TH400 Shift Early & Soft / No KickDown

[85k10383]

OK all you Tranny gurus. My TH400 needs something, and I would like your help to decide what. The PO never rigged up the electric kickdown to gas pedal mechanism, so the trans will not downshift, pretty much ever. Secondly it shifts very softly like it came out of great grannies Coupe Deville. Thirdly, it seems allergic to high revs in any situation, I have not seen better that 3500 on the tach with the new engine. I have swapped out the fluid for B&M Hotshift, which i know won't do much, and replaced the pan with a B&M + 2. My questions are: What can/should I do first? Is it worth the dough to get a better torque converter (higher stall etc.) without also doing more to the trans? Will just rigging up the kickdown help a little? Why won't the trans hold gears under load? Like when getting on the highway, it upshifts at about 3200 1-2-3, seems like I wouldn't burn as much fuel if the damned trans would just let me keep 2nd gear up to about 4500 RPM. All opinions and advice very much appreciated!
Truck is running a Blueprint 383, TH400, 3.08 gears 33-12.50-15 tires if that helps at all

 

[davbell22602]

Hotrodpc should be able to help you.

 

[85k10383]

But he doesn't like me apparently LOL HRPC where are you? A lowly trans rookie needs your sage advice.....

 

[crazy4offroad]

He should be chiming in soon. In the meantime can you manual shift it to hold 2nd longer? I know it's a PITA but so is listening to it shift too soon lol. On a side note, did yall get in on any of that hail/tornado action last week? We had a nice gentle rain here but after it was over I was getting all kinds of calls about roofs/siding etc damaged.

 

[HotRodPC]

Ahh hell, I like everybody, just was doing the parttime thing tonight.
Your vac modulator is way out of adjustment, shifting way early and to soft. Take the vac hose off, insert a small standard screwdriver into the vac line hole, there is small screw in there. As far out as yours sounds, I'd give it 2 full turns at least. Put the vac line back on, Take it for a ride see if that helps the issue. If that helps, then add more turns, or back it off to where you like it and that's that issue fixed.
Look at your gas pedal assembly for a Th400 kickdown swith to see if its even got one first of all, then we'll go from there.

Drivers side of trans toward the tailshaft just above the pan.

 

[HotRodPC]

I got a bit of time now for a better more detailed explanation, and to answer all your questions.
NO, Kickdown switch on a Th400 will NOT change your problem. Now it would if you floor boarded the gas pedal everytime you took off, cuz the Kickdown switch will override the vac modulator. Hooking up you kickdown on a Th400 will do nothing but give you kickdown. Its electronic, and its either on or off. No in between. As said above, lets find out what you have in the truck as far as the swtich and the wires, and we'll go from there on hooking that up.
No, do not need to change fluid, pan, converter or nothing like that. Your problem with early soft shifts is nothing more than a vac modulator adjustment or failure. There are 3 types of Vac Modulators. One is a biggo dude about the size of a coffee cup with a small very noticeable screw in the back of it, usualy painted black but can be other colors. Those are old style and not used or seen much today. Then there is small bulb type that is usually brass. Some are not adjustable. You can tell these models becuase if you look in the end of it where the vac line connects you will not see a small screw. If you have this type without the screw and having this problem, you'll need to replace it with the adjustable cousin. The 3rd type looks just like the 2nd type, small brass bell or bulb type as I have pictured below, and usually has a red stripe around it. I'm assuming that is what you have since its most common now days.
Th400 relies on 2 things to detemine shift points. 1 is the governor which monitors tailshaft RPM, #2 is Vac modulator. That's it. The vac modulator is what tells the trans what kind of strain is being put on the engine and when and for how long to wind out the shift points. Yours was probably never set. It was just taken out of the box and bolted on without adjusting it. So its set real sissy like with a very very light spring, so the vacuum overcomes the spring very easily thefore the vac mod valve is telling the trans to shift way early. Remember, less vacuum actually holds the gear longer. So when you romp on it, low engine vac will not allow a shift, but governer will take over so you don't go to high of rpm. And if you had the vac disconnected, like no vacuum, then it would shift very very late and very hard. So you have the opposite problem. Just adjust the modulator or put an adjustable type on and adjust it, you're problem is solved. Vac Mod is on the passenger side of the trans just above the pan. It'll take you longer to find a screwdriver small enough to fit the hole that it will to adjust it. It'll take you a painless 15 seconds to adjust.

