Grinding rear end - bad differential or other problem?
 

Hi everyone,

I'm experiencing a nasty sound in the rear of my car when i back up while turning. I've been told that it sounds like the posi unit in my differential is going bad or possibly something else back there. I'd like to know if anyone has any suggestions as to how I can narrow down the problem and if I might be able to fix it myself. I don't have much experience working on cars but since I have SPF 1683 I thought you guys could give me some advice. I also plan to buy a 4 post lift in the next few months so that should be a big help with any work I need to do.

Thanks,
Bryan

The differential in your car is probably a Ford 8.8. You should have independent rear suspension with half shafts containing CV joints. The half shafts have splines at both ends. The differential side set in place with a flexible C clip. On the outside they bolt in place when the wheel is off.

You may have a bad CV joint. I bought a complete new set of stronger half shafts from The Drive shaft shop. http://www.driveshaftshop.com They're familiar with broken Superformance half shafts and would be able to produce correct fitment.

I also replaced my Ford 8.8 limited slip innards with a Detroit Truetrac. http://www.detroitlocker.com/DT.htm It works better.

I got out of the entire job with upgraded parts for about 2000.00

You may also have a broken differential mount that causes the differential to rotate when in reverse and binding up the drive shaft.

Thanks for the feedback guys.
Greg, was the $2000 tag for the parts and labor? Just wondering if you did any of the work yourself.

I'm all for upgrading if I have to replace anything so I really appreciate that you included the vendors for the parts you used.

Any other recommendations from the viewing audience???

If you are going to pull all that out, you may want to reconsider the gear ration of the rear as well. I think most were usuing 3.73 ratio that is a little tall for most guys liking. I put mine to 331s and some guys even do 327s and like them. I can say changing was great for me. I had my guy put an Eaton posi unit in mine, he said it is bullet proof (he does a lot of 4x4 stuff here locally). there are a couple recalls you should check out too, one about the axel nut coming loose and a seconda about A arm misalignments....depending on your car number. Earlier cars apparently had problems snapping the passanger side diff mounts on some of the early cars as well. I had the diff gone through, changed gears, and all the bearings changed (kept the same shafts) and had the whole thing redon and it went for 1200 parts and labor. I have also read/heard that this diff does make noise that some dealers and service guys say is normal. I have never had sounds that I know of, pretty loud car. My bearings were toast on both sides and I had broken an A arm bolt which gave me a little wild shimmy and a shake at times. thought it was front end problems or tire balance, but turned out to be ass end weirdness. lots of help here, so keep asking questions and check those more common problems first

Buzz

It may me a much simpler problem.
The "Traction-Loc" (posi) clutches will sometimes "groan" when turning at low speed. You might want to try changing the gear oil and adding the proper amount (4 ozs.?) of "Friction Modifier" that is required for the Ford T-L differentials to keep the clutches quiet. You can get it at Ford/Mercury dealers.
It has cured the noise on three Fords I've owned. You may have to drive a couple of hundred miles before it quiets down.

I did the removal and install of half shafts and rear end as I wanted to learn how it works and clean everything in there while I was at it. It was a reasonably smooth learning curve with the club cobra and internet at my finger tips and the helpful shops which the parts came from. For me this is part of making the hobby fun. I just like to know what happened when something occurs on my car.

The Detroit Truetrac was 450 dollars however I did pay labor to have it take the position of the limited slip at SVC Motorsports in Scottsdale, AZ. It still looks like a Ford 8.8 on the outside, but the guts are just different now. SVC is an extremely detailed shop that digs into detail. I wanted to be certain gear tolerances were perfect and they let me see the job being performed.

The Detroit Truetrac is superior out of corners and in a straight line without doubt. If you do a search on the internet the opinion is about 99% the same. The new half shafts are stronger than the original type that came on the Superformacne with the Ford 8.8 rear end, but not everyone needs to go stronger.

If you make the change you'll have the option to go to 31 spine parts from 28, but it will cost a bit more as you'll need to change the wheel bearing with splines on the outside. You should change the inside bearing anyway. The drive shaft shop would call it their stage 5 instead of stage 2. Actual Ford parts are stronger than the Superformance parts, but the center bar needs to be the correct length. I went with the 28 spline again at the advice of the drive shaft shop saying it would be fine for a car and driver of 2550 lbs.

If you do a search on the forums you find people have used the Superformance half shafts without failure while others have not. I'd estimate much of this has to do with actual usable power on the road, how sticky their tires are, how much side wall flex the tires have and how they start from a stopped position. Engine HP is irrelevant as there are way too many variables by the time it's in the car. Trap speed at the strip or even time would provide a better estimate of strength necessity.

Here's a drag race calculator to estimate usable power. The trap speed calculation would be most useful as times vary greatly while speed only varies slightly at the end of the 1/4.

http://www.pro-system.com/calc.html

Oh, wanted to say, www.buckshotracefab.com makes a strengthening bracket for the differential if that's your problem or you wanted one for safe measure.

I have the drive shaft parking break which looks as if it would add additional differential rigidity, but not the part made by Buckshot Racefab.

FYI, I installed this one from Steeda...30 minute job max. They are a good idea, as the alum cover is a weak link.

http://www.steeda.com/products/cobra...over_brace.php

-Dean