TH400 Rebuild Questions

jfwireless · 10-05-2007, 06:31 AM

My 32 year old TH400 is popping out of third gear now, most likely seals are all dried up. Time for a rebuild. Not bad service 32 years out of an auto tranny.

Jeep is a 1976 CJ-7 with TBI 304 and quadratrac, runs good, planning on a 360 transplant. Rebuilt the transfer case a few months ago, too bad I didn't know than I need to pull the auto out. I should have just pulled it than and had it rebuilt.

So what options do you guys recommend for the rebuild?? I was thinking about a mild shift kit, nothing radical. The TH400 is a tough cookie, I think it will handle a 360 up to 300 HP with no issues or additional rebuild considerations?

Any suggestions would be appreciated. I did rebuild everyting else on this CJ, but I just do not feel comfortable rebuilding a TH400 automatic, I know their are videos and lots of info for the do it youreslf guy, but automatics have lots of parts and most likely you need some experience to rebuild one of these.

Jim

**redcj576** · 10-05-2007, 08:20 AM

i want to hear this too ,my th400 is 25 years old now still strong.i hope it goes like this.

**JeepHammer** · 10-05-2007, 10:52 AM

Just clarifying something here,
It's impossible to 'Pop Out' of gear, since there is no mechanical linkage keeping an automatic transmission in gear...

The clutch pack is 'Slipping' due to (but not limited to) reduced hydraulic fluid pressure on the application piston at the 3rd gear clutch pack.

Starting with the clutches themselves and working backwards,
Warn clutches, or clutches that have drawn moisture and come apart.
Warn or cracked seals on the apply piston,
Worn or cracked seals in/on the center support snout,
Worn center support bushing, allowing the forward drum to grind away at the center support seals (this would be my guess, especially if the trans slips in reverse also),
Low line pressure from weak bypass valve.
Crud caught in bypass valve,
Weak pump,

You get the idea!
Anyway, this is the #1 death of TH-400 transmissions in my shop,

The center support bushing gets worn, lets the shaft it supports drop,
The forward drum contacts the center support snout and grinds away at the seals.
Sorry about the pictures, I don't do may Transmission now that I'm retired, and these are junk parts so they stay outside...

jfwireless · 10-05-2007, 11:16 AM

Jeephammer:

OK, I guess my description of popping out of gear is not correct, but I think you got the idea, it does stay in gear, but the trans slips and the engine races up. If I drop the throttle down, the trans will stop slipping again and off we go for anywhere from 10 seconds to five minutes. This only happens in third gear, if I go in second, first or reverse this never happens, anyway I need a rebuild, and after 32 year that is just fine, no complaints.

So do you recommend any modifications to the TH400 during the rebuild?? That is the question?

Jim

**tankcj** · 10-09-2007, 05:50 PM

The TH400 in our Jeeps has the same internals as the TH400 GM put behind the 454 LS6 (I think). That had 450 hp and around 500 ft lbs of torque. I'm guessing a stock rebuild will be more than sufficient for a 360 (even a 401)...a shift kit would get you a nice 'chirp' when it shifts though...I think the TH400 will do that anyway if you let it wind out in low...

Faw · 10-10-2007, 07:36 AM

Speaking of automatic transmissions anyone tried to put a TH700R4 in their Quadratrac jeep? Are there any 5-speed automatics that can be adapted to a jeep?

jfwireless · 10-10-2007, 09:30 AM

So I went ahead and had the mild shift kit installed, I also have an adjustable modulator so I can adjust the upshift where I want it. The mechanic also changed out the spring and made a few changes on the pump to get alittle more pressure.

Should have the Jeep back in a few days, I'll post back on how it all worked out.

Interesting, while in the shop today the mechanic showed me a few transfer cases he had laying around, Dana 300, NP208, and 209, and also one of those asian transfer cases they put in the Japan Jeeps, never saw one of those before. If you need one I know where one is.

Jim

**JeepHammer** · 10-10-2007, 05:16 PM

Quote:

"If I drop the throttle down, the trans will stop slipping again and off we go for anywhere from 10 seconds to five minutes. This only happens in third gear,..."

The factory Teflon seal is flapping around on the center support nose. When it gets displaced, you loose high forward (third) gear.
Pretty soon you will start having problems with reverse...
Nature of the beast.

See, everything in the automatic trans is turning,
Except for the center support.
So that's where the hydraulic fluid is transported to from the valve body that decides what gear you are going to be in.

The fluid goes up through that center support in passages, and is distributed to the clutch packs to make first, second, third or reverse work.
Since it's liquid, it has to have seals between the different mechanical parts.
When the seals fail, the transmission fails to do what you have come to expect it to do...

But since that center support is stationary, and everything else up in that part of the trans is moving, the center support literally supports the lions share of all the weight,
AND,
Works as a hydraulic 'Couplers' or 'Distribution Block' for all the moving parts...

