# TH400 Kickdown Operation/ 67 GTO



## armyadarkness (Dec 7, 2020)

Those of you following my exploits will be familiar with my state of the union. While tweaking an resolving my 67 GTO, I've determined that the vacuum line for the TH400, may have a slight vac leak. I've had a few of these trannies over the years, but I forget their kickdown behavior. The one thing that I DO remember, is that when my 66 Lemans kicked down at the same time the 4bbl opened, it was the single moment that made me a Pontiac fan for life.

Unfortunately, that likely had more to do with luck, than tuning prowess.

I readjusted the valves on my engine and it sounds A LOT better. Makes great power, but I can tell that its not being tapped properly. I'm not overly concerned, because I still have to jet the AVS2 800, which I won't do until I weld in the AEM 02 sensor, and install the roller rockers. But I definitely don't think that the trans is shifting and kicking down as it should.

I thought that there was a cable which controlled this, but I don't have one. So is it vacuum or electric?

If I'm cruising around 10-15 MPH and mash the throttle; if it kicks down, it lights the tires and goes sideways! But if it doesnt kick down, I just watch gas $ go out the tailpipe


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## O52 (Jan 27, 2019)

T400s have an electric kick down solenoid that is controlled by a switch either on the carburetor or the gas pedal inside the car. 

T400 still has a vacuum modulator for normal shifting.

Photo from my El Camino


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## armyadarkness (Dec 7, 2020)

O52 said:


> T400s have an electric kick down solenoid that is controlled by a switch either on the carburetor or the gas pedal inside the car.
> 
> T400 still has a vacuum modulator for normal shifting.
> 
> ...


So kickdown is electric only, and not vacuum related? Is operation adjustable?


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## O52 (Jan 27, 2019)

The switch is adjustable. But there's not to much there to adjust.

Photos from PY forums





Gas Pedal Bracket - PY Online Forums - Bringing the Pontiac Hobby Together


Gas Pedal Bracket 66-67 GTO Tempest & LeMans TECH



forums.maxperformanceinc.com


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## armyadarkness (Dec 7, 2020)

My issues are that under normal driving conditions, it shifts into second almost instantly, and then kickdown is inconsistent. Oddly enough, there's a disconnected wire in the vicinity of that switch! In my vette, the PO bypassed almost every switch on the car, so I simply never bothered to investigate.


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## O52 (Jan 27, 2019)

My El Camino shifted into second at about 10 mph under light throttle which was considered normal. You can adjust your shift points but I'll leave that to someone with more knowledge in that regard.

But realize there are different types of downshifting in a TH 400; 
The normal off throttle downshift controlled by the governor and vacuum modulator
A forced partial throttle downshift controlled by the governor and vacuum modulator
A forced full throttle downshift controlled by the throttle switch and downshift solenoid inside the transmission


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## Colorado67GTO (Sep 11, 2014)

One way to “adjust” the downshift switch is to cut a piece of hose and put it over the part of the switch that is hit by the pedal bar. That will cause it to downshift sooner. My 67 had such a hose piece on it when I got it and the place where it downshifts in the travel of the pedal seems just right on mine.


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## armyadarkness (Dec 7, 2020)

Colorado67GTO said:


> One way to “adjust” the downshift switch is to cut a piece of hose and put it over the part of the switch that is hit by the pedal bar. That will cause it to downshift sooner. My 67 had such a hose piece on it when I got it and the place where it downshifts in the travel of the pedal seems just right on mine.


Thanks guys. I can see that the switch mounting screws seem to be elongated, so (I assume) there is some adjustment. As predicted, I worked on the car on Saturday and then went out test driving... No amount of effort got it to downshift on its own. So just as with my old 66, this new GTO seems to be very finicky with its downshifts, and it's based on luck, not science. I'll search for some diagnostics on the switch.


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## O52 (Jan 27, 2019)

Pull the plug off the transmission, stick a voltmeter probe into the plug terminal, other probe to ground. Turn the key to On (Don't Start) and manually operate the switch. 
If you show 12 volts at the plug you probably have a bad solenoid in the transmission.
If not start measuring voltages at the switch. One wire should be hot when key is on. The other when the switch is activated.


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## armyadarkness (Dec 7, 2020)

Thanks! I assume that testing the switch is easy as well. It seems as though the switches do fail. Many Chevelle users have reported that replacing it cured their issues. My trans was just rebuilt, but I'm replacing the pan, so I'll check the switch and solenoid before hand.


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## 1967pontiac400 (Dec 10, 2020)

To piggy back here, I'm having a similar issue. I'll mash the gas at lower speed, say 30mph and it just stays in 3. 

Can anyone advise on precisely how much travel the electric switch should have? When I got under the dash tonight I noticed that fully depressing the gas only actualed the electric switch about 50% of the avaliable travel, so I'm wondering if I need to re-do the initial adjustment?

Thanks!


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## O52 (Jan 27, 2019)

Procedure as shown. Its a ratcheting mechanism that self adjusts


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## GTOJUNIOR (Aug 7, 2011)

Here's a '66 SB proceedure is the same for '67 as shown above.


