TH400 gurus, direction needed.

QBuff02

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OK, so back story as I have never been inside of this th400 in my truck, but I do know it was rebuilt probably less than 5,000 miles before I bought the truck. It has always been a very solid unit and not something I felt I had to mess with as it was supposedly built for pulling. when I had the pizza cutter tires on it, it would squall the tires going into second gear and bang drive nicely with a heavy dose of throttle. never really had any shifting issues. But back when I was playing around with it like that I one, didn't have the engine built like it currently is yet, and two, still had the factory column mounted shifter. But the trans was always just a nice all around performer. with that said-Fast forward to now, with the 454 rebuilt and roughly 500hp/500ft/lbs tq on tap, and a floor mounted shifter (the stock linkage was completely worn out and basically fell apart when I went to take it apart) to go with the newfound performance and certainly not wanting to hit neutral under heavy throttle. Anyway, i'm getting off track.. Back to the trans, it runs and drives just like it did (only change to trans was added a 2,400rpm stall during engine rebuild) before rebuilding the engine during part throttle, in town, cruising, moderate load, never misses a beat, just works... until I try to do full throttle jaunts and shifting with my foot on the floor. It will flat out NOT shift to second gear if my foot is on the floor. Like misses the mark and hits the rev limiter at 6,500, but during anything from light to part throttle driving, if I even breathe on the lever it hits 2nd, if I am doing spirited driving it will shift as soon as I hit the lever, but if my right foot is anywhere near (WOT) the carpet, you can bet your ass I will have to lift to get it to hit second. Drive doesn't seem to be phased, going from 2-3 it will shift literally as soon as I lean on the shifter no matter where my foot is and how hard i'm driving it. I had changed the vacuum modulator previously and have moved it some each way with no change under wot, however I did notice a slight change in part throttle driving in town. With the cam that's in it, it only has about 10-11" of vacuum at idle. I have tried it both ways with the kickdown switch hooked up and disconnected and have verified power to it. There is no change to the "condition". So is this a governor issue? or do I have something else possibly going on? Should I test the kickdown switch? or is it not even a player at these conditions? I've looked into the adjustable weight kits, but would like a possible direction to travel before I just start ordering parts. I mean, I have a feeling the extra power and the possible lack of vacuum may have something to do with it. Been in hundreds of engines, but only a handful of transmissions in my life and I figure someone here can probably easily point me in the right direction.. Help!
 

Matt69olds

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First, because it’s cheap and easy, try a new vacuum modulator.
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This is the part number for the adjustable modulator from AutoZone. About 15 bucks, install it as is and see if anything changes


Find a pressure gauge that will read 250ish psi. The line pressure tap is above the shift linkage. Check the pressure in each gear, with and without the vacuum modulator connected. Hopefully you have 170ish psi with the modulator unhooked. If not, you might need to change the pressure regulator spring.

From your description, it will shift as soon as you let up on the throttle? That suggests a valve body issue. Do you know anything about what was done or who built the trans?


The kickdown switch doesn’t have any effect on line pressure or full throttle shifts. However, if the solenoid itself leaks, that may be a issue. Take off the pan, the kickdown solenoid is held to the case by 2 bolts. Make sure they are tight. The oval can style solenoid takes a thin stamped steel gasket. The later model open type has a built in o-ring style seal. I have seen people use the steel gasket with the late style solenoid, sometimes they will leak. Without getting too technical, when activated the kickdown solenoid changes governor pressure to the shift valves, forcing them into the downshift position. If line pressure is good, changing the modulator doesn’t help, and the solenoid is tight, before messing with the valve body you might want to replace or at least inspect the kickdown solenoid. There is a tiny hole in the solenoid, use a rubber tipped blow gun to apply air to that orifice. Don’t use more than about 70-80 psi. It shouldn’t leak. If you apply power to the wire, air should vent thru the solenoid and then immediately seal when you remove power.


If everything checks out, it’s probably time to look at the valve body.
 

QBuff02

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First, because it’s cheap and easy, try a new vacuum modulator.
You must be registered for see images attach

This is the part number for the adjustable modulator from AutoZone. About 15 bucks, install it as is and see if anything changes


Find a pressure gauge that will read 250ish psi. The line pressure tap is above the shift linkage. Check the pressure in each gear, with and without the vacuum modulator connected. Hopefully you have 170ish psi with the modulator unhooked. If not, you might need to change the pressure regulator spring.

From your description, it will shift as soon as you let up on the throttle? That suggests a valve body issue. Do you know anything about what was done or who built the trans?