 

[85k10383]

HRPC, you are a god in my eyes! I never had it explained to me in a way I could get my brain wrapped around, very well done sir, and thanks very much. I will go out to the driveway this evening and see what I find. I don't believe the truck had the TH400 kickdown switch on the pedal assembly, as it had a 700R4 originally, but I could be remembering it wrong. Is there a hard/fast rule to which way to turn the screw for which result? i.e. clockwise for early shift, ccw for later? I am so happy right now, I'll bet that adjusting this little bugger is gonna make a huge difference.

 

[HotRodPC]

Yep, turn the screw IN CLOCKWISE to tighten the sprng and make the shift later and firmer. For sure this is what you want to do. If you turn it in to much, back it out CCW to shift earlier and softer. No doubt its going to make a differance, and I'd be shocked as can be if it doesn't fix the problem.
You're right, if the truck was originaly a 700r4, then you won't have the switch. But its nothing that can't be swapped out and in.

 

[85k10383]

OK the modulator must be toast, I put a total of four turns clockwise on it and basically nothing happened. It is the second of the three types you described (no red stripe) And when i pulled the vac line off there was the little screw. My vacuum line is about a foot of rubber then about three feet of hard line with another foot or so of rubber connected to the carb, I'm guessing I should just replace that with new as long as I'm replacing the modulator correct? I'm also assuming that if the mudulator were good that four turns would defineately have shown some measurable resullts. One last question, I want the most recent modulator that is still adjustable correct? My local NAPA probably has them in stock.... Thanks again for the help you are a lifesaver!

 

[oneluckypops]

OK the modulator must be toast, I put a total of four turns clockwise on it and basically nothing happened. It is the second of the three types you described (no red stripe) And when i pulled the vac line off there was the little screw. My vacuum line is about a foot of rubber then about three feet of hard line with another foot or so of rubber connected to the carb, I'm guessing I should just replace that with new as long as I'm replacing the modulator correct? I'm also assuming that if the mudulator were good that four turns would defineately have shown some measurable resullts. One last question, I want the most recent modulator that is still adjustable correct? My local NAPA probably has them in stock.... Thanks again for the help you are a lifesaver!

Hold the phone. Theres no need to throw parts at it. You need to verify you even have vacuume at the modulator with a gauge, Also when you pulled the vacuume line off was it wet or did and fluid run out of the modulator?

 

[HotRodPC]

He's getting vacuum or it would shift way late as if a modulator was not connected at all. I am beggining to suspect that maybe, just maybe, the vac modulator valve is stuck and not moving. Go ahead and pull the modulator out by moving the retaininng bolt. Should take a 1/2 in wrench. Then get some needle nose pliers and pull the valve out. Try to feel if it feels like its dragging or stiff. It should be move very very freely and easy. If not, then that is your problem. Take the valve out, clean it off with carb or brake cleaner. Look for any burrs on it. If there are any, file or emory cloth them off. Then put vaseline on the valve shove it back in its place and work it back and forth with the vaseline to clean any debris out of the bore. I'll never say never, but its rare a vac modulator goes bad. And when they do, its a hole in the diaphram thats sucking trans fluid. This is why Pops is suggesting to check for wet or fluid. My guess will be no since you didn't mention a loss of trans fluid, or putting out nasty white smoke which would be trans fluid burning in your intake system.
Sure its possible your vac mod is bad, but usually when they go, its late or no shift, not soft or early. This is why I'm suggesting to make sure the valve is not hung up. Another way to verify, is the unhook the vac line from the modulator under the truck, and put a bolt in it the vac line so you don't have an engine vac leak. Take it for a drive. It should shift very very late without the line hooked up. If it still shifts way early and soft, then that will about guarentee the valve is hung up in sissy shift position. Vac mods are not alot of money, $20 or so for the good adjustable brass style, so replace if you want, but I think I'll lean on stuck valve first and at least check it before I spent the funds.

