Author Topic: Rear end HELP  (Read 4150 times)

Offline 1979SIERRAGRANDE

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Rear end HELP
« on: April 26, 2017, 11:09:32 PM »
hey everyone greeting from Texas i have a 1979 Gmc C15 with a th350 and a 12 bolt. When i bought the truck it came with 275x55R17s all the way around recently it started making a clunking sound when going into reverse or drive.I decided to change the u joints since they did have play and some wear once i changed them the clunk happened more often and worse.today and i took the diff cover off and inspected the teeth and counted them 17 on the pinion gear and 47 on the ring gear both have excessive wear as well as the spider gears. stamped on the pinion gear are the numbers 10 78  . i have 2.76s should my rpms be lower for having such lower gears , all this time i thought i had high gears in the rear because going 60 down the high way i'm at almost 5k in 1:1 my question is why is my truck about to explode on the highway since the stock wheel size is around 29 inches tall and mine is 28.9 nothing is adding up on why i drives like that maybe going to a higher gear ration would help or would it worsen it the worsen the prob if any one could give me input on hot to fix this issue or advice that would be great thanks

Offline zieg85

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Re: Rear end HELP
« Reply #1 on: April 27, 2017, 07:01:43 AM »
Do you feel your transmission shift into drive?  Can you manually verify that?
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
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Offline 1979SIERRAGRANDE

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Re: Rear end HELP
« Reply #2 on: April 27, 2017, 11:24:16 AM »
yes it does soon as shift into drive it wants to go. when you say manually verify you  like watch the linkage from underneath?

Offline Joegibbstruck

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Re: Rear end HELP
« Reply #3 on: April 27, 2017, 11:28:01 AM »
Do you feel it shift through all 3 gears? If so I'm guessing either your converter or trans is slipping. There is nothing else that will effect rpm besides trans, gears, and converter. Or maybe your tach is reading wrong.

Offline 1979SIERRAGRANDE

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Re: Rear end HELP
« Reply #4 on: April 27, 2017, 11:40:15 AM »
yes the trans feels and shifts good when i first got it the guy i bought it from thought it was built well after check the vacuum modulator was unplugged for who knows how long once i hooked up that and the kick down cable it works great build was bright red when i first got it leads me to think it hadn't been too long before it was rebuilt or serviced. i can feel it shift into all 3 gears i can feel it kick down as well now fluid is still good now but honestly how would i check if its a converter is bad . the guywas clueless about the truck he traded a low rider for it and with all the paper work i got with it seems like mechanics did all the work and no paperwork about trans work so im on my own on this one makes me.

Offline 1979SIERRAGRANDE

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Re: Rear end HELP
« Reply #5 on: April 27, 2017, 11:48:18 AM »
concerning the tach may be wrong it could be but that doesn't Answer the fact i cant go past 70 without feeling like i may break something. ??? clunk itself is from the wear of the spider gears and Ring and pinion. what I'm concerned about if i replace with a higher gear will it actually worsen my MPG and my top speed in theory shouldn't the higher gear use less engine power to move the wheels ? Right now i get about 11 on a good day thats with my foot out rebuilt the carb dual exhaust headers intake and tune up was done other than that its pretty much stock.thanks again for the quick reply

Offline zieg85

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Re: Rear end HELP
« Reply #6 on: April 27, 2017, 02:42:57 PM »
2.56 is the only "higher gear" available.  You have something transmission wise going on if you feel all 3 gears distinctly.  Torque converter makes the most sense.  You should be mid 2K cruising at 70 with a 2.76
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline 1979SIERRAGRANDE

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Re: Rear end HELP
« Reply #7 on: April 27, 2017, 03:18:58 PM »
well the i wanted to put 373s prior to knowing it had 2.76s along with a posi i dont know if its a good idea now.anyway to test the torque converter prior to taking it all apart the truck is my daily driver.

Offline 1979SIERRAGRANDE

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Re: Rear end HELP
« Reply #8 on: April 27, 2017, 10:29:15 PM »
when i said a higher gear i meant 373 gears why would the torque converter have to do with final drive rpms wouldn't it be stalling or locking up if it was failing ?

Offline blazer74

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Re: Rear end HELP
« Reply #9 on: April 28, 2017, 12:58:09 AM »
Gear ratio that is best for your type of driving.
3:73 will give more torque to the wheels with less power used, but engine will see higher rpm at faster speeds with 3:73.
It's a balance of what you want.
Poor gas mileage is inherent.
Clunking at 70, definitely priority to find the cause first.
Loose torque converter bolts, mounts, exhaust rattling , u joints, something loose.
Clunk in and out of gear, motor mounts, driveshaft slip joint worn, ring pinion / pinion bearing worn.
Loose yoke at rear end.
Even loose u bolts at the springs.

Offline Joegibbstruck

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Re: Rear end HELP
« Reply #10 on: April 28, 2017, 08:43:05 AM »
The torque converter is the coupling between motor and trans, if its slipping( stalling) your engine rims would be higher, if its not locking up the same would occur.  Do a stall test and see what that looks like, if it looks ok I would say  It's not locking up if its even a lock up version, it wasn't introduced until late 79.

Offline 1979SIERRAGRANDE

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Re: Rear end HELP
« Reply #11 on: April 28, 2017, 08:54:34 AM »
well the clunking is coming from the ring and pinion the spider gears i sat there spinning the tires jacked up in there air just not sure where im getting such high rpms from i'm sure thats not normal.when i first got the truck he told me it might have been built because the trans shifted hard did some looking turns out whoever installed it never bothered to hook up the vacuum modulator so it was shifting off the governor i'm sure thats was not good for the trans don't know how long the guy drove it that way but it shifts fine now and goes into gear other than the high rpms i see i'm worried about that just cause i daily drive it i try not to get on the highway as much but the times i do engine trans and rear end see high rpms for half an hour at times. thanks again for the responses if there is anything i am leaving out let me know.

Offline 1979SIERRAGRANDE

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Re: Rear end HELP
« Reply #12 on: April 28, 2017, 09:01:03 AM »
how would i do a stall test ? and to fix  the clunking already looking at  New old stock posi. As much as i would like to buy from other brands i would rather buy something that was  proven to work. along with a master rebuild kit anyone tried richmond gears or any gears they have had good experience with? Need to find a good ring and pinion set dont know what ration but point in the right direction would be great thanks.

Offline Joegibbstruck

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Re: Rear end HELP
« Reply #13 on: April 28, 2017, 01:17:44 PM »
To do a stall test you basically power brake without spinning the tires lol. You put it in gear, put the parking brake, hold the brake pedal as hard as you can and give it gas. Watch your tach and that will tell you what your stall speed is.

As far as Richmond goes, they make great stuff. Usually you can find a guy who works at a differential shop and pay him on the side to do it. Last rear end i had rebuilt was for my 08 Silverado that I toasted the spider gears in. Found me one of those guys that do side work and paid him 350 for a limited slip, and 4.11 gears installed.

Offline 1979SIERRAGRANDE

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Re: Rear end HELP
« Reply #14 on: May 09, 2017, 11:40:38 PM »
quick update i could not do the stall test truck started running poorly starts to break under a load new DUI distributor ordered to replace the Chinese special that came with the truck. But fortunately I switched tachometers to test the accuracy of my autometer its about 2-3k off so it was the tach that was the culprit t.the rear end is still going to get a rebuild just picked up a New old stock 12 bolt eaton posi part number #30060 cheaper than what they go for now a days still working on finding a good set of ring and pinion gears.  thanks for everyone's suggestions much appreciated!