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## armyadarkness (Dec 7, 2020)

I have to say, I spent several hours trying all of the diagnosing... days researching where to buy a new switch and solenoid... and in the end, all I did was push the switch in toward the firewall (to reset it), then push the gas to the floor all of the way, and now the car kicks down and roasts the tires at 20 mph. Prior, all it did was rattle and spit gas.


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## Muttley (Jan 22, 2021)

Did you ever find a good affordable source for the switch ? My car may need one. Also, does anyone know where to get the plug that attaches to the side of the trans? I touched mine the other day and it just fell off in my hand ... Or can I possibly re-attach it if it's still in decent shape ?


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## armyadarkness (Dec 7, 2020)

Muttley said:


> Did you ever find a good affordable source for the switch ? My car may need one. Also, does anyone know where to get the plug that attaches to the side of the trans? I touched mine the other day and it just fell off in my hand ... Or can I possibly re-attach it if it's still in decent shape ?


I did find many sources for the switch. It's readily available by simply "googling th400 kickdown switch". I didn't replace mine, however I will be removing and cleaning it with Deoxit, because it does seem intermittent. The connector on the trans is just a female spade with a boot, so you can easily crimp up your own, if you can't find one at the resto sites.. I'm sure you could also rig any boot to it.


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## armyadarkness (Dec 7, 2020)

1967pontiac400 said:


> To piggy back here, I'm having a similar issue. I'll mash the gas at lower speed, say 30mph and it just stays in 3.
> 
> Can anyone advise on precisely how much travel the electric switch should have? When I got under the dash tonight I noticed that fully depressing the gas only actualed the electric switch about 50% of the avaliable travel, so I'm wondering if I need to re-do the initial adjustment?
> 
> Thanks!


It seems that in later years, the switch was moved to the carb linkage on the Rochester... I found that Holley/ B&M make this... I'm LOVING THIS!!!!!






B&M 20297 Kickdown Switch Kit For TH400 Transmission


When swapping the TH400 transmission into any vehicle, this kit makes installation of the transmission's kickdown feature a snap. This switch mounts to either a TH700R4, TH200, TH200-R4 or TH350 throttle cable bracket.




www.holley.com


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## Muttley (Jan 22, 2021)

armyadarkness said:


> It seems that in later years, the switch was moved to the carb linkage on the Rochester... I found that Holley/ B&M make this... I'm LOVING THIS!!!!!
> 
> 
> 
> ...


I wonder ho hard it would be to adapt to a 66 tri-power setup? it would sure make wiring easier ...


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## armyadarkness (Dec 7, 2020)

Muttley said:


> I wonder ho hard it would be to adapt to a 66 tri-power setup? it would sure make wiring easier ...


In my opinion, working under the hood is better than working under the dash. However, nothing is easier than replacing the OEM switch, if possible. The grey area here is "which is better"... not easier. I want what's most reliable.


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## ylwgto (Oct 27, 2009)

Having a similar issue with almost immediate 1-2 shifts under normal driving (400/TH400) after changing carbs. Also getting some slight flaring in 2nd. 

I currently have the vacuum line to the tranny modulator going to one of the vacuum ports on the base of the carb (Holley 4160 750cfm). Should this line be coming from full manifold vacuum and not the carb base? 1-2 shifts used to be hard and chirp the tires, now it bogs down and feels like driving grandma's car.


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## deanhickey (Apr 6, 2015)

ylwgto said:


> Having a similar issue with almost immediate 1-2 shifts under normal driving (400/TH400) after changing carbs. Also getting some slight flaring in 2nd.
> 
> I currently have the vacuum line to the tranny modulator going to one of the vacuum ports on the base of the carb (Holley 4160 750cfm). Should this line be coming from full manifold vacuum and not the carb base? 1-2 shifts used to be hard and chirp the tires, now it bogs down and feels like driving grandma's car.


On my 1968 with a Quadrajet it is set up for ported vacuum from the carb, and works fine.


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## ylwgto (Oct 27, 2009)

deanhickey said:


> On my 1968 with a Quadrajet it is set up for ported vacuum from the carb, and works fine.


Thanks. Maybe I'll double check the hose or replace the modulator. Gotta get my crisp shifts back!


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## armyadarkness (Dec 7, 2020)

ylwgto said:


> Having a similar issue with almost immediate 1-2 shifts under normal driving (400/TH400) after changing carbs. Also getting some slight flaring in 2nd.
> 
> I currently have the vacuum line to the tranny modulator going to one of the vacuum ports on the base of the carb (Holley 4160 750cfm). Should this line be coming from full manifold vacuum and not the carb base? 1-2 shifts used to be hard and chirp the tires, now it bogs down and feels like driving grandma's car.


I'm trying to figure this one out, too. 
See this thread:








67 GTO Stalls at Idle While Braking


I finally got my car running like an animal. Low end, mid, and top are perfect, however the car jumps all over at idle and the minute that you put it in gear, it dies. Reverse seems worse than drive. The vacuum is great and I've checked all of the hoses for leaks. Fuel pressure is perfect, carb...




www.gtoforum.com


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## ylwgto (Oct 27, 2009)

armyadarkness said:


> I'm trying to figure this one out, too.
> See this thread:
> 
> 
> ...