 

[HotRodPC]

BTW, if the valve is stuck, it is possible to not be getting vacuum too, but I say highly unlikely. You can actually start the truck, pull the line off that modulator and put your finger over it. You'll know if it has vacuum. I am almost wondering if its possible the valve was never put in too and what affect that would have. Late shift of early shift? I am thinking early shift since the vavle is not in there to block the passages and cause late shifts. I say check for valve in place, or stuck before you go buy and Vac mod. If it don't have a valve in it, I have one here that shouldn't be to expensive to mail. I'd say it only weighs a few ounces. So don't spend any money just yet. We'll get this figured out.

 

[oneluckypops]

He's getting vacuum or it would shift way late as if a modulator was not connected at all. I am beggining to suspect that maybe, just maybe, the vac modulator valve is stuck and not moving. Go ahead and pull the modulator out by moving the retaininng bolt. Should take a 1/2 in wrench. Then get some needle nose pliers and pull the valve out. Try to feel if it feels like its dragging or stiff. It should be move very very freely and easy. If not, then that is your problem. Take the valve out, clean it off with carb or brake cleaner. Look for any burrs on it. If there are any, file or emory cloth them off. Then put vaseline on the valve shove it back in its place and work it back and forth with the vaseline to clean any debris out of the bore. I'll never say never, but its rare a vac modulator goes bad. And when they do, its a hole in the diaphram thats sucking trans fluid. This is why Pops is suggesting to check for wet or fluid. My guess will be no since you didn't mention a loss of trans fluid, or putting out nasty white smoke which would be trans fluid burning in your intake system.
Sure its possible your vac mod is bad, but usually when they go, its late or no shift, not soft or early. This is why I'm suggesting to make sure the valve is not hung up. Another way to verify, is the unhook the vac line from the modulator under the truck, and put a bolt in it the vac line so you don't have an engine vac leak. Take it for a drive. It should shift very very late without the line hooked up. If it still shifts way early and soft, then that will about guarentee the valve is hung up in sissy shift position. Vac mods are not alot of money, $20 or so for the good adjustable brass style, so replace if you want, but I think I'll lean on stuck valve first and at least check it before I spent the funds.

As engine RPMS increase, Vacuume Increases so. When the modulator is doesnt have any vacuume supplied to it it would be soft shifts, FULL Vaccume is what makes it have Longer shifts.

You should be ale to unplug the module, PLUG the Hose, and install a vacuume pump to the module then put it under vacume and it should increase the shift points.

 

[HotRodPC]

I see what you are saying, but I thnk you got it backwards. If that were the case, when you take the vac line off completely, and have no vacuum, the shift are way late and hard. Vacuum actually increases after the rpms have built up and motor strain is reduced. If you romp on it taking off from a light, you will have ZERO vacuum for quite some time and not allowing a shift. You got the right idea in theory, but I think you might be backwards on the vac modulator operation. No vac is no shift. Line disconnected or no vac at all, it would shift way late, not way early.

 

[oneluckypops]

I see what you are saying, but I thnk you got it backwards. If that were the case, when you take the vac line off completely, and have no vacuum, the shift are way late and hard. Vacuum actually increases after the rpms have built up and motor strain is reduced. If you romp on it taking off from a light, you will have ZERO vacuum for quite some time and not allowing a shift. You got the right idea in theory, but I think you might be backwards on the vac modulator operation. No vac is no shift. Line disconnected or no vac at all, it would shift way late, not way early.

been a long time since i have messed with cars that are that old BUT from what i remember is line un hooked = early shift (no vacuume). You got any 350's or 400's running that you can plug the hose and verify which way it is positively?