having a friend sit in the car and shift the tranny from reverse to drive to second to first and back. I could see the mount move a good inch downward on all down shifts and slam into the front diff cross member creating the loud clunk. The strap cushioned the upshifts.

 

I fixed it by putting in the solid diff mount from MSA. Fixed the downshift clunk instantly. As far as more road noise, It really isn't that bad and I can only here it more when the windows are rolled up.

 

There still is some clunks in the rear but they are miner in comparison to what I was experiencing with the stock mount. I figure new u-joints will help with those. I was able to get the car out on the track and had no problem with hard shifting in the turns either up or down.

 

Just thought you might want to know a certain fix to a specific clunk.