Army, any luck on solving your issue?
I still have a car that thifts to 3rd almost immediately (under soft or part throttle when in D) and has weak shifts under full throttle. 
When I manually control the shifts with my ratchet shifter, they are a bit better but still not tire blipping like they used to be. 
The car will not automatically kick-down under any circumstances
frustrating!


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## armyadarkness (Dec 7, 2020)

ylwgto said:


> Army, any luck on solving your issue?
> I still have a car that thifts to 3rd almost immediately (under soft or part throttle when in D) and has weak shifts under full throttle.
> When I manually control the shifts with my ratchet shifter, they are a bit better but still not tire blipping like they used to be.
> The car will not automatically kick-down under any circumstances
> frustrating!


I did have success by resetting the electric kick down. I didnt need to adjust the modulator, but I think that I will try to. It shifts so early... Mine does have a shift kit in it and that helps a LOT! I think that I'll be replacing the torque converter soon.


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## armyadarkness (Dec 7, 2020)

NEWS NEWS NEWS!!!!!!

Okay... So, I pulled off ye old kick-down switch and I sprayed the bloop out of it with "Deoxit". I then worked it several times and maybe I saw an improvement... but maybe not. It still seemed inconsistent at best. So, I finally just bought a new damn switch.

It didn't adjust the same, so I wasn't optimistic, but 4 minutes later, HOLY COW! Now, whenever I floor it, it down shifts and is gone! I chirped 3rd at over 60mph, and now it's such a rocket that I have a bog/ flat condition... So I need to make sure that my rev limiter isn't too low and that I have enough jets!

If your down shift is erratic, just buy the damn switch! So far, it's the best $30 I ever spent on the car.


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## armyadarkness (Dec 7, 2020)

ylwgto said:


> Having a similar issue with almost immediate 1-2 shifts under normal driving (400/TH400) after changing carbs. Also getting some slight flaring in 2nd.
> 
> I currently have the vacuum line to the tranny modulator going to one of the vacuum ports on the base of the carb (Holley 4160 750cfm). Should this line be coming from full manifold vacuum and not the carb base? 1-2 shifts used to be hard and chirp the tires, now it bogs down and feels like driving grandma's car.


I run mine to full manifold.


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## ylwgto (Oct 27, 2009)

ok, cool. thanks. i'll just get a new switch too and see what happens!


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## 1967pontiac400 (Dec 10, 2020)

Nice! Mine arrived today, can't wait to see if I experience the same results! Right now, even after readjustment, there is no downshift ever in "d".


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## O52 (Jan 27, 2019)

If the car still doesn't downshift to 2nd after installation and adjustment of the switch, check for 12 volts at the switch and the transmission connector when the switch is activated. If you have voltage at the connector but still no downshift, the downshift solenoid inside the transmission may be bad or has a bad ground. They're not expensive and are easy to replace.


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## armyadarkness (Dec 7, 2020)

ylwgto said:


> ok, cool. thanks. i'll just get a new switch too and see what happens!


I'm a musician and I've been restoring electronics for several decades. Usually a switch can just be cleaned up, but I gues 55 years of wear, mixed with moisture, dirt, and fuzz were just too much. It's insane how much power this thing has now.

When driving normally, my car goes through 1st and 2nd within 20 feet... So if I ever wish to roll-on race, I really need the kickdown to work. It's the difference between a classic muscle car showing the world what it's made of... or throwing $20 in gas out the window and watching a Honda Accord beat me. Well worth $30

If you're reading this, I hope it helps you, too!


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## armyadarkness (Dec 7, 2020)

1967pontiac400 said:


> Nice! Mine arrived today, can't wait to see if I experience the same results! Right now, even after readjustment, there is no downshift ever in "d".


That was me until I adjusted it, then it was intermittent and in consistent. So I cleaned it and readjusted it a few times... Most of the time, when I floor it, it just goes brrrrrrrrrrrrrrr and I watch the gas gauge drop.

This time... I was out for my casual Sunday cruise, the traffic light changed, I took off from Pole Position nice and easy... Saw that the road ahead merged down to two lanes... and so OF COURSE!!!!!!! coming up fast and passing me on the right is Johnny Jack Ass... looking to cut me off. Well, I'd let anyone in front of me, but when you pull a move like that, you're just asking for it.

I didn't expect the kickdown to work, but halfway through the intersection, it dropped, both tires broke loose, smoke everywhere, and jack ass wont try that again. My girlfriend needed a new pair of pants afterwards, and it was all worth it.


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## armyadarkness (Dec 7, 2020)

O52 said:


> If the car still doesn't downshift to 2nd after installation and adjustment of the switch, check for 12 volts at the switch and the transmission connector when the switch is activated. If you have voltage at the connector but still no downshift, the downshift solenoid inside the transmission may be bad or has a bad ground. They're not expensive and are easy to replace.


Thanks Ed. I saw/ read all of the diagnostic procedures, and I have to say, I was never able to get the solenoid to click in the trans (that I could hear anyway).


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