72 GMC K2500 TH350 w/NP205 dilemma

[kmoore]

HELP!!

Looking for some insight from you experts with knowledge of what was original in my 72 K2500 GMC Sub. This is out of my league, know enough about an automatic transmission to be.......well in the position I am in now!

Problem started with a leak that was spraying fluid all over the exhaust while I was driving and smoking pretty bad, looked under the truck (after cleaning everything up to find the source) and drove it again with the same problem. The only place that it was obviously coming from was the transfer case, well that did not seem right.....that would be gear oil....right? Tightened everything up that I could on the transmission and transfer case again and still seemed to be coming from the transfer case. I had Transmission issues anyway so time to spend some money on the Burb since it has been a while right? Towed it to an AAMACO dealer who stated that they had experience with 40 year old transmissions and this would be a piece of cake.

Fast forward to now: There should be a seal between the transmission and the transfer case but there is none. I do have fluid in my transfer case but they have never seen that before nor his sources/advisors. "They can put it all together and should be all right"

If there was not a seal it has been that way during the many miles I have driven it with no issues before. He said everyone stated (his transmission sources) that it should have been a TH 400 since it is 4 wheel drive and 3/4 ton. I am not the original owner but my build sheet and all paper work shows TH transmission with an * but no other denotation of what the* means or 350 or 400 listed.

The question is should this have been a TH400 or did they put TH350 in 3/4 four wheel drive units? All mounting holes/brackets and geometry match to what is in there now and no sign of any changes.

What about the seal issue and tranny fluid in the transfer case? May have made a poor decision in AAMOCO but I thought I was doing good with national brand and resources.

If there was an seal/plate available that would change all the spacing of the mounts and would then lend one to think it was not originally set up that way....unless the TH400 was a tad bit shorter. Now I am confused.

First 3 pics are my transfer case/TH 350, and following 2 are from his source of a TH400/TC with a seal or plate.

I do appreciate any help or insight that anyone may have.

[k5kev]

Definately no factory Turbo 400 4x4 in 67-72. A 3/4 ton auto 4x4 would have a Turbo 350.

The old tranny fluid in the tranfer case has happened to me a couple times over the years, I would say it is somewhat common.

I'm definately not a tranny guys but I seem to remember a o-ring or something between the tranny and adapter, although I always thought that the leaks was from the tranny ouput shaft seal? Or there is a seal on the backside of the adapter, between it and the T/C?? Somebody else chime in and enlighten us both!

[57NAPCO]

I did not know Chevy "never" put a turbo 400 in any of their 67-72 3/4 ton 4X4 burbs.
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[Steven B]

I had the same problem a few years ago. The seal between the trans and transfer case was leaking. Trans fluid was going into the transfer case and leaking out where the shift rods go into the T-case. New seal and all is good. Also never heard of T400 in 72 4X4's.

[DirtyLarry]

There were no TH400’s in 67-72 4x4 Suburban’s period. You can rule that out completely.

They need to remove the tcase to trans adapter from the tcase, pull the front bearing, slide out the adapter sleeve and there will be two seals in there. Replace those seals then add gasket material to the transmission spines and tcase splines during assembly to keep fluid from following the splines. Pretty straight forward repair actually. For some reason, I am not surprised AAMCO can’t figure this out but I suppose the guy working on this thing wasn’t even born yet when 205 tranfercases were in 75% of all 4x4’s.

The last 2 small pictures you posted are not relevant at all as that is a TH350 trans with an aftermarket adapter to fit what looks like a Toyota transfer case.

Here is a TH350/NP205 adapter right after I put new seals in it. When these seals leak trans fluid seeps into the tcase overfilling it then blowing out the tcase vent. Very common failure mode on these old girls.

[csb#356]

I would opt to take my transmission someplace else. If I know more (and thats not a lot) than the people trying to do the work to your transmission, I'd be nervous

[bbaxter51]

As the others mentioned. The th350's dont have a rear output seal per say like you would imagine. It's sealed from the tcase with a seal in the adapter. I just did mine due to the same issue you're having.

I must have folded over the delicate seal lip when installing the coupler because it still leaks like a sieve.

[kmoore]

Thanks to all that took the time to post on this. It gave me ammunition to get this done right!

Update is that yes in fact it was found that there was a seal in the adapter but he missed that because he thought the adapter was actually part of the transfer case. The information you all provided along with more research was passed on to him. No I did not get a discount for solving the problem. The scary thing is that before I inquired here they wanted to put it all back together and for me to just deal with tranny fluid in the case…………hell I could have done that without paying them.

[donut]

Good you got it fixed, even if you did have to teach the tech how to do it. Now you both know for the next time.

[WhiteWhale]

Originally Posted by DirtyLarry

There were no TH400’s in 67-72 4x4 Suburban’s period. You can rule that out completely.

Is this true? A buddy of mine had what seemed to be an all original 72 GMC Jimmy that needed a trans when he got it. When I helped him take it out I was surprised that it was a TH400. He had some original promotional brochures on the Jimmy and on the options list it did show a 400 available, in fact standard on models equipped with the 350.

Here is a copy of the specification page which shows the Auto 400