I've got an 89 IROC 350/700 (see mods below) with a rear end out of a 92 with 3.73 after market gears.Through some searches and previous posts/threads, I have been told it is the 7 5/8"--28 spline with the Auburn cone posi unit ( have not confirmed that by visual inspection, but will soon).

A few weeks before X-mas, I went out to the local 1/8 mile strip to make a few passes. When doing my burnout before pulling up to stage, I noticed that the left tire was not spinning like normal (right was), I went ahead and made the run anyway (mistake), when I took off at the light only the right tire was pulling/spinning which let it over rev the engine, which broke a rocker arm. I tested it doing a couple more burnouts and only the right side was pulling. Once I discovered I had hurt the engine, I limped it home, pulled the heads for a valve job, replaced the rockers with some Proform roller tips. It took a while for me to get the car back together because I got covered up at work, and the car sat for about 6/7 weeks.

Now that it is back up and running, it was time to look into reparing the posi unit, but in it taking it for a few test drives, I have done a number of second gear burnouts, as well as first gear launches. The posi seems to be pulling on both sides--leaving equal burn marks on the pavement--pulling out of the whole like it used to.

My questions are:
What the heck is going on here?
Is this particular rearend/posi unit known to sometimes not work?
Is it possible for something to get out of whack in this rearend and correct itself?
Is this a sign that it is on the way out.

Since I have been told, that this rearend is not rebuildable, I was ready to buy a new posi unit, but now I'm not sure what to do!!!

Yeah, I know-- "If ain't broke, don't fix it", but I'm not sure if it's broke or not!

HELP!!!

PS One thing I wanted to add is that at the strip u have to make a hard left to pull onto the strip and only pull up about 5 to 10 feet before you are in the water box. Is it possible that the rear/posi unit "unlocks" to make a turn like that and it had not yet locked back up when I did the burn out and the run? (I realize that may be a stupid question, but I'm not real sure exactly how this particular style posi unit functions.)

 

Don't assume anything. First of all pull the rear cover off the diff and have a look inside to see if everything is ok. It's possible the clutches are worn out and when the oil gets hot, they don't grab. There's a specification to test a posi by jacking up one wheel and using a torque wrench to see how much torque it takes before the clutches slip. If they slip below the minimum torque then the posi isn't going to work very well. I have no idea what the spec is.

 

ANYBODY?

 

HHmmm.. I was thinking the Auburn did not use clutches?

Anybody shed more light on the procedure Stephen 87 IROC is referring to here?

 

I had the same problems with my rear last year at the track. while doing a burnout the LR would stop. Same thing happened when launching at the track, but not on the street. the clutches were getting worn. i just went ahead and installed an auburn pro series posi unit... garunteed not to slip.