The weight of all that spinning steel around the clutch packs (and don't kid yourself, that rotating mass is about 60 pounds!) is bearing down on that center support every second of operation...
SO,
It's 'Bearing' or 'Bushing' is going to get VERY worn!

When that bearing wears out, the shaft inside drops down, the forward drum drops down on the center support nose, and wears away the seals (and the seal supports for the seals) on the center support snout.
Some times you have seal, sometimes you don't...
As the center support bushing continues to wear away, the forward drum will continue to drop down on the snout of the center support, and continue to wear away the seals and their guides...
Finally resulting in total loss of High (3rd) gear.

By that time, you can expect reverse to be giving out or gone also, so watch where you park!
You probably won't get any warning with reverse since it gets full pump pressure!
The forward gears have other accessories that bleed off some line pressure, but reverse gets full pressure, and when the seal fails, it usually gets blown out all at once!

These are Wholesale prices...
You are looking at about $200 to $250 for a very good rebuild kit with seals, clutches, ect.
You are looking at about another $75 for bushings throughout.
You are looking at $125 for the forward drum and center support if it's worn to that point...
You are looking at about $35 for cleaning supplies to clean everything up.
If you leave the forward drum continue to drop until reverse doesn't work, you can add $35 to $50 for a new forward roller clutch to go on the new forward drum.

Fly-By-Night places will just slip in a new center support bushing, seals and a few gaskets, and you won't know the difference for about 2K to 5K miles... When the rough surfaces of the snout/forward drum wipe out the new, probably cheap cast iron seals, and do REAL damage...
But since you paid for a 'Repair' instead of a 'Rebuild' or 'Remanufacture', you are on the hook for the new guts...

**JeepHammer** · 10-10-2007, 05:43 PM

Quote:

"The TH400 in our Jeeps has the same internals as the TH400 GM put behind the 454 LS6 (I think). That had 450 hp and around 500 ft lbs of torque."

It is the same transmission, but the 'Performance Series' was a little more stoutly and tighter built.
Besides, the LS6 was factory De-Tuned to keep you from breaking parts until that vehicle was out from under it's 12 month/12,000 warranty.
Then anything you broke was money in the bank for the dealership.
-------------------

Quote:

"Speaking of automatic transmissions anyone tried to put a TH700R4 in their Quadratrac jeep?"

It's a common conversion to the 700R4, and the 700R4 is every bit as strong as the TH-400, except for shock loading, where it's only slightly less robust.
The TH-400 is an energy hog in all gears, where the 700R4 is only an energy hog in over drive. The 700R4 builds TONS of heat in overdrive, so make sure you have a really good transmission cooler if you are going to run one!
(It's not nearly as bad in a light weight jeep as it is for a full size vehicle towing a trailer!)

There are adaptors (expensive adaptors!) sold for putting a Q-track behind them, so I'm assuming that there is a market for them...
----------------------

Quote:

"Are there any 5-speed automatics that can be adapted to a jeep?"

Nothing common that I'm aware of...
But then again, 5 speed auto isn't very common.... Yet!
----------------------

Quote:

"So I went ahead and had the mild shift kit installed,..."

Please tell me that's along with getting the actual problem fixed!?!?!
Cheap shift kits just raise the line pressure, and often prematurely wear out parts, expensive parts, like the front pump!

Quote:

"I also have an adjustable modulator so I can adjust the upshift where I want it."

Not to point out the obvious, but ALL TH-400 trans had adjustable vacuum modulators.
That's how they got the transmission to shift about where they wanted them, no matter if it was in a light weight car or heavy truck...

Quote:

"The mechanic also changed out the spring and made a few changes on the pump to get alittle more pressure. "

GM pumps can ALWAYS use a little help!
Mass production and good, tight tolerances don't lend themselves to getting along very well!
I spend as much time with the pump as I do the rest of the rotating mass!
Pump is the very heart of the auto trans, and it has to work right for 100K or more!

He changed to a stronger regulator/bypass valve spring in the pump.
Line pressure changes have consequences!
Springs also die with heat and use! If the line pressure was down, it's probably the bypass/regulator spring giving up!

I've started using chorme-silicone springs instead of the usual steel 'music wire' springs.
They keep their tension a lot better over the years and movement cycles, and really seem to handle the heat better! They are about twice the $ as music wire, but at only $15 it's a real deal in my opinion!
Nothing like a pump that is still factory spec pressure after 200K on the clock!

Sounds like you may have got a mechanic that knows what he's doing...
But STILL! Get your warranty in writing! Good fences make good neighbors!
----------------------

I'm always looking for D-300 T-cases out of Jeeps, how much did he want for it?
If I could get it cheap enough, it would be worth shipping...?...

jfwireless · 10-10-2007, 06:33 PM

Jeephammer:

I will ask him about the 300 transfer case when I pick up the Jeep Friday.

We did do a full rebuild, the only hard parts that needed to be replaced were the sprag and band.

