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Old 07-23-2005, 19:01   #1 (permalink)
cprice@here.com
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LONG: Help diagnose rear diff noise - 1997 Cobra



1997 Cobra, 60K miles, bone stock. Rear diff is an 8.8, same at GT for
past many, many years.

I have been noticing quite a bit of noise coming from the rear diff on
the car. At slow speeds (<15mph), the noise is either not there or is
barely noticeable. At higher speeds, it becomes a kinda loud noise,
which is constant in pitch and level a a specific speed. The noise does
not change pitch, level, or go away when I step on the brakes. I do not
have a point of reference for how loud a rear should be in a late model
GT/Cobra, so it might be completely normal.

I've had rear diff gear sets installed incorrectly in the past which
resulted in 'howl' coming from the rear end. The noise I am hearing is
definitely not the same as the 'howl' I have had in the past, its more
like a dull grinding noise. Like something is rubbing, or a bad bearing
(eg: outer axle bearing).

I took the rear apart today to try and narrow down the problem. With
the rear on jackstands, I spun the rear wheels. It seems like I can hear
something coming from the rear end while I turn by hand. Its not
obvious, and the rear turns smoothly, but does require a moderate amount
of effort. Again, I do not have a point of reference for how loud a rear
should be, so it might be completely normal.

Figuring that there might be something wrong, and feeling motivated
(maybe stupidly), I removed the rear wheels, rear brake calipers, rear
caliper mounts and the rear discs. I then turned the rear axles by hand
and the noise was the same. I then pulled the driveshaft and turned the
axles again and still the same noise. I then pulled the center-section
cover, removed the wrist-pin and c-clips and pulled the axles. Neither
wrist-pin or c-clips show any signs of abnormal wear (grooves, galling,
etc). I then turned the gearset by hand, and while it turns smoothly, it
seems to make a moderate amount of noise while turning.

So basically I *think* I've eliminated rear brake noise, U-joint and
outer axle bearing as sources of the noise. Now I am down to:

1) Pinion bearing/race
2) Carrier bearing(s)

Can I use a torque wrench to see how much resistance the rear gear set
is providing?

I'd like to hear some thoughts on this.

Do 8.8 traction-lok rear make a good deal of noise? I had the rear diff
fluid changed at a 'Mr Lube' about a year ago (similar to Jiffy Lube).
They use a vacuum to suck the old fluid out and then put new fluid in.
Now I know that 8.8 trac-loks need the friction modifier gunk from ford.
I know I asked the guy at the Mr Lube if they were going to put the
friction modifier in with the new fluid, and I am pretty sure he told me
that since they were putting in GL-5* 80-90w I didnt need the friction
modifier.

TIA for your responses.

Chris

P.S. I am also posting this in the SVT Tech Stangnet Forum.
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Old 07-24-2005, 14:02   #2 (permalink)
toytrd89
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Re: LONG: Help diagnose rear diff noise - 1997 Cobra

Is there a change in pitch when you let off of the gas at higher speeds? If
the noise changes pitch with heaver throttle it sounds like the pinion
bearings. And if you have the rear end already torn down you should also
check your backlash on you ring. "shouldnt be more than .010".

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Old 07-25-2005, 00:01   #3 (permalink)
Ashton Crusher
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Posts: n/a
Re: LONG: Help diagnose rear diff noise - 1997 Cobra

On Sun, 24 Jul 2005 01:20:25 GMT, "cprice@here.com" <cprice@here.com>
wrote:

>
>
> 1997 Cobra, 60K miles, bone stock. Rear diff is an 8.8, same at GT for
>past many, many years.
>
> I have been noticing quite a bit of noise coming from the rear diff on
>the car. At slow speeds (<15mph), the noise is either not there or is
>barely noticeable. At higher speeds, it becomes a kinda loud noise,
>which is constant in pitch and level a a specific speed. The noise does
>not change pitch, level, or go away when I step on the brakes. I do not
>have a point of reference for how loud a rear should be in a late model
>GT/Cobra, so it might be completely normal.
>
> I've had rear diff gear sets installed incorrectly in the past which
>resulted in 'howl' coming from the rear end. The noise I am hearing is
>definitely not the same as the 'howl' I have had in the past, its more
>like a dull grinding noise. Like something is rubbing, or a bad bearing
>(eg: outer axle bearing).
>
> I took the rear apart today to try and narrow down the problem. With
>the rear on jackstands, I spun the rear wheels. It seems like I can hear
>something coming from the rear end while I turn by hand. Its not
>obvious, and the rear turns smoothly, but does require a moderate amount
>of effort. Again, I do not have a point of reference for how loud a rear
>should be, so it might be completely normal.
>
> Figuring that there might be something wrong, and feeling motivated
>(maybe stupidly), I removed the rear wheels, rear brake calipers, rear
>caliper mounts and the rear discs. I then turned the rear axles by hand
>and the noise was the same. I then pulled the driveshaft and turned the
>axles again and still the same noise. I then pulled the center-section
>cover, removed the wrist-pin and c-clips and pulled the axles. Neither
>wrist-pin or c-clips show any signs of abnormal wear (grooves, galling,
>etc). I then turned the gearset by hand, and while it turns smoothly, it
>seems to make a moderate amount of noise while turning.
>
> So basically I *think* I've eliminated rear brake noise, U-joint and
>outer axle bearing as sources of the noise. Now I am down to:
>
> 1) Pinion bearing/race
> 2) Carrier bearing(s)
>


