I finally got the car (1990 Z28, 355) back together well enough to take it out on the highway. Mildly built 700R4 (Redline clutches, Beast shell, .50 TV boost valve, heavy duty pump spring, wide HD bands, etc.)...

Around 2004, I swapped in a low-mileage 2000 TA 4th-gen rear end. I didn't have any trouble for the year or two (stock 350 & 700R4) before I took it off the road and decided to change engines, transmission, brakes, suspension, etc... I'm 99% sure the 4th gen rear-end is 3.23, same as the stock rear end. Tire diameter is same as stock (235/40/18 vs stock 245/50/16, a difference of maybe 1/4").

So here's the problem: now I'm seeing very high RPMs ... at 65 MPH I'm turning 3000 RPM. 3500 RPM at 80 MPH. At first I thought maybe I have a taller rear-end gear than I realized, but per the online calculators I'd have to be somewhere in the 4.50 range to account for those RPMs. And of course, there is the little matter that it wasn't winding itself out with the stock engine & transmission. It's definitely shifting into 4th, too.

A friend of mine damaged the rear-end in his pickup truck and it did weird things, and he suggested maybe that's the problem. So I'm wondering if there is a way that a broken rear end could cause this.

Were all 4th gen TA rear-ends posi? I thought this one was, but as I said, it hasn't been on the road for a couple years until now. I ask that because I jacked it up and the rear wheels spin in opposite directions, which I'm pretty sure is a sign of an open or limited-slip. With one wheel on the ground, the jacked wheel still turns. As I understand it, if it's a posi, that's bad news.

Did I blow up my rear-end already?

 

What is your torque converter's stall? Bad sign if thats a posi... just replace the clutches?

 

Most likely your tach is off.

 

2000 stall.

I haven't jacked it up to get numbers off it yet, I'll do that this weekend (I did my little wheel-spin test using a pair of floor jacks, obviously I'm not crawling under it that way). It's a 10-bolt. All I can see is laying on the floor next to it is "08 36" stamped into the cover. Does that tell me anything useful?

I dunno about the tach being off, it sounds like it's winding up pretty good...

 

how do you know it is shifting into 4th? are you sure it isn't the torque convertor locking up that you feel? sounds like you aren't shifting into 4th.

 

If you consider stuff like the date/shift code from the cover stamping plant "useful", yes. Otherwise, probably not.

Check your tach. Don't just blow that off. You're new here: you likely have NO CLUE yet how many people have posted the same sort of thing in the past, and it ALMOST ALWAYS turns out their tach is wrong. These tachs always fail, every one (I guess it's just like hard drives: there's 2 kinds in the world, those that have already failed, and those that will fail later), and they always fail by reading high. Sometimes twice the actual RPM or occasionally even higher. Usually they kind of creep up over a period of time, but who knows.

Also check your gears. Am I correct in guessing that you actually put a JUNK part in your car and DIDN'T EVEN CHECK THE FLUID???? You need to do that, regardless. While you're catching up on what should have been done as basic inspection of a newly-acquired used part from a random source, make a point of counting the teeth, or locating the stamping in the edge of the ring gear.

 

Fluid was changed prior to installation. Seller was well-known to me, significantly reducing concerns with it being a "junk" part. Not to mention that I drove around on it for nearly two years without any issues at all.

I didn't "blow off" anything. I was actually reading the tach threads before it was even mentioned here. I suppose it's probably wrong too (they all are, it seems), but nonetheless, as I've already described, it's very obviously revving quite a bit. No, I don't specifically know how much yet.

I agree, it sounds a lot like it isn't shifting into 4th. I know it shifts just fine into 4th at lower speeds (40-45 MPH). I'm assuming it shifts under harder acceleration because the RPM drop is considerably greater than the roughly 200 RPM decrease due to lockup.

Am I correct about the diff behavior when one and when both tires are off the ground? A posi should spin both in the same direction, or not at all with one tire on the ground?

 

You're correct about the posi behavior, except in the case of a 98-newer, zexel-torsen type posi. If you have a torsen posi in there, the wheels will spin opposite directions, just like an open diff. You really should pull the rearend cover. That way you can determine what type of diff you have, and you can also verify the gear ratio for certain. Whatever the case, your high rpm's aren't because of anything broken in the rearend. A burnt out posi won't cause that, and if you had broken ring/pinion teeth, that'd be obvious from the noise, jerking, etc. The only way the rearend would cause excessively high rpm's would be due to really low gears, such as 4.10 or 4.56. I'd say you're either not going into 4th gear, or possibly have a problem with the shift governor. Also might want to check the adjustment on your shifter cable and the TV cable.

 

Thanks, I'll pull the cover in a day or two. The guy I bought the rear-end from said it should be a Zexel, and he said it was bone stock, and that all the rear-ends he's ever owned were 3.23s -- and of course, I didn't have high RPM troubles with the stock engine/trans.

I appreciate the input from everyone.