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

[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

[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.

[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.

[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.