Something's busted, thought it was my rearend

'87 Caprice

Last night I'm driving and getting on it pretty good when all of a sudden when I'm giving it a good amount of gas, BAM! and the engine revs and car will only coast. Coast it into a parking lot to find when I give it gas the motor just revs, no power is transferred to the wheels. Looked under the car and there wasn't anything leaking or hanging. I was blocking a car that wanted to pull out so put it in neutral and attempted to push it backwards but it wouldn't budge. So I had to push it forward into a spot and it pushed forward no problem. Could this be a trani issue? Or a rear end issue? I pulled the diff cover this morning and there's nothing I can see obviously wrong but I'm no expert.

This things got a 383 with about 307 ft lbs of torque and I put in a rebuilt 700r4 with a shift kit. The rear end is what came in this '87 and the driveline is a donor straight from a junkyard 700r4 wagon. I have plans of getting a rebuilt posi in there so maybe now's the time? Was hoping to wait til winter when I'm parking the thing to prep for paint.

When you turn a tire does the driveshaft spin?? Is it a 7.5" or 8.5" axle? The 8.5" is stout, the 7.5" is weak.

the 700 line of trannies requires more than average skill to properly assemble. A LOT of shops do a LOT of bad things to them which can result in hard parts failure. If the tranny fragged then blame the shop because the parts can work. My car makes a hell of a lot more power than your's and I have no tranny problems now that I got a good tranny from www.pro-built.net

When you turn a tire does the driveshaft spin?? Is it a 7.5" or 8.5" axle? The 8.5" is stout, the 7.5" is weak.

When I turn a tire the other tire spins in the opposite direction and the driveline doesn't turn. But if I rotate the tire back and forth a few inches repeatedly the driveline turns as I'm doing it.

Not sure the rear end size, the diff cover measures about 10.5"x11.25", not sure if that helps decipher if it's a 8.5" or 7.5". My lug configuration is 5x4.75 not 5x5 if that helps. There's a code in the trunk that tells what it is isn't there?

I am not completely up on my boxy specs but I suspect the 4.75" pattern means the 7.5". Easiest way to identify it for sure would be to look online at gears or the like and they usually ahve the cover shape shown to help identification.

What you are describing though sounds like the axle is working normally.

Have you tried putting it in gear on jack stands?? If the wheels turn then I would have someone keep an eye while someone steps on the brake lightly to apply load. Of course never getting under the car running.

If a shift kit is set up too harsh or accumulation is blocked whichh a lot of bad shops like to do it can and will break hard parts in this tranny.

Again if you have a major tranny problem it is the shops fault. My 4L60E is an electronic version of what you have, almost 6 years old and stands up to enough power to lift the driver's front tire at 4200lbs.

Have you checked for a broken flex plate?

When I turn a tire the other tire spins in the opposite direction and the driveline doesn't turn. But if I rotate the tire back and forth a few inches repeatedly the driveline turns as I'm doing it.

Not sure the rear end size, the diff cover measures about 10.5"x11.25", not sure if that helps decipher if it's a 8.5" or 7.5". My lug configuration is 5x4.75 not 5x5 if that helps. There's a code in the trunk that tells what it is isn't there?

Some of them have a code, some do not. If you see a "G87" in the trunk, it's an 8.5", but if you don't see that, it doesn't mean it isn't. G89 is definitive proof of a 7.5". The rear cover on the 8.5" seems to stick out a good distance and the pan itself is oval shaped, while the 7.5" has a less protrusive cover and is more of a rounded square shape. The easy way to tell the two apart is by looking at the bottom of the differential casting - a 7.5" will have, on the bottom, on either side of middle, a machine pad with a 'hook' shape while the 8.5" will have square shapes sticking out below the cover.

This is an 8.5" (it's the diff on my car):

http://www.silicon212.org/diffcover1.jpg

closeup of the square machine pad:

http://www.silicon212.org/diffcover2.jpg

and here is a 7.5" for comparison:

http://www.accuratelawn.ca/Diff.jpg

http://www.accuratelawn.ca/Diff2.jpg

http://www.silicon212.org/diff75.jpg

I'm pretty sure it's the trani. Talked to thetrani shop and they said bring it in since the rear end looked fine. I've been skeptical of this shop ever since they overcharged immensly to hook up the Painless vaccum torque converter lockup (I got rid of computer in this car) and since it's never functioned right in overdrive and since they told me it was normal how it woul make a loud bang whenever it shifted from 3rd to 2nd slowing down to a stop.

Anything I can ask them about or have them do while the're messing with this trani? I read somewhere on here about 3-4 creep with overdrive bein solved by removing some valve or something like that.
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Yep trani shop says something busted inside there. When I asked them about addressing the 3-4 undecicivness they didn't know what I was talking about. This thing has a Painless vaccuum lockup and has never worked properly in overdrive. Any suggestions? I'm supposed to be getting the car back on Friday or Saturday so I'm hoping to have something to tell these guys while the trani's on the bench.

Some of them have a code, some do not.

Thanks for the pics Silicon, I'll check once I get it back from the trani shop. I think I do have an 8.5" but not sure.