Vacuum boost canister

[Mrturner1]

Talked with a transmission guy about getting my stall converter and tranny cooler done, and explained about my th400 shifting from 1st to 3rd when at wide open throttle, but not when just cruising. When cruising it goes through all 3 gears perfectly. He says my cam is large enough that I need a vacuum boost canister, and the lack of vacuum is why the transmission is shifting too soon.

So who makes a good canister setup? And how hard is it to install, I'd like to save on the labor costs

[j_cst_10]

At wide open throttle you aren't making any vacuum. There is a WOT electronic switch that needs to be activated when the pedal is mashed to tell the transmission to exert maximum line pressure.

You may want to change transmission guys haha.

[Mrturner1]

I don't understand. My tranny shifted from first to third when I'm on the gas hard, but if I'm cruising it uses second just fine. Now if I put the gear selector in 2nd and floor it from a stop, it hits second gear nice and hard and then third when I manually shift it. He is saying that I don't have enough vacuum because of the big cam, and that makes the tranny shift into 3rd instead of second.

[GASoline71]

The kickdown on TH400's is an electric switch under the dash. If it's faulty, it won't kick down to 2nd on hard acceleration or "passing gear".

Gary

[Mrturner1]

I'm not having trouble with kickdown, my issue is when I'm at a dead stop and I floor it , it goes through first and when it should hit second it skips it and goes to third

[Ironangel]

Originally Posted by Mrturner1

I'm not having trouble with kickdown, my issue is when I'm at a dead stop and I floor it , it goes through first and when it should hit second it skips it and goes to third

If you post this in the Engine & Drivetrain section you'll get an expert to chime in. "Clinebarger" is the transmission guru over there...

[RichardJ]

>>So who makes a good canister setup?<<

Ah, GM?


Go to any wrecking yard and pick any vacuum reservoir from any car that suits your fancy. Most cars with AC have one to prevent problems with AC/heater doors when accelerating. You'll need a vacuum "T" fitting, so grab that as well. Insert the "T" fitting into the vacuum modulator hose and you'll find out very shortly if the trans guy is correct.

Make sure you don't already have a vacuum leak anywhere including the vacuum line to the modulator.

[davepl]

While you're at that wrecking yard, ask yourself why no other TH400 vehicle has a vacuum cannister (unless needed for HVAC or headlights or whatever).

The TH400 has a vacuum modulator but it maximizes pressure when there's no vacuum. You don't need, want, or expect vacuum at WOT. I'd be surprised if THM made a transmission that needed vacuum at WOT.

Here's everything I know about automatic transmissions: my bet is there is a GeeGaw A that has to build enough counter pressure against Whatzit B until the Gee Gaw forces the Whatzit to slide back allowing for a 1-2 upshift. Or something like that.

[Mrturner1]

Originally Posted by davepl

While you're at that wrecking yard, ask yourself why no other TH400 vehicle has a vacuum cannister (unless needed for HVAC or headlights or whatever).

The TH400 has a vacuum modulator but it maximizes pressure when there's no vacuum. You don't need, want, or expect vacuum at WOT. I'd be surprised if THM made a transmission that needed vacuum at WOT.

Here's everything I know about automatic transmissions: my bet is there is a GeeGaw A that has to build enough counter pressure against Whatzit B until the Gee Gaw forces the Whatzit to slide back allowing for a 1-2 upshift. Or something like that.

while your on here acting like a smartass, ask yourself why you have so much free time in your life that you post crap like this on the internet when someone is trying to get some help.

Here's everything I know about transmissions...."theres a GeeGaw A that has to build enough counter pressure against Whatzit B until the GeeGaw forces the Whatzit to slide back allowing for a 1-2 upshift....

I learned that from you, because I'm on this site asking questions to learn what I don't know, so in English please, if what your saying is even going to help

[Ol Blue K20]

Originally Posted by Mrturner1

while your on here acting like a smartass, ask yourself why you have so much free time in your life that you post crap like this on the internet when someone is trying to get some help.

Here's everything I know about transmissions...."theres a GeeGaw A that has to build enough counter pressure against Whatzit B until the GeeGaw forces the Whatzit to slide back allowing for a 1-2 upshift....

