TH400 Issues. Lookin for some advice

[rastaangel]

I am by no means a auto tranny guru so I am lookin for a bit of advice as I have never owned a vehicle that has a auto tranny.
My friend just bought a 1987 3/4 ton truck to use as a mud rig. It has a TH400 tranny in it and he is worried about a few things and weither or not it will be able to handle the power of the built 454 BBC he plans on putting in it. It has 263k on the powertrain and is all orignal acourding to the orginal owner he bought it from
The tranny in his truck engages and cluncks really hard when put in reverse. It also will not shift into 2nd gear till you hit 3,000 RPM, same thing to hit 3rd. It will not shift unless you hit that rpm and it is a stiff shift as well when it does shift. At WOT it shifts at about 4300 rpm and he would like to change that as well to a higher shift point.
As I said I have no idea about the magic of automatic transmissions but I thought that changing out his valve body would prob fix some of those issues.
Any tips or advice would be greatly appreciated!

 

[76zimmer]

need to verify the condition of the trans...is the 263K on the trans too?
Does the trans shift automatically if left in D?
If the trans is put in 1, will it shift to 2 under heavy throttle by 4500 rpm?
The valve body is built to make that shift whether it is in 1 or 2.
A manual v/b will make it full manual control. You start in D, your in D, you start in 1 and rev it til the engine blows, you'll still be in 1.
The hard hitting reverse is typical in my experiences with the T400, but check your idle speed, that can make it seem harsh if its above 700rpm.
If the fluid is opaque looking and bright red, it sounds like its ok internally as far a clutch packs are concerned. There are a few changes that can be made cheaply along with a manual v/b to make a T400 stand up to some pretty stout power.

 

[diesel4me]

GM put TH400's behind almost all of their V8's including big blocks like 454's,455's and Caddy 500's,they were the largest automatics ,and ones most able to handle high torque outputs,so I would say they are the best non overdrive automatics to use in high performance applications..
I have had very good luck with all the ones I've had in my trucks and cars,and I didn't baby them one bit!..

The one you mentioned may have had a shift kit installed at some point,by the sounds of it ...but even a vacuum modulator thats faulty or lacking vacuum supply to it can make it shift hard and late as you described...

 

[rastaangel]

Thats 263,000km not miles to haha
It will only shift in D when you hit 3,000 RPM, if you go to 2800 and let off it doestn shift either. You have to hit 3000
If you put it in 1 it will rev to 4500 then shift to 2nd
His idel is about 1000 RPM, its a TBI engine swapped in so he cant reall change his idle
The previous owner need said anythng about a shift kit, the truck was used to haul on a farm so it wasntused in a hi peformance application to need a shift kit.
How can he check the vacuum modulator you mentioned??

 

[Greg Ducato]

Make sure vacuum source is the direct port at the back of the intake, not the throttle body, and be sure the line is not restricted

 

[454Sub]

Slamming into reverse is a sign that a (cant remember the name) spring is broken. Its under the valve body. Mine was broke and it slammed into reverse like a drag car. Or, you just have a wicked shift kit in it, or your vac line is messed up. I have a pretty stout shift kit in mine, as I enjoy the slamming gears, and tranny life in a mud truck is not important. I can rebuild mine myself and parts are cheap so for me its no big deal.

Vac modulators are adjustable as well, or atleast mine is. Takes an itty bitty flatblade screwdriver. Turning it clockwise harshens the shift, opposite softens it. It also delays or quickens shift points as well.

 

[Fordum]

Probably the single most common failure of the modulator is a pinhole in the diaphragm.
This causes transmission fluid to get sucked into the engine and burned.
That is only a problem long term. The symptom you notice is the weird shifts because the transmission fluid swells the short piece of rubber hose that hooks onto the modulator.

After a week or so, the hose swells so much it either causes a vacuum leak that keeps the modulator from working properly, or else falls off completely.

Back when the 400s were common, I used to hear the same complaint all the time. "That darn hose keeps falling off the transmission."
"I cut the swelled end off, and it fits great but then falls off again after a couple of weeks."
If yours is a tight fit, pull it off and see if its wet inside. If so, you need a new modulator.
Also check the fluid, since it will eventually suck it dry.

 

[rastaangel]

If this is the vac modulator...

Then it is not hooked up to anything! You can see where the hose ends in this pic by the rotor and intake. I have no idea where its supposed to plug in and theres no sign of a busted line where it should have been, my guess would be the post behind it. Where is it supposed to be plugged to?


And the port on the bck of the intake goes into the valve cover as it alwyas has been

 

[Fordum]

Well, theres your trouble right there!!

Yep, that is the modulator, should have a piece of steel line between the two rubber lines.
As to where it goes, you are going to have to wait for someone more familar with those setups than me. Its been years since I worked on one, and I don't remember ever working on that end.

What is that small black thing with the nipple on top just to the center right in the picture?
Looks like it might be screwed into the intake, and have a place for a hose.

 

[454Sub]

It can hook anywhere where it is MANIFOLD vacuum. If that nipple is actually threaded into the intake and leaking, that is a massive vacuum leak. That is where the modulator is supposed to hook up to.

 

[76zimmer]

yep hook up the mod to that fitting, the breather pipe from the v/c is supposed to go to the air filter housing, and the pcv should go to the front of the base of the carb to a large vac port.

 

[Fordum]

The truck is going to drive like a completely different machine once you get the line hooked up.
Right now the transmission thinks the engine is heavily loaded down and straining. So it stays in the lower gears as long as it can.
Don't know what part of the world you are in, but look in the end of that hose before you hook it up and make sure a bug has not built a nest in it.

Down here where I live, you leave a round opening like that open for long in the spring or summer, and a dirt dauber wasp will build a nest in it.

 

[rastaangel]

Well I stopped by my buddys place with that info in mind.
As soon as I put my finger on that nipple it was obviously a MASSIVE leak as the idle instantly smoothed out!
So I added a few inches to the line and hooked it up to that nipple and the idle improved even more as well as lowering the idle a bit
Then we went for a drive.... and holy cow! His truck drives like a different vehicle! It shifts so smooth now you can barely feel it and at any RPM.
It still hits like a cannon ball shifting into reverse but he says he can live with that and he likes the fact he can change his shift points when were offroad to reduce how much this tranny shifts and only let it shift at high RPM
He is greatful for the help and I told him to join this forum
I too thank you for the help.

 

[diesel4me]

I would say its fairly normal for it to go into reverse kind of harshly,all of my TH400's did...its when you notice a delay, or a soft "sliding" shift into reverse,that you should start worrying,thats the first symptom a TH350 or TH400 is on its way out--usually reverse fails first,then the forward gears...its probably in good shape still..