[68elky396]

I have been noticing a mild shutter that lasts for about 1/2 second when leaving from a dead stop. This only seems to happen after about 5 miles or so of driving when it gets warmed up. If I quickly give it about 1/3 travel of the gas pedal and put a load on the drivetrain, I can feel a shutter which feels like it may be coming from the rear end, but I'm really not sure exactly where it's coming from. If I were to take off normally, there is no problem. The transmission, torque converter and rear end have less than 3,000 miles on them. I don't think that it would be driveshaft related, because it doesn't happen until I drive it a bit and everything gets warmed up. I broke in the new ring and pinion just like the shop advised, changing the fluid and refilling it per their instructions. Posi additive was also used. I am just baffled. Could the transmission be the culprit? I just thought I would throw this out in the forum to see if anyone knows what might be going on, or how I may be able to diagnose the problem.

Steve

 

[70 Flint $$]

might want to check torque on pinion nut, possibly loosened up.

 

[68elky396]

might want to check torque on pinion nut, possibly loosened up.

Thanks, I appreciate the response. If it was the pinion nut, wouldn't I be feeling it right away and not after putting 5 to 10 miles on it?

 

[Capt Casey]

Vacuum advance or a vacuum leak?
Check those hoses she has been sitting for a while...

 

[68elky396]

Vacuum advance or a vacuum leak?
Check those hoses she has been sitting for a while...

Thanks Casey, but there are no vacuum leaks. Keep in mind that this only happens after I drive it for a while, it does not happen right away. It kind of feels like differential posi clutch chatter. I kind of hate to, but maybe I need to drop the diff cover and take a look. Maybe it doesn't like the fluid that's in there? The oil I have in there is Valvoline 80W90 with a Posi additive that I purchased from my local Chevy dealership.

 

[427L88]

The diff would shudder around turns, not straight, but... defintely check your fluid.

And your transmission mount. And finally your trans AND converter. You might find a shop with a transmission dyno in your area.

 

[69-CHVL]

Does sound tranny related. Maybe check the fluid levels both hot and cold. Check vac modulator for leaks etc. May not be a bad idea to drop the pan and inspect.

Based on the description my guess is that some clearance is opening up with heat (clutch pack, sprags etc).

 

[68elky396]

The diff would shudder around turns, not straight, but... defintely check your fluid.

And your transmission mount. And finally your trans AND converter. You might find a shop with a transmission dyno in your area.

Thanks for your reply. Transmission mount is a brand new Energy Suspension part and it checked out good, besides if it was something like that, it would act up right from the start and not after driving it for a while. I will be removing the diff cover sometime soon to take a look. And then from there if need be, I guess I'll be looking at the transmission.

 

[billdini]

Steve sounds like you changed everything and did things correctly. One question, did you replace the U joints. Had that happen with my 69 Chevelle once and changing them fixed it.

 

[68elky396]

Does sound tranny related. Maybe check the fluid levels both hot and cold. Check vac modulator for leaks etc. May not be a bad idea to drop the pan and inspect.

Based on the description my guess is that some clearance is opening up with heat (clutch pack, sprags etc).

Thanks for your response, I guess I will be looking at the transmission after I look at the differential. There is one thing that comes to mind now that I'm thinking about it. During the break in procedure of the new ring and pinion, I remember making a tight U-turn in a neighborhood and the rear end started binding and making noises. So I stopped to get out and take a look (why, I don't know) got back in and continued to make the U-turn and I heard a pop sound like it had broke free.

I then drove it back home and changed out the oil. Everything looked good at that point, nothing unusual about the oil. So this time I used the GM Posi additive instead of the other brand I used the first time, which at the moment I don't recall that other brand. Maybe the clutch pack got messed up even though the differential seems fine otherwise. I will still be pulling the cover to take a look.

 

[68elky396]

Does sound tranny related. Maybe check the fluid levels both hot and cold. Check vac modulator for leaks etc. May not be a bad idea to drop the pan and inspect.

Based on the description my guess is that some clearance is opening up with heat (clutch pack, sprags etc).

Thanks for your response, I guess I will be looking at the transmission after I look at the differential. There is one thing that comes to mind now that I'm thinking about it. During the break in procedure of the new ring and pinion, I remember making a tight U-turn in a neighborhood and the rear end started binding and making noises. So I stopped to get out and take a look (why, I don't know) got back in and continued to make the U-turn and I heard a pop sound like it had broke free.