I learned that from you, because I'm on this site asking questions to learn what I don't know, so in English please, if what your saying is even going to help

Easy now...a little humor never hurts.... I'm no expert but my experience tells me that if it skips second then something is wrong with the trans. I had a th350 do that and had to get it rebuilt. Mine didn't have to be WOT though but would hit second if I shifted manually.

[Gromit]

I assume you are aware there are adjustable modulators for the TH350 and TH400?

I've not had to adjust one but they are easy to remove and maybe there is a way to bench test yours using a hand vacuum pump with a gauge.

Also you could check to see what vacuum you are getting at the hose end that connects to your vacuum modulator.

I think it is the ones with a red stripe that are adjustable Like I say I've never done one but they are fairly simple in operation

Here is something I just found not looking too hard

http://forum.73-87chevytrucks.com/sm...?topic=25669.0


And here is something from the TCI site: (a new one is only 16 bucks)...

Adjustable Vacuum Modulators
The vacuum modulator is a vital component of automatic transmissions. It tells the transmission what kind of load is being put on it, allowing the transmission to react with the proper line pressures and shift points. Over time, modulators can develop leaks, suffer ruptured diaphragms, become bent, etc. Not only can this cause annoying drivability issues, it can lead to premature transmission failure.

TCI® comes to the rescue with this line of adjustable modulators. Renew performance, and gain the ability to slightly raise/lower part throttle shift points and line pressures. Simply adjust the screw inside the vacuum nipple for more or less diaphragm pre-load.

Allows you to fine tune your shift points and shift feel.

[Mrturner1]

Originally Posted by Gromit

I assume you are aware there are adjustable modulators for the TH350 and TH400?

I've not had to adjust one but they are easy to remove and maybe there is a way to bench test yours using a hand vacuum pump with a gauge.

Also you could check to see what vacuum you are getting at the hose end that connects to your vacuum modulator.

I think it is the ones with a red stripe that are adjustable Like I say I've never done one but they are fairly simple in operation

Here is something I just found not looking too hard

http://forum.73-87chevytrucks.com/sm...?topic=25669.0


And here is something from the TCI site: (a new one is only 16 bucks)...

Adjustable Vacuum Modulators
The vacuum modulator is a vital component of automatic transmissions. It tells the transmission what kind of load is being put on it, allowing the transmission to react with the proper line pressures and shift points. Over time, modulators can develop leaks, suffer ruptured diaphragms, become bent, etc. Not only can this cause annoying drivability issues, it can lead to premature transmission failure.

TCI® comes to the rescue with this line of adjustable modulators. Renew performance, and gain the ability to slightly raise/lower part throttle shift points and line pressures. Simply adjust the screw inside the vacuum nipple for more or less diaphragm pre-load.

Allows you to fine tune your shift points and shift feel.

Just recently aware Im learning so much by asking questions on this site and going back and forth to my truck to see whats what, and whats where. Its awesome.

Tommarro I'm going to find this adjustable modulated and turn the screw IN about two turns (as suggested above) and see what that does. ALSO, I'm going to walmart and buying my first timing light and I'm gonna learn what timing is and how to set it where I want, and figure out what my truck likes best. Its gonna be a fun day, of work and time to tinker

[Mrturner1]

Originally Posted by Ol Blue K20

Easy now...a little humor never hurts.... I'm no expert but my experience tells me that if it skips second then something is wrong with the trans. I had a th350 do that and had to get it rebuilt. Mine didn't have to be WOT though but would hit second if I shifted manually.

Had a rough day combined with the frustration of not knowing these things about my own truck, and we have a baby due yesterday who should be coming out any time now. ill try to be a little more thick skinned, I apologize for that, to davepl

[Alex V.]

Any actions your TH400 takes, that are altered by vacuum change, at WOT are in response to a loss of vacuum. Some Turbo 400's (particilarly trucks, but not all trucks if I'm correct) don't have any kind of kickdown mechanism, just the vacuum modulator to hold it in the present gear longer before shifting up to the next. (A friend who's messed with cars, Rangers, etc. and went to school for auto tech tried to tell me my '85 C3500 should have a kickdown cable/linkage on it, and I was crazy to think it would've come from the factory without one - hence my research that concluded it is correct and my friend needs to study up more on old heavy Chevys.) BUT there is, indeed, no mechanism to force shift the transmission down a gear if it's already shifted up. If I'm right, all TH350's do have a kickdown cable, 400's do not - only a switch on the accelerator that kicks down when WOT is reached, and that's only on some of them. So, unfortunately, if your mechanic meant what he said about a lumpy cam "not providing enough vacuum" when you've got it to the floor, he doesn't quite grasp how all this works.