My guess is carrier bearings.


> Can I use a torque wrench to see how much resistance the rear gear set
>is providing?
>
> I'd like to hear some thoughts on this.
>
> Do 8.8 traction-lok rear make a good deal of noise? I had the rear diff
>fluid changed at a 'Mr Lube' about a year ago (similar to Jiffy Lube).
>They use a vacuum to suck the old fluid out and then put new fluid in.
>Now I know that 8.8 trac-loks need the friction modifier gunk from ford.
>I know I asked the guy at the Mr Lube if they were going to put the
>friction modifier in with the new fluid, and I am pretty sure he told me
>that since they were putting in GL-5* 80-90w I didnt need the friction
>modifier.
>
> TIA for your responses.
>
> Chris
>
>P.S. I am also posting this in the SVT Tech Stangnet Forum.


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Old 07-25-2005, 19:01   #4 (permalink)
cprice@here.com
Guest
 
Posts: n/a
Re: LONG: Help diagnose rear diff noise - 1997 Cobra


Thanks for the replies. I think its pretty much impossible to figure
out whether its pinion or carrier, so I am going to replace the whole
schmeer to be sure.

New questions:

1) Does a new pinion bearing require a new race? I have always assumed
they get installed in pairs.

2) If I get a new pinion bearing, do I need to reset/check pinion depth?

3) (as above) Do new carrier bearing require new races, and if so, do I
need to confirm/set backlash with the new bearings/races?

I am all set to replace the outer bearings. While I own a mag base
stand and a decent quality dial indicator, I think that the pinion and
carrier bearings are beyond my current abilities. Besides, this might be
a great time to go to a 4:11 gearset. :)

Ashton Crusher wrote:
> On Sun, 24 Jul 2005 01:20:25 GMT, "cprice@here.com" <cprice@here.com>
> wrote:
>
>
>>
>> 1997 Cobra, 60K miles, bone stock. Rear diff is an 8.8, same at GT for
>>past many, many years.
>>
>> I have been noticing quite a bit of noise coming from the rear diff on
>>the car. At slow speeds (<15mph), the noise is either not there or is
>>barely noticeable. At higher speeds, it becomes a kinda loud noise,
>>which is constant in pitch and level a a specific speed. The noise does
>>not change pitch, level, or go away when I step on the brakes. I do not
>>have a point of reference for how loud a rear should be in a late model
>>GT/Cobra, so it might be completely normal.
>>
>> I've had rear diff gear sets installed incorrectly in the past which
>>resulted in 'howl' coming from the rear end. The noise I am hearing is
>>definitely not the same as the 'howl' I have had in the past, its more
>>like a dull grinding noise. Like something is rubbing, or a bad bearing
>>(eg: outer axle bearing).
>>
>> I took the rear apart today to try and narrow down the problem. With
>>the rear on jackstands, I spun the rear wheels. It seems like I can hear
>>something coming from the rear end while I turn by hand. Its not
>>obvious, and the rear turns smoothly, but does require a moderate amount
>>of effort. Again, I do not have a point of reference for how loud a rear
>>should be, so it might be completely normal.
>>
>> Figuring that there might be something wrong, and feeling motivated
>>(maybe stupidly), I removed the rear wheels, rear brake calipers, rear
>>caliper mounts and the rear discs. I then turned the rear axles by hand
>>and the noise was the same. I then pulled the driveshaft and turned the
>>axles again and still the same noise. I then pulled the center-section
>>cover, removed the wrist-pin and c-clips and pulled the axles. Neither
>>wrist-pin or c-clips show any signs of abnormal wear (grooves, galling,
>>etc). I then turned the gearset by hand, and while it turns smoothly, it
>>seems to make a moderate amount of noise while turning.
>>
>> So basically I *think* I've eliminated rear brake noise, U-joint and
>>outer axle bearing as sources of the noise. Now I am down to:
>>
>> 1) Pinion bearing/race
>> 2) Carrier bearing(s)
>>