I then drove it back home and changed out the oil. Everything looked good at that point, nothing unusual about the oil. So this time I used the GM Posi additive instead of the other brand I used the first time, which at the moment I don't recall that other brand.
Maybe the clutch pack got messed up even though the differential seems fine otherwise. I will still be pulling the cover to take a look.

 

[Hotwire]

Could the u-joint straps be too tight and the joint binding once warmed up?

 

[68elky396]

Steve sounds like you changed everything and did things correctly. One question, did you replace the U joints. Had that happen with my 69 Chevelle once and changing them fixed it.

Bill, thanks for your response, the driveshaft went to a driveshaft shop back around 1990 for a check and balance. I had them also replace the U-joints. Since then, I put about 25,000 mile on the vehicle and have lubricated the joints regularly. I'm thinking not the U-joints because it doesn't act up until it's been driven a bit.

 

[68elky396]

Could the u-joint straps be too tight and the joint binding once warmed up?

Thanks for the reply hotwire, but I doubt it, they were torqued to spec when I installed the driveshaft.

 

[billdini]

Bill, thanks for your response, the driveshaft went to a driveshaft shop back around 1990 for a check and balance. I had them also replace the U-joints. Since then, I put about 25,000 mile on the vehicle and have lubricated the joints regularly. I'm thinking not the U-joints because it doesn't act up until it's been driven a bit.

I know we don't think 31 years is old anymore Steve but unfortunately it is. I get what you are saying about being hot. That is usually what happens when the clutches start sticking as the fluid starts to thin. I know its a auto. If you are comfortable its not the joints then you are probably looking at a converter or tranny with converter being my best guess but it's only a guess.

 

[68elky396]

Update on the rear differential. First, I would like to thank everyone again that has taken the time to try to help me out. I called DTS, the shop that built it. I explained everything to him and he said that I did everything right. He explained that sometimes people have had to add more limited slip additive than it calls for to take care of sticky clutch plates. He said to start there because it would be the least expensive, then if need be, move to the transmission. I was in complete agreement with that, just as most of you are, it just makes sense. So I pulled the plug and drained the fluid that has about 1,200 miles on it now and it looked pristine.

So then I start reading online and people are using this brand and the other for gear oil. They were using 80w90, 85w90 and 90w and they all pretty much used the 4oz. bottle of GM additive. To recap from earlier, the oil that I had in there was Valvoline 80w90 with a 4oz. bottle of the GM additive. I also read that if you put too much additive in there, it becomes too slippery and the rear will act like an open diff. And if you don't put enough additive in, the clutch pack will bind when turning.

I really don't want to be throwing money around blindly, so what the hell should I do, now that I have drained the oil? If I pull the cover, I really don't expect to see anything that would say "here's the problem". Should I go with a different weight oil? I don't know. Some people online reccomend EP gear oil (extreme pressure) and have had good luck with it after experiencing a clutch pack that's too grabby. It very well may be the transmission, I don't know. What I do know is before the shop put in my new gear set, bearings and seals, everything was fine. They did not put a new clutch pack in there at that time because it was only a couple thousand miles earlier, that those were replaced along with a different set of gears. What would you all do at this point, now that the oil is drained?

Steve

 

[68elky396]

Has anyone ever ran this oil in their 12 bolt posi with good results?

 

[vortec710]

Used to be a GM shop cure for “grabby” posi units, find a parking lot like place, drain the rear, refill with kerosene, drive 5-10 right hand circles, then 5-10 left hand circles...STOP...drain kerosene out throughly, refill with posi lube and drive...for grabby posi units only.

 

[68elky396]

Used to be a GM shop cure for “grabby” posi units, find a parking lot like place, drain the rear, refill with kerosene, drive 5-10 right hand circles, then 5-10 left hand circles...STOP...drain kerosene out throughly, refill with posi lube and drive...for grabby posi units only.

Thank you, I appreciate your input. But just to be honest, that may be a fix for some, it is not something I will be doing. Today I will be finishing up with my new door weatherstrips from Metro while waiting for my fresh gear oil and additive to show up at my door. I ordered online this time and saved a good amount of money. After that, I will be looking towards the transmission for answers.

Steve

 

[Hotwire]

I doubt the shudder you feel is from limited slip clutches when you're accelerating in a straight line? Only way that would be possible is if you have 2 completely different sized rear tires. Your clutches are not doing anything unless your turning, accelerating with one tire in dirt other on pavement. I could understand them being in a bind but it wouldn't be repeatable as you're suggesting. If you're hearing/feeling popping in the rear when you're making a turn, then add additive.

You're looking at brakes grabbing, u joints binding from pinion angle, or trans issues. Sorry.