I had all the seals replaced in the TH400, all of them were originals, not teflon, but they are all teflon now. The seals were old and stiff, not flexible at all.

All seals, bearings and clutches were replaced, including the front center support bushing. I had a bad sprag, he showed my how all the springs were weak. He knew that from just feeling the pressure on each spring on the sprag by hand.

We also got a rebuilt torque converter.

Everything you are saying I heard from my mechanic, so this really makes me feel good about this rebuild.

Thanks for such an informative post. I am sure many others will appreciate this information, it is just so hard to make sense of repairs you know very little about.

Most of the transmission places I talked to just said "yea, bring it on in , I will fix it for $1500" and did not want to talk about exactly what they were going to do.

I only found two places that had knowlegable mechanics I could talk to. The one I picked just lets me walk in and see what he is doing during the rebuild. I learned a lot the past few days on how automatics work, I needed that education, and I got a better price than I expected.

Jim

Rollbar · 10-10-2007, 07:15 PM

My TH-400 started to slip in my '79 but all I did was replace the "O" ring in the pick up tube (I guess that's what it's called), the plastic tube that is located on the drivers side in the upper left corner. I pulled the plastic tube down/off & the "O" ring was broken, replaced it & never had a problem again.

**JeepHammer** · 10-10-2007, 11:14 PM

Quote:

Originally Posted by Rollbar

My TH-400 started to slip in my '79 but all I did was replace the "O" ring in the pick up tube (I guess that's what it's called), the plastic tube that is located on the drivers side in the upper left corner. I pulled the plastic tube down/off & the "O" ring was broken, replaced it & never had a problem again.

Sucking air is not uncommon when a dipstick does maintenance and doesn't replace the O rings and seals when he services the transmission...
Sucking air is REALLY hard on the front pump!
A dry pump or cavitating pump is normally a dead pump in short order, so you must have found it early!
--------------------------------

jfwireless, sounds like you found one of the few shops left that is actually doing the job instead of doing the customer!

Now, the guy did you a solid, you do him one too, make sure ALL your friends know about this guy. Pass out his business cards like they were free!
A good independent shop is REALLY hard to keep running without a stead flow of NEW customers!
Since we actually fix the problem, we don't see people having to repeat time and time again, so it takes new customers to keep the doors open!

If you charge off the Motor Parts & Time Guide, no matter how quick you got it fixed, and just repair things instead of fixing the problem, you have repeat customers...
You would be surprised how often you can screw people and they sing your praises and come back over and over again.

I just fixed a loose coil connector terminal for a guy...
That had spent $1,700 at the local Chrysler dealership to get that stalling problem fixed and get a valve cover leak fixed!
They ordered one of the cast aluminum valve covers, stuck it down with silicone 'form-a-gasket', then broke the casting by over tightening the bolts!

When I met him, he was broke down along side a seldom used county road!
Took me about 20 minutes to diagnose the ignition problem (stalling), no time or equipment except for my mulit-tool to fix so he and the misses could drive it home, and about an hour to weld up and flatten out the valve cover enough to reuse it with a PROPER gasket torqued to CORRECT specs for the GASKET!

I Charged him $35 an hour with a 1 hr, minimum...
And I supply common 'Shop Supplies', but I don't include special cleaners like for transmission parts (mineral spirits work well) or cleaning off the general crud, like engine degreasers, ect.
He pays for lubes.

Now he's been back to get his brakes working correctly, since the local dealership had 'Fixed' them too, that took about a half day, they were REALLY screwed up!

Get his front differential rebuilt (and boy, did it need it!) where 25 years of wear was catching up to it!
We caught it in time, so all the hard parts were still serviceable, but it did need every seal, bearing, gasket, U joint, ect...
I serviced the rear diff when we did brakes, so now he has a set that works.

I've serviced the trans & Tcase also, good clean Synthetic in there, and now he has vents with tubes that actually go somewhere besides drag on the ground... If they were attached at all!

Now he wants his heater & gauges to work! Of all the nerve!

Guess I've created a monster!
He wants to trade me his 'Old Jeeps' in the pasture behind his house for the labor/work...
I'm up for that since I'm technically retired!

You should see the 'Jeep Farm' he has!
He has about 1,400 acres of farm land and a HUGE home, but he's retired too and rents out the tillable acreage.

Three CJ's with hard tops and doors in good shape, one tub that is pretty well salvage...
Check this out,
http://www.JunkYardGenius.com/jeep/images/jeepfarm1.gif

Yup! I'm up for trades!

jfwireless · 10-11-2007, 06:23 AM

Jeep hammer:

Ironically the way I found this transmssion mec hanic was through one of my friends in the roofing business. He had several of his truck transmisions rebuilt by this guy and they lasted forever. Word of mouth is one of the best ways to bring business into a shop. You can be sure I will pass this shop on to all my friends just as one of my friends passed it on to me.

After I talk with him about the 300 transfer case I will just PM you and you can talk to him directly.

Thanks again

Jim