Trying to diagnose a new driveline clunk

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  1. #1
    Senior Member Array torchred04's Avatar
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    Nov 2004
    Calgary Canada

    Default Trying to diagnose a new driveline clunk

    Been searching on here and svtperformance trying to see if I can pin point what might be causing my issue. Sounds like most times its the rubber front and rear diff case bushings, mine are aluminum FTBR bushings and everything seems solid and tight on that end.

    Now my IRS subframe and control arm bushings are the 15 year old stock rubbers.
    My driveshaft u-joints are still original.
    I have rebuilt the clutch packs in my stock differential with the Ford rebuild kit, but the differential unit its self is original. Stock gears.
    Stock half shafts

    I have a clunk noise I did not have prior when I go to get moving in first gear. You can hear it as you let the clutch out. I can make the sound all the time if I clutch in/clutch out on flat ground, moving up my driveway, etc.

    I am going to replace my u-joints with spicer 5-1350x units, cheap item to change first.

    My next question is how often do the side gears go/get too much play and same with the stock half shaft CV's? I have a bit of play with the park brake on and car in first if you go under and rotate the driveshaft by hand, but its not a large amount.

    Video below is park brake on and my foot on the brake. Letting clutch out in first gear to try and replicate the sound I get with the car on the ground. Decent amount of movement in the upper control arms.

    Just trying to get some opinions on where to target in on. I am not sure if its the side gears in the diff or a function of the rubber bushings finally getting too old.

    Let me know what you guys think.

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  3. #2


    Although it is difficult to positively anchor the upper control arms without adding an additional link at the top to restrain them, what you are experiencing is fairly significant movement even at very low power levels, Schyler. I would expect to see significantly more movement under power and that's not good.

    To get that type of movement, your lower bushings are probably asking to be replaced in addition to your uppers. If you wanted to take the time and effort you can make a radius rod that connects from the frame behind the axle and to the top of the spindle. The radius rod would need to have freedom of movement in the vertical plane to accommodate the range of vertical wheel movement the suspension allows.

    Once installed it would preclude the forward movement of the upper portion of the spindle. The bushings would still need to be maintained on a regular basis however, because of the loads they would have to accommodate under power. Although this would certainly tidy up the monkey motion on the rear wheels it is probably not the source of the clunk you are hearing.

    My suspicion is the clunk is a ring & pinion backlash related issue. It is possible to reassemble the reared after a clutch refresh and leave a whisker too much backlash. That will create a noticeable clunk of the type you are describing.

    Sadly to get to the R&P you have to pull the pumpkin — as you already know. If you do, set it up on your workbench and measure the backlash at 90˚ intervals around the ring gear. You will probably get different measurements. If you did, when you last assembled the rear end, (and you removed the ring gear) remove it once again and check for clean the surfaces on the carrier and the back of the ring gear. Reassemble the ring gear to the carrier and torque down the gear.

    When you do the first trial assembly check backlash every 90˚. If it is not uniform remove the carrier and ring gear and rotate them 90˚ or 180˚ to equalize the backlash. Once you have uniform backlash then begin the process of closing it up to Ford specs. If you like drag strip style launches then you want to go to the low end of the Ford spec. You can go tighter than the spec but not more than 0.001" tighter.

    Additionally you want to check backlash every 90˚ after your final lash adjustment to make sure the lash is still consistent and not binding anywhere.


  4. #3
    Senior Member Array torchred04's Avatar
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    Nov 2004
    Calgary Canada


    Ed thanks.

    Would the backlash issue be right from the get go? I drove a decent amount last year before the "clunk" noise appeared. I don't remember removing the diff or ring gear, I want to say we just removed it all while still in the housing. We did check the backlash after, I am by no means an expert at rebuilding diffs though.

    I think the bushings are long over due for a change out. Just want to make sure when all the crap is on the floor I don't leave anything out!

    If I open up the diff to check all this is there anything else I should replace?


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  6. #4


    If you didn't remove the ring gear from the diff and just disassembled the diff to replace the clutches you may have introduced contaminants at the differential housing parting lines which would have a similar effect as to getting them between the ring gear and the drive flange on the diff.

    Before disassembling everything in sight, I would do a backlash check on the ring and pinion just as they are currently installed. I have attached a Ford Installation guide (Ford 4209-8.pdf) to this post for additional information. Check on pdf page 17 for backlash specs and other interesting stuff. The Ford spec calls for 0.008" to 0.011" backlash. I would not go tighter then 0.007" for a street driven car.

    The bulletin suggests that the lash should vary no more than 0 to 0.004" as you check every 90˚ on the ring gear. My experience with previously used R&P's is that you will be able to get even less variance than Ford is asking for because the two gears have worn into each other over thousands of miles polishing off any high (tight) spots. I would not be surprised to find that you can get a backlash variation that repeats with a 0.001" to 0.002" max difference all the way around the ring gear, possibly even less.


    Ford 4209-8.pdf

    p.s. A good rule of thumb in these events is don't fix anything that is not broke. It makes your job easier and does not detract from the quality of the finished product effort. Be sure to pay attention to case preload in the Ford doc.
    Last edited by eschaider; 03-25-2019 at 02:51 PM. Reason: Broken Attachment Link

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