The kickdown switch doesn’t have any effect on line pressure or full throttle shifts. However, if the solenoid itself leaks, that may be a issue. Take off the pan, the kickdown solenoid is held to the case by 2 bolts. Make sure they are tight. The oval can style solenoid takes a thin stamped steel gasket. The later model open type has a built in o-ring style seal. I have seen people use the steel gasket with the late style solenoid, sometimes they will leak. Without getting too technical, when activated the kickdown solenoid changes governor pressure to the shift valves, forcing them into the downshift position. If line pressure is good, changing the modulator doesn’t help, and the solenoid is tight, before messing with the valve body you might want to replace or at least inspect the kickdown solenoid. There is a tiny hole in the solenoid, use a rubber tipped blow gun to apply air to that orifice. Don’t use more than about 70-80 psi. It shouldn’t leak. If you apply power to the wire, air should vent thru the solenoid and then immediately seal when you remove power.


If everything checks out, it’s probably time to look at the valve body.



I had thought about checking line pressure on it just to see what it is, apparently that's a good place to start. I do not know who built the trans, but according to the guy I bought the truck from I do know they made some modifications/additions to the clutch packs and valve body. I would venture to say one of the somewhat local to here shops did the work, but without knowing who or where it might be a crap shoot. especially since this is now going back probably several years for the rebuild. And in answer to the question, yes if I plant the throttle, it revs through the rpm range and go to hit second gear on the shifter and it does NOT shift, when I back off the pedal it will finally go into second. but it only does that at WOT. nowhere else in the rpm range or amount of load on the driveline does it seem to matter. any normal driving, cruising, and i'd say less than 75ish% pedal it will shift to 2nd as soon as I hit the lever. Now, I've heard and read that th400's would automatically upshift to second even if held in first manually at a certain speed/rpm for engine protection, but that's assuming stock or near stock internals? With this truck being setup for cheater stock pulling before I bought it, and i'm assuming this trans built with the pulling in mind is it possible that whoever did the trans made some sort of modification to the trans/valve body to make it so it never comes out of first under full throttle conditions? That is something I've wondered a lot since discovering this little issue. When I finally DO get this thing back on the track and hooked to a sled, it would be nice to hold it in first without worry it jumps to second when i'm not wanting it to. And at this point I don't feel like it's going to. Lol But when i'm on the street or having fun I WANT it to shift when I tell it to! Nothing is more poser cringe worthy than all that power and it banging up on the rev limiter and/or having to lift when it doesn't shift. I've even tried short shifting it at around 5,200-5,500 while my foot is on the floor and it does the same there as when I run it to 6-6,200, stays in first gear until it hits the limiter and/or I lift up on the throttle. Now i'm not actually letting it bang up against the rev limiter all day, but it has gotten into it several times but at this point generally I am anticipating it not shifting and trying to lift my foot before it gets to that point but it does rev pretty quick through first gear. I did buy a pan gasket and new fluid and was planning on pulling the pan and investigating, but aside from a visual I wouldn't have known to look at a few of the thing you mentioned. One other question, I've read about low vacuum style vacuum modulators.. Are they worth a damn? I ask just because with 10-11" vacuum at idle I've wondered if that might be a small contributing factor and if it would be worth trying one out. with all i've done, Automatic Transmissions are pretty voodoo to me.
 

Matt69olds

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I assume you have a pretty hairy cam, resulting in low manifold vacuum? That is low, but not low enough to cause your problem. If anything, the low vacuum would move the shift points up slightly.

Tru the modulator, if possible find out what the line pressure is. If the modulator didn’t help, drop the pan and swap the kickdown solenoid. After that, if nothing helps, it’s time to look at the valve body
 

Matt69olds

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Another thought, on the pass side of the trans there is a square cover with 4 bolt. The governor is inside. Make sure you have a replacement gasket on hand, just in case it tears. Remove the governor, post a pick of the weights and springs. Make sure the bore the governor spins in isn’t chewed up, make sure the govenor gear is in good condition.
 

QBuff02

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I assume you have a pretty hairy cam, resulting in low manifold vacuum? That is low, but not low enough to cause your problem. If anything, the low vacuum would move the shift points up slightly.

Tru the modulator, if possible find out what the line pressure is. If the modulator didn’t help, drop the pan and swap the kickdown solenoid. After that, if nothing helps, it’s time to look at the valve body

Yes, the engine is pretty healthy. And that's why I asked if low vacuum signal could cause part of this issue. I am going to pick up a modulator and governor gasket later today and hopefully if I get a little time this weekend I can tinker around with it in the garage. If I do i'll post some pics and report my findings.
 

SquareRoot

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I was watching a show this weekend and the guy had a squarebody with a 500 CI Merlin pushing 700ft lbs backed by a th700. Yes, I said 700! That blew my mind.

I know that doesn't help your situation but I thought I'd just throw that out there.
 

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If this is a local truck, I would take it to the trans shop known for beefing transmissions. These guys all know each other and someone(the older guy at the shop) will be familiar with the work done and maybe who did it.
 

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