However, your problem isn't with kickdown behavior. I'm not nearly an expert on auto transmissions, but to agree with at least one prior poster it doesn't sound like something that would be caused by a modulator/kickdown malfunction, rather something internal.

[68c10owner]

Im guessing you have a valve body issue. You dont need a vacuum canister for your transmission. You can try the adjustable modulator. I saw someone posted a link to a ebay unit for a TH350. I honestly dont know if they interchange with a TH400 but make sure you get one that will work with a TH400. Im a little shocked that the tranny shop said that. Maybe you dont want to use them after all, lol.

[j_cst_10]

I am not 100% sure that all TH400's have an electric kickdown, but everyone I have seen does. Make sure everything in the system is operating properly.

Play around with a google image search of TH400 kickdown and it will show you where the sensor is located on the transmission and also look on your throttle linkage to see if you have a switch. If you do have all of this, make sure it is operating properly.

This may not be your issue but you can rule it out if it is functioning properly.

[davepl]

Originally Posted by Mrturner1

while your on here acting like a smartass, ask yourself why you have so much free time in your life that you post crap like this on the internet when someone is trying to get some help.

Because I'm rich and good looking and like to help people. You're welcome. If you want it dry and humorless, ask it where I won't see it. But if you have a really simple problem and I give a really simple explanation, I have to jazz it up a little or I'll get bored talking about vacuum modulation.

Originally Posted by Mrturner1

I learned that from you, because I'm on this site asking questions to learn what I don't know, so in English please, if what your saying is even going to help

Read the post, donut hole. It explains why a vacuum modulator doesn't need a vacuum reservoir on a TH400, in plain english.

Now go home and kick your dog or do something to get that aggression out because if you hassle me you're going to get me banned, and then I won't be able to help people like you.

[Mrturner1]

Originally Posted by davepl

Because I'm rich and good looking and like to help people. You're welcome. If you want it dry and humorless, ask it where I won't see it. But if you have a really simple problem and I give a really simple explanation, I have to jazz it up a little or I'll get bored talking about vacuum modulation.



Read the post, donut hole. It explains why a vacuum modulator doesn't need a vacuum reservoir on a TH400, in plain english.

Now go home and kick your dog or do something to get that aggression out because if you hassle me you're going to get me banned, and then I won't be able to help people like you.

Pretty tough talk through a keyboard man. I wouldn't be surprised if the rich and good looking stuff was just through the keyboard too. I apologized, but obviously your ego had to be restored with a response like that or you just wouldn't get to feel that false toughness over the internet anymore. I understand.

Be good now and don't get banned, what a shame that would be. Then you'd only have 4 other internet profiles to troll on

If this question was too simple for you, just leave the thread, it will do fine without you. You don't HAVE to chime in on everything you see with sarcasm

[GASoline71]

"Lighten up Francis." ~ Sgt. Hulka

Both of you'se

Gary

[rcazwillis]

I don't think your transmission guy is not off his rocker. The modulator controls shift points via vacumn levels. Big cams kill vacumn at wide open throttle. The screw in the modulator will affect shift points up to 5% or so. That is 250 rpm at most. Your description is a complete failure to use 2nd shift, not shifting too early. I would check the things recommended on this thread.
1. Is the vacumn system, every line, in good shape without leaks.
2. I don't think the kick down is the issue, but check it anyway. Is there a kick down switch on your throttle pedal? Does it work? An ohm meter on the outputs would indicate if the switch is working.
3. Is the wiring from the switch to the transmission good.
4. Is the vacumn module good? Take it out of the trans, it will leak a little fluid so be ready to catch it. I test them by sucking on the hose end with a vacumn pump. The plunger on the other end should pull in as the vacumn is applied. A tool placed against the plunger side will allow you to feel it pulling in. You can use your mouth for the suction, just wipe it off good, jokes here...
5. If you have a way to check wide open vacumn, gauge, you could check that. A wrecking yard canister would allow you to test the theory too.
After these steps, you are into the valve body issues. You have to eliminate one at a time.

[davepl]

Originally Posted by Mrturner1

Pretty tough talk through a keyboard man. I wouldn't be surprised if the rich and good looking stuff was just through the keyboard too.

