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epsylon · Posted: Thu Jan 20, 2005 11:45 am Post subject: Rear End Knock

My 931 has developed a nasty loud knock coming from the rear. I was in first gear pulling from a light when I heard the sound and drove it to the shoulder. Car has been towed and is now in my garage. I jacked up the rear and rotated the rear wheels by hand and I get 2 knocks per revolution. The sound sounds like it is coming from the tranny. The car is in nuetral. Any ideas as to what broke.
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'82 931
'78 928
Howell Henderson
South Padre Island

924RACR · Posted: Thu Jan 20, 2005 11:32 pm Post subject:

Start by checking the simple stuff, are all the CV bolts still firmly torqued?
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Zuffen · Posted: Fri Jan 21, 2005 7:42 am Post subject:

have someone else spin and get down there with a mechanics stethoscope

I had knock in a CV joints under load from the passenger side

you always remove the cv axles and run the tranny from the engine if you think it may be a bad diff.

also drain you diff fluid and check for metal, run a magnet down the bottom of the trans
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Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951

epsylon · Posted: Fri Jan 21, 2005 7:42 am Post subject:

CV Joints appear fine. Boots are still intact and bolts are tight. I thought it might be a bad joint but I think the sound is coming from insde the tranny case. What are the symptoms of a failing/failed diff?
_________________
'82 931
'78 928
Howell Henderson
South Padre Island

Zuffen · Posted: Fri Jan 21, 2005 7:52 am Post subject:

noise, followed by metal, followed by more noise, followed by lock up

the internal condition of the CV's is unknown though.

if they are original and never been repacked then your looking at old hardened grease and wear.

You can rebuild cv's by reversing the inner knuckle, the balls have to be in good shape.

might be time to pull them out and take a look

you can buy jus the cv's or the entire axle prepacked and ready to install.

removing cvs from the axles isn't the easiest....it has been years since I did it, pretty messy and the grease has lead so you need to wear gloves
_________________
Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951

Smoothie · Posted: Fri Jan 21, 2005 8:39 am Post subject:

Yeah I'd think if the noise was bad enough to make you pull over and get a tow, it's probably something more serious than CV joints. I've driven for months on clicking/clunking CVs - the cheap-easy fix for that is to just turn up the music.

There are a few things you can do to further isolate the problem in addition to listening with a stethescope. -
1) jack up just one side of the rear, put it in neutral and spin the jacked-up wheel forward, listen, then spin it backward and listen. -Then do the same with the other side jacked-up. If you get the same sound spinning just the left as you do spinning just the right, then it's not a CV joint problem (this also rules out a diff side-gear problem).
2) Then you can jack up both rears at the same time (assuming it's not a LSD for this step as well as step 1), put the transmission in gear. This will hold the ring gear and input pinion of the differential from moving. spin one of the rear wheels (the other one should spin in the opposite direction by itself). If you get the noise now, you know it's not a ring and/or input pinion problem and the problem is with the cage-pinion/side-gear assembly. If no noise, then it is a ring-gear and input-pinion problem (or a problem elsewhere in the transmission).
http://auto.howstuffworks.com/differential3.htm
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

numbbers · Posted: Fri Jan 21, 2005 11:08 am Post subject:

Well, CV joints will make a loud clicking sound. So, I would disconnect the inner CV joints, and then rotate the tranny via the input shaft and see if your noise goes away.
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1980 924 Turbo

epsylon · Posted: Sat Jan 22, 2005 5:19 am Post subject:

The sound is present when I spin either side while keeping other the from moving.

I will check to see how it sounds with the car in gear. Although the car was in gear when I was puilling from the light and driving to the shoulder and the knock was very much present.

Back to driving my gas eating F-150
_________________
'82 931
'78 928
Howell Henderson
South Padre Island

Zuffen · Posted: Sat Jan 22, 2005 5:32 am Post subject:

I have an 82 931 tranny cleaned and ready to ship, if you need it. Check for Forward Air or Old Dominion trucking in your area. Shipping would be about 50 bucks, name a fair price and I'll get it to you.

bob
_________________
Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951