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Old 05-25-2008, 07:19 PM
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THM 400 rebuild

I have recently rebuilt the engine and transmission in my 86 GMC K2500 4WD. The engine rebuild went great so far. .30 over on the block, new cam, lifters, timing set, pistons - all at stock specs. Balanced the crank, had the q'jet remanufactures, runs really sweet. Now if I can get it to move; the transmission, 3L80, was working before the rebuild, yes my wife has told me if it ain't broke..... but that has never stopped me before. I purchased a kit and the ATSG manual. I have followed very diligently and everything seemed to go together well. After starting up the engine and setting the idle and timing I have found that I have no movement in any gear. Nada, nothing. Just sits there. I am hoping to hear some so possible places to start looking. I lowered the transmission and removed the pump to check the boost valve assembly, as well as double check the pump to case gasket position. I have replaced the new modulator with the old one as well. I have not purchased a pressure gauge yet, hoping to borrow one this coming week so I don't know if I am getting pressure. I have double checked the fluid level, no leaks, no sounds. I have the correct end-play and the inner shaft will spin by hand. Getting ready to pull it out (again) any suggestions?

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Old 05-25-2008, 08:52 PM
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I won't ask if the converter is bolted to...nah

Did you make any changes to the valve body? Have all the check balls in the correct location? Separator plate and the correct gasket in place? Linkage tight, when apart did the rooster comb go in correctly? Just some 'simple fix' thoughts, were you able to air test it?

I rebuilt my T-400 as well, but not a true guru, we have some here though.....
I'm the one your momma never told you about...

Semper Fi

Midnight Sun Street Rod Association
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Old 05-26-2008, 07:06 AM
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I know the converter was/is bolted because I installed my flexplate backwards and had to drop the tranny (again) to correct. No changes to the valve body, however, I did have an extra gasket. (2) gaskets exactly alike and a third that was different to go around the detent solenoid. I left out one of the two since the book only shows one on top of the steel and one below. Maybe my gaskets are missaligned? I did not remove any of the valves in the valve body.

Not sure how to air test. As far as I know, and that ain't far, I checked and triple checked the check ball locations. The roster comb seamed fine, as I never disassembled those parts only the spring roller was removed, and I can "feel" the bumps as I work the shift lever.

A couple of things that I expected to be in the kit were not; such as new seals in the rear servo/accumulator assembly, the front servo, the three "ring" seals the go around the neck of the center support and the same type of seals around the neck of the pump cover assembly, these are delicate little things, beveled on both ends, and they fit loosely, not something that I would suspect to hold a great deal of pressure.

Sounds like I will be pulling it back out and going thru again, evidently I need the practice.
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Old 05-26-2008, 11:58 PM
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VB gaskets

1 thing to check. The Valvebody gaskets are usually marked VB & Trans. VB marked gasket goes between Valvebody & Plate, Trans goes between Plate & trans case.
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Old 06-18-2008, 07:08 AM
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The truck is now running. It seems that one should read and follow ALL of the sentences in the manual. Especially, the one that states "make sure the manual detent pin is engaged to the manual valve". One pet peave is the fact the the trouble shooting tips refer to this as the "rooster comb"; be nice if they called everything the same thing througout the book and all the pictures. Anyway, thanks for all the advice. See you on the road.
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