Rear-Diff Diagnosis

[belize1334]

My 4-bolt is in a bad way (I think) but before I pull the trigger on a new one I'd like to make sure and determine whether it can be rebuilt.

Is it possible to just pull the rear cover off of a 4-bolt and visually inspect the gears / shims to determine their condition? Could somebody give some pointers on what to look for and where? Maybe a picture with one of those fancy red circles indicated where the shims are inside the housing?

 

[Barnes]

What is happening that makes you suspect the rear diff?

 

[belize1334]

It's a loud shudder that I hear and feel going through the car which clearly comes from the rear-end. It's gotten progressively worse and it first showed up the day I put this thing in. It's worse when I'm engine braking but I can still hear/feel it anytime I'm not deep in the throttle.

There is the possibility that it's transmission related but, IIRC, this started happening before I swapped to my DSM transmission and was not affected by the swap. It's also possible that it's x-fer case related but I've had two different units in there and there was no change so it seems unlikely that they'd both cause this. And believe me, if you road in the car you'd say, "it sounds like your rear diff is about to blow up", cause that's what it sounds like.

 

[bazeng]

You could probably try and feel the slop in it by jacking it up and holding onto the tail shaft (driveshaft) and turning the axel and seeing how much slop is in the diff externally.

 

[belize1334]

I'll try that. I'm also gonna drain the fluid and look for shiny stuff. Still, I know there are some around here who've dealt with the shims on their diffs and I'm wondering if anybody can offer some insight on what the insides of these look like and which bits go where.

 

[jepherz]

Horrible picture, but it gives you an idea. Imagine the rear diff with the rear cover off. The center gear section (Don't know the technical term) is sandwiched between the shims on either side (blue) .




When you are shimming the rear diff, you are moving the center section either left or right, in order to make more or less contact with the pinion. Moving it left, makes more lash.

Rebuilding the diff pretty much involves replacing the bearings and re-shimming (assuming all internals are good). On a mitsubishi diff, everything comes out the back.

 

[Rausch]

Quote:
(Don't know the technical term)

Carrier?

 

[belize1334]

Well I pulled it into the garage last night and checked the slack in the rear diff. I don't know the spec but it seemed excessive. I then drained the fluid and found no metal debris, which is promising. So, scientist that I am, I swapped the rear diff for the 3-bolt LSD out of my '90 Tsi part-out. Today I take it for a test drive and the sound is STILL THERE. It's not quite as loud as it was but there's still a pronounced growl from the rear-end. But, since it isn't quite as loud as it was, I am also now able to hear some nasty grinding noises from my transmission. I'm thinking that the rear-diff was just acting as a sounding box for some transmission noises and the reason that it's not quite as loud now is that the 3-bolt has less slop and thus doesn't communicate quite as much noise (for some reason).

Next step is to try my other transfer case and if the noises persist (which I expect they will) I'll just send all my transmissions (I have 3 at this point) to Lucas English and have him apply their core values to the cost of a stg-2 rebuild.

 

[belize1334]

Quick update:

I decided to check my output shaft nut to see if maybe that was loosing and causing lash in the transmission. Before I even got that far I drained the oil and guess what? It looks like glitter glue. It's a suspension of little bits of aluminum. So, the moral of the story is, even if you think it sounds like your rear diff is about to explode and you're certain you remember this happening before you swapped in that new-used transmission you bought on craigslist ... it's probably your transmission.

F**K!... That's all I need right now is to drop a grand on a transmission.

cheers

 

Roger, I feel your pain. When my transmission's input bearings started to go (synchros were long gone) the redline oil came out looking like metallic flake paint. I haven't put the other transmission in yet; how's yours doing?

 

[belize1334]

Loving life. It's dripping purple oil now...which is odd since the new oil I put in is greenish and the oil that came out is reddish and green+red is not purple. Oh, well...just chalk it up to bad things happening inside. I don't drive it much at this point so I expect it'll last 'till summer and I'll deal with it then.