>
>
> My guess is carrier bearings.
>
>
>
>> Can I use a torque wrench to see how much resistance the rear gear set
>>is providing?
>>
>> I'd like to hear some thoughts on this.
>>
>> Do 8.8 traction-lok rear make a good deal of noise? I had the rear diff
>>fluid changed at a 'Mr Lube' about a year ago (similar to Jiffy Lube).
>>They use a vacuum to suck the old fluid out and then put new fluid in.
>>Now I know that 8.8 trac-loks need the friction modifier gunk from ford.
>>I know I asked the guy at the Mr Lube if they were going to put the
>>friction modifier in with the new fluid, and I am pretty sure he told me
>>that since they were putting in GL-5* 80-90w I didnt need the friction
>>modifier.
>>
>> TIA for your responses.
>>
>> Chris
>>
>>P.S. I am also posting this in the SVT Tech Stangnet Forum.

>
>

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Old 07-28-2005, 08:01   #5 (permalink)
toytrd89
Guest
 
Posts: n/a
Re: LONG: Help diagnose rear diff noise - 1997 Cobra

When you replace your carrier and pinion bearing's you always need to
replace the race because it has worn to the previous bearing and would
either cause scoring or noise. When replacing the pinion bearings you will
have to reset pinion depth, but usualy you can reuse the same shims and be
pretty close. The carrier is a little more tricky, after you install new
bearings and races you need to check your back lash and you are going to
need marking compound to see your gear mesh pattern. Is your cobra stock?
A taller gear is always a good Idea for modular cars do to the heaver
weight, go with 4.10 or 4.30 but you will loose top end. Now if you have
boost then I would go with 3.73's

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Old 08-04-2005, 07:01   #6 (permalink)
D
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Posts: n/a
Re: Help diagnose rear diff noise - 1997 Cobra

I just discovered the source of that noise in my own '89 5.0 LX Sport. Mine
had gotten to the point of being shaky at 90 mph. I kept hearing noise then
I noticed clunk clunk as I got on and off of the gas. I thought gearset and
bearings but I could find no slop in mine and the patterns were dead on
front and back so..... I thought it is just the polygraphite bushings making
the noise and 'gear clunk' and high speed vibration more pronounced. I knew
that wasn't right and I was getting ill about it. I had even spun the axles
up on a lathe to make sure they weren't twisted. Then one night I was
adjusting the valve lash on my 1965 11:1 293 (289) CID fuel injected Ford
small block that is hooked to this rear end thru a T5 and it came to me.
The differential bushing indirectly supports the inside end of the rear
axle by directly supporting the side gear. With the side gears and the
clutches retained by the spring, it isn't evident that there is any play
inside the carrier. I had even taken my clutch pack apart and restacked it
still not noticing the slop, in one side in particular, of the side gears
because I didn't think to set the side gears in place without the clutches
there.

I had already changed my outer rear bearings and checked the axle races for
wear and concentricity.

I have allot of nine inch stuff and a big steel table, I was thinking
about jigging the table up with the 8.8 and putting nine inch ends on it.
The reason for that sudden change of subject was side load. The retainer
grooves and ends of my 8.8 axles were blue. There were marks in the gear
pin where the axles had briefly touched it at some point before. It seems
to me that this side loading of the carrier bearings is just not good. Not
to mention the possibility of popping one of those lil buttons off of the
end of an axle.

My two Cent.
D


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Old 08-04-2005, 07:01   #7 (permalink)
Big Al
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Posts: n/a
Re: Help diagnose rear diff noise - 1997 Cobra


"D" <ddelamar@spamcharter.net> wrote in message
news:SnoIe.3372$_41.2469@fe02.lga...

____SNIP___
>
> The reason for that sudden change of subject was side load. The retainer
> grooves and ends of my 8.8 axles were blue. There were marks in the gear
> pin where the axles had briefly touched it at some point before. It seems
> to me that this side loading of the carrier bearings is just not good. Not
> to mention the possibility of popping one of those lil buttons off of the
> end of an axle.
>
> My two Cent.
> D
>


Well you lost me:) The axle is kept in the housing by the pin and the "C"
clip. Pin keeps it from going in, "C" clip keeps it from going out, correct?
What side loads are you talking about?

Al


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