Oh, on the Internet that's about a guarantee.

We have Telephone Tough Guy but no one has come up with a good one for online. Flexing your Web Muscles? One day, someone will hit it.

Anyway, wasn't trying to rankle you - just my suggestion followed by a (humorous to me) disclaimer that I don't know a great deal about the internal workings of automatic transmissions except - and this was the part you didn't like - that generally they operate by having relative pressures override each other.

Consider that the upshift from 1->2 might only happen when the pressure from the driven pump plus the force of he vacuum modulator exceeds some other threshold; at cruise there's lots of vacuum so it upshifts early. At WOT there's no vacuum so it upshifts much later.

If something else were wrong in that circuit, the combined pressure may -never- get high enough to shift 1->2 without vacuum, but the kicker is there's never any vacuum at WOT anyway, to speak of, and that's what the TH400 is designed to live with, so there's an electronic kickdown to signal WOT since there's no detent.

I used parts terms of Whatzit and Geegaw to indicate that my description was an analog only, not authoritative.

Now if someone knows how the 1->2 upshift in a TH400 actually happens, we'd have something.

[snipescastle2]

When I last visited a recycling yard, I found a bunch of those spun aluminum "Oxygen" tanks, they were recycling, after they are hydro tested so many times, they have to recycle them. I found a small one (about the size of a large coffee thermos) I chucked it up in a lathe, took a small skim cut to remove the clear powder coating and the green paint on the top and neck of the bottle. Since it was already threaded, made it a breeze to use pipe thread bushings for a tee fitting allowing me to plumb the vacuum booster hose directly to the "tank" and then run the other hose with a check valve inline to the intake. I'm polishing the tank right now, that's why I haven't posted any pictures yet.
I'm going to be running a pretty stout solid lifter cam in my 383, so I'll need the vacuum reservoir too.
I also had my welder buddies at work make me a coolant overflow canister out of a piece of aluminum pipe and had them weld plate aluminum on the ends, I tapped the top and bottom ends for 3/8" pipe fittings, I again used the lathe to clean up the rough ends, then polished that too, added two (2) 90 degree compression elbow fittings and some clear tygon tubing to check the level in the tank, I added a drain valve too. These are just some ideas I used.....
Ben

[Mrturner1]

Originally Posted by davepl

Oh, on the Internet that's about a guarantee.

We have Telephone Tough Guy but no one has come up with a good one for online. Flexing your Web Muscles? One day, someone will hit it.

My post was actually bringing out the fact that YOU were "flexing your web muscles" (don't make me get banned by REALLY telling you off, donut hole) which isn't even necessary, and you continue to post bs on this thread and say things that don't make sense, like someday someone will hit it. Just let it die man, you said yourself that your knowledge of the TH400 is limited, so why beat a dead horse?

I apologized, there was no need for more bs after that. Now I'll thank you for your input. Let it go

[davepl]

Originally Posted by Mrturner1

My post was actually bringing out the fact that YOU were "flexing your web muscles" (don't make me get banned by REALLY telling you off, donut hole) which isn't even necessary, and you continue to post bs on this thread and say things that don't make sense, like someday someone will hit it.

Yeah, I was trying to be nice, but it was lost on you I guess.

There's nothing to let die. If you read my posts you'll learn stuff. If I read yours they're about me. It's a win-win in my mind.

But if it's important to let it go, just do so (replying yet again wouldn't be letting it go). I only want to talk about trucks and transmissions anyway, not about you, which is why I've been staying on topic.

I didn't want to ruin the suspense, but I do know a fair bit about transmissions, even if I tried to dumb it down because it's still a "bit", and not a "lot". If the electric kickdown is indeed functioning, and it's not the vacuum modulator as I initially thought, odds are the governor is sticking. It's on the passenger side of the transmission near the back, under a 4-bolt cover.

I'd pull the transmission pan and look for fine metal (maybe stick a strong magent to the pan for a day first).

Generally with the governor it won't go down into first when you stop, though, and you said it goes 1-3 under cruise fine, so I still think modulator.

I'll bet you $10 that if you just buy a new vacuum modulator and install it, your problems are solved. Actually your approbation and thanks would be enough, keep the $10. But that's the most likely and easiest place to start. You can likely get it at any auto parts store, they made a lot of these.