Coogan351w

I bought a 1989 Mustang GT last year as a project car, and I'm having some issues that are making me scratch my head.

The car has: 351w / t5 5-speed / bbk shorty headers / bbk offroad h-pipe / flowmaster american thunder catback / Edelbrock 650cfm 4bbl / Edelbrock performer intake manifold / 4.10 gears / (not sure on internals)


Backstory: I drove the car to work on a Friday, with no issues. However, on the drive home I got caught in some serious traffic. While down shifting from 2nd to 1st, I heard a loud "BANG!" followed by loud clunking that increased/decreased in frequency based on the engine RPM.

At that point I lost all power to the rear end, as if I was in Neutral. So I shifted from 1st to Neutral and rev'd the engine a little to see if the clunking was coming from the engine and it was not. The clunking goes away while in Neutral.

So I tried to put the car back into 1st, and as soon as I put it into gear, the clunking continued. Again, the frequency increased/decreased with the engine RPM's. I tried every gear, and the same clunking noise happened in each gear.

My thinking was that the power was either being lost between the engine and trans, or trans and rear diff.

I call the tow truck and get the car back home and in the garage. I jack the car up and remove the trans.

Once the trans was removed, I noticed ALOT of slop in the input shaft. I could take the shaft and rattle it up/down and left/right with ease. So I decided that since it was already taken apart, I would replace the flywheel, clutch, pilot bearing, throw out bearing, clutch fork, clutch cable, and transmission.

When installing the new parts, I took my time and made sure that everything was aligned, torqued and installed the correct way.

Then, I re-installed the drive shaft and started the car while it was still on stands.

Put the car in 1st gear and the tires rotate slightly and then BANG! I hear the clunk again. I feel the clunk in the shifter and can hear it coming from the rear end.

The best way to describe it would be that it feels like every time it hits 12 0'clock, there's a clunk. The tires don't want to turn as if the rear diff is seized up. HOWEVER, when the car is in neutral, the tires spin forwards and backwards just fine. The clunking only happens when the trans is delivering power to the rear.

When the car was able to be driven, there was a hum from the rear. I know that's a good indication of trouble. I just don't know enough about rear diffs.

Does anyone have any ideas? Could a failed LSD cause this?

Urambo Tauro

Welcome!

Sounds a lot like a rear end issue. I'd pull the rear cover (as if doing a fluid change), just to have a look around in there. Even if you're not familiar with differentials, a catastrophic failure should be fairly easy to spot (look closely at the big vertical ring gear's teeth). Posts some pics of your findings and we can have a better idea of what it is you're up against.

Coogan351w

thanks!

Yeah I was thinking it was the rear end, but just couldn't say for sure. I'm out of state at the moment, but I'll be back by the end of the week. I'll post pics then.

88 orangepeel notch

Welcome to the forum. Sorry it's under these circumstances.

I'd like to see those pics when you get them up. Sounds like you'll have a couple extra pieces sitting in the housing when you pop the cover off.

Coogan351w

Got home last week and was able to take the Diff cover off.


I found ALL 8 ring gear bolts sitting at the bottom of the case. 1 of which was stuck between the case and the ring gear. That must've been the initial failure, that trickled down the line and ended up blowing the trans (or at least I think so).


1 out of the 8 bolts was SHEARED, the rest were just walked off. (grade 8 bolts)


I'm thinking either 1) No locktite 2) not enough torque when they were installed.

Urambo Tauro

WOW! That's awful. But not unheard of.

I think you hit the nail on the head. Improper torque, lack of threadlocker... maybe even the presence of gear oil in the threads during installation, compromising the threadlocker's effectiveness.

dawson1112

Just to help ease your mind a bit, that transmission is fine. Your not gonna hurt it at all if the rear end blows up. That slop in the output shaft is normal, unless once you install the drive shaft it is still sloppy. The reason is that the ouput shaft is only supported by the front input shaft bearing and the main shaft bearing that sits in the main case just in front of the tail housing. The output shaft and main shaft are one piece. There is no support at the tail end untill the drive shaft is installed. The tail bushing provides support for the drive shaft, which provides support for the output shaft at that end.

rmodel65

id take this opportunity to find a 94-95 gt roller and take the rear and the front brakes and wheels for a 5 lug swap. ought to be able to find one for about 1k$ or less