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Old 01-04-2012, 12:34 AM
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Can rear trans. seal leak into transfer case?

I have a 1991 S-10 4x4 with an automatic transmission. I have to add transmission fluid almost every gas fill up but it doesn't leak a drop. I was checking the transfer case fluid level (it holds 2.2 pints) and it was way overfilled. It had almost a gallon of trans. fluid in it. Can the rear transmission seal leak into the transfer case? If so, do I have to remove the transmission to replace the seal? Another thing is that the vacuum line from the transmission to the engine feels soft and spongy all the way up close to the vacuum resevoir. Can fluid be sucked up the vacuum line into the engine? Every now and then the truck puts out an embarrassing amount of smoke when I'm stopping coming down one particular hill that has a steep grade with a stop light at the bottom. (So much smoke that I can't see the car behind me while sitting at the light!) Can all of this be related? This must be why it takes so long for it to shift out of 4 wheel drive as well. ( transfer case vacuum switch full of fluid?) This one really has me stumped! Any help on this would really be appreciated!

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Old 01-04-2012, 01:43 AM
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the tranny doesnt have a output sealits got a front seal on the t case wich is probly bad
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Old 01-04-2012, 09:53 AM
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Absolutely ATF can be sucked up the vac mod line and the symptoms are exactly what you detail. You need a new vac modulator. Not as familiar with the S series but the full size blazers had a seal in the T-case adapter which is about 1-2 inches thick between the trans and t case. Those seals would go bad and the fluid would transfer, but the majority of yours is going out the tail pipe, IMO.
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Old 01-04-2012, 12:31 PM
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"CAN OF WORMS"

Crap! The transmission and the transfer case are the two parts that I've always stayed away from. Is the transfer case seal accessible from the outside without opening it up? Does the vacuum modulator prevent fluid from coming up the vacuum line or do you think that it has fluid in it therefore ruining it? Do you think that the vacuum switch on the transfer case is shot as well? Thanks again for your help!
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Old 01-04-2012, 01:52 PM
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I can't say about the S series 4x4's. If they build it like the full size you can split the trans from the transfer case and the seal is there in the adapter. Lots of work, but not too complicated. The vac modulator controls part throttle shifts through manifold vacuum. It is normally sealed from fluid, but when they break, it can allow fluid to be sucked into the engine. If it's me, I'd replace the vac modulator and line and start out with a full trans and a full trans case and drive it for a while. When you see the trans fluid getting a little low, check the transfer case and drain as needed. It might be so slow that it would require service every oil change or so. That's the lazy way out, but I can be lazy with jobs like that. Don't know about the 4 wheel drive switch, it's not related to this other problem.
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Old 01-04-2012, 06:01 PM
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Quote:
Originally Posted by hinges56
Crap! The transmission and the transfer case are the two parts that I've always stayed away from. Is the transfer case seal accessible from the outside without opening it up? Does the vacuum modulator prevent fluid from coming up the vacuum line or do you think that it has fluid in it therefore ruining it? Do you think that the vacuum switch on the transfer case is shot as well? Thanks again for your help!

doing the t-case input seal is not that bad of a job with the right tools and equipment. yes, the transfer case does need to be removed to access the seal as its at the front of the t-case. you do not have to open the t-case up to change the seal. a slide hammer with a screw will pull it out no problem.
as far as this talk of a modulator goes, there isnt one as the s series never came with a th350. your s-10 came with a 700r4 and it does not have a modulator, never did.

the t-case will also require a new gasket to mate back up to the trans or it will leak. make sure the surfaces are clean and dry. it should have a new process 231 t-case in it. theres a round tag on the backside of the t-case that will have the model #.
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Old 01-04-2012, 06:08 PM
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tnsmith is absolutley correct. And the tranfer case isnt that hard to get off.
I think the s trucks have 5 bolts in the transfer to adapter housing, the top one being the toughest as clearance is the issue.
If you are doing the job on your back it is easy to get the transfer in and out as you can bench press the thing, they are aluminum and arent too heavy, maybe 90-100 pounds. I shove them in and out by myself, and I am old .
The fluid is getting sucked up by the vacuum switch on top of the transfer case, replace it while you have it out, they are only like 15 bucks. Also you should check the line to the vacuum servo mounted under the battery box, if it gets ATF in it (which it probably does) it will ruin the rubber diaphram.
The servo has the cable coming from it that goes to the front axle that locks the axle to the differential when 4x4 goes active. The vacuum lines will most likely have atf in them also. It would be smart to flush them with brake clean and compressed air.

Last edited by latech; 01-04-2012 at 06:13 PM.
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Old 01-05-2012, 12:35 AM
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Many Thanks!

Thanks again for all your help. It all makes a lot more sense now. My truck has a small body lift so maybe it might make the upper transfer case bolt more accessible. I just changed the ball joints so I have to play catch-up on other projects before I start this one. Then it looks like I'll replace the transfer case seal & gasket, servo under the battery if needed, vacuum switch at the transfer case, and the vacuum line. Until then, I'll keep checking the fluid so the transfer case doesn't overfill again. It's probably wishful thinking that it's a slow leak because of how much tranny fluid I've been pouring in. You guys are awesome!!
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Old 01-05-2012, 03:19 AM
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The transmission (TH700-R4) does NOT have a rear seal.
The transmission (TH700-R4) does NOT have a modulator.

Transfer case HAS a front seal.
Transfer case seal CAN be replaced without opening the transfer case.
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Old 01-05-2012, 05:37 PM
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That small body lift makes a TON of difference for removing things like the transfer case and / or transmission.
You would be amazed how much a 2 or 3 inch body lift makes on those trucks..
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Old 09-15-2012, 03:05 PM
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Did the tranny fluid ruin anything else?

I had my radiator reservoir and blower motor out and figured I might as well check the vacuum reservoir for tranny fluid. I could not believe how heavy it was! It was about 3/4 full of tranny fluid! That's it, it's time to replace the seal! Other than working in the driveway (my 56 belair project takes up the whole dang garage and it's stripped down to bare metal) and being one of the hottest days of the year, (Labor day weekend, 110 degrees) it went smoothly (with the help from from all of you!) The 5 transfer case bolts and the linkage bracket that covers the transfer case bolts were the worst but not too bad! The first seal that I bought was a Moog from a local auto parts store. It was the same size inside and out but it wasn't as thick. They said it wouldn't matter but the more I looked at it, the more I new that it wouldn't work. I bit the bullet and went to the Chevy dealer. It was the same price but it had to be ordered. It came in the next day and I bought a gasket, vacuum switch and a roll of vacuum hose as well. I haven't bought a vacuum servo yet but I think that's next because the front axle is hesitant and sorta clunks when it shifts out of 4 wheel drive. Is there any thing else that the tranny fluid could have ruined? Thanks again to everyone that helped answer my questions and for steering me in the right direction!!
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