Bum Posi unit?

[moparmaniac59]

Well,
I am like Pink Panther in his post titled "I am Pissed"  :swear:I bought an 8 3/4, 742 case 4:10 posi unit from a local racer in the spring. He said if was fine and he had maybe 10 passes on it. It was very clean and turned freely & smoothly. It has no apparent unusual wear or visable damage. I spent all day changing out the unit. I put in synthetic rear end gear oil and also the posi unit lubricant. When I backed out of the driveway and put it in gear it made a loud clunk/pop noise. I actually thought the parking brake had stuck, but it wasn't engaged. Then I took a up the road cruising up to about 45-55. It seemed a little whiny but nothing alarming. I hammered it at a stop light and when the torque converter (2800 stall) came up to speed the rear end made a very loud clunk/pop. I though it was gonna fall out of the car. I was about 5 blocks from the house and cruised into the yard & parked it. It did the "clunk" again when I pulled into the driveway.  What do ya think?? A trashed posi unit?? I paid $250.00 for the unit not to mention my entire day wasted. Should I just trash it and look for another or look into fixing it?? I know a 742 case has no crush sleeve as the pinion depth is set with shims, so  that can be eliminated as a problem unless of course it was wrong to begin with, but that would cause a wear problem, not what I am experiencing.

                                                             Matt B.

[moparmaniac59]

I've been scratching my head on this.  :pullinghair:I've got to check some things tomorrow. I was wondering if anyone knew how much clearance there should be (ball park) from the top of pinion snubber to the body. I've got a tall adjustable snubber (3 settings retained by a pin). It's all the way down & got about 1 1/2 to 2 inches from the rubber snubber to the body. I was thinking it might be part of the problem, but I don't think it would cause the clunk noise. I've also got to recheck the axle shaft end play. I didn't look to see if the pins were still there for properly setting the axle end play. If there was no end play, would this cause my problem?? The rear end is the factory setup and I believe the 2:76 open carrier I removed was the factory gear set. I've got a 4:56 489 case rear end in the garage, but I don't want that much gear or to install it without knowing what the problem was with the 4:10 gears. Any troubleshooting tips would be greatly appreciated.


                                                       Matt B.

[bb71challenger]

That loud popping sound you talk about does not bode well for your sure grip. I read this thread last night and although I cannot give any clues I just wanted to let you know that you are not the first one to get a bad unit. I know it probably does not help much but you are not alone 

[Chryco Psycho]

is it just clunking when cornering ?
it could be as problem with the axle gears

[moparmaniac59]

No,
It's more than that. When I was on a straight stretch and went to hammer it, I thought the rear end was gonna drop out of the car!!




                                                  Matt B.

[Moparal]

Not the ujoint coming out right?

[moparmaniac59]

Hi Al,
No, as a matter of fact, both U-joints have been replaced. That was one of my hold-ups installing the third member was I had to order a U-joint. As I had mentioned, after installation, I backed slowly out of the back yard and then put it in drive. Thats when I heard a loud "pop". I almost thought my axle shaft had snapped,as it was loud (cracked spider gear?). I thought my parking brake had stuck. So I decided to try a little drive and went down the hwy (city driving) and got cruising up to about 45-50 mph. It did fine. Maybe a little on the whiny side. I went to a local cruise-in, left and drove the same routh home. Again, it did fine. I pulled onto a local secluded straight strip and nailed it. The engine sped up (2800 stall) and then "pow", the whole rear end jumped. It scared the crap out of me. I immediately let off of the accelerator and drove about 5 blocks home. It didn't make any noise going the 5 blocks home. When I turned into the driveway, I heard the "pop" again. I wanted to go through the troubleshooting guide in the Mopar Chassis manual and also check axle end play again. RDF had a thread on third member changeout where the issue of the center pins came up. I was looking at another 3rd member I have, 489 case and there are no pins or holes, I guess it has a  center block?? Anyway I was thinking the problem lies in the posi unit. I'll know more later this week. Any suggestions would be great!! Thanks.

                                                          Matt B.

[Moparal]

Is it a cone or a powerlok?   Someone may of had it apart and got something backwards. I know on a sure grip there is a plate you could have in the oposit direction if you didnt pay attention putting it back together. It may even be loading up and releasing . Did you say synthetic and additive or gear oil and additive?

[moparmaniac59]

Thanks Al,
I used 75-90 Royal Purple synthetic and also the sure-grip additive. I am not positive, but I think it's an Auburn sure grip unit. The guy I bought the 3rd member from I don't believe was trying to cheat me. He said he ran it in his Dart (maybe 10 passes) and then went to a taller gear. After removing it, it sat on a shelf for 5 years. Maybe the springs, shims, cones ect sitting for that long in a hot garage may have caused them to loose elasticity/deflection ability??


                                                         Matt B.

[71chally416]

I know that synthetic lube is supposed to be the shizzle, but the only time I ever used it in a rearend this was the result. It was 3.91 742 unit and this happened with STREET tires. It's old school gear lube for me from now on.

 
 

[MrMopar440]

Yeah, but would that type of failure be caused by the oil? Or maybe just a weak spot that finally let go? Just asking

[71chally416]

Don't know, but in 30 years of driving Mopes on the street it was the first gear failure I ever had. Track cars yes, street cars NO. Maybe just a coincidence?   

[ShelbyDogg]

Did you adjust your axle end -play correctly?  Shake the car's rear end side to side and tell us what you hear.

Rob

[moparmaniac59]

I haven't had a chance to check it, but I am going to pull it and I'll post later what I find. I am curious about the pinion snubber. The low (non-adjustable) snubber and the adjustable are a world of difference in regards to height, even with the adjustable at the lowest setting. I have an adjustable installed and it has about 1 1/2 inches of clearance to the body. I'm not sure what is proper and know it's a lot of trial & error to get the car to hook up right. Any input??


                                                         Matt B.

[moparmaniac59]

I checked the axle shaft end-play and had to adjust. Don't know if this would have caused any of my problems. I'll be pulling it out to look for "pieces" next week. Thanks for all the input!!

                                                       Matt B.