TH400 Kickdown operation

[ranger danger]

I see that the trans has a vacuum connect and a power wire from a key-on source. The connection on the trans has 2 wire connections. I read that the vertical one is for the kick down operation. Whats the horizontal one for? How does the trans know when to down shift? Is there another switch some place?
My trans wont kick down for passing etc. and shifts up very early until its in 3rd. I checked that the vacuum is connected (it is) and then checked for power at the vertical trans terminal with the key on. No power. My first correction to try to fix this will be to run a new key-on power wire to the vertical terminal on the trans. If this solves the problem then no issues. If it does not fix the problem, where do I go next?

[chevy_mike]

You need a kickdown switch, which could be either at the carb or in the cab where the gas pedal would hit. Earlier trucks used the switch at the carb but these are really hard to find the switch and correct bracket. Later trucks used a switch mounted up under the dash and the gas pedal arm would hit it. You can mount a manual switch if you want. Basically you give it 12 volt to down shift. I haven't seen the switch on the trans with 2 connectors before. Likely something from a really late model that tied into a computer or emissions stuff. All of mine have only had 1 connector.

[ranger danger]

Thanks Mike, Ill look to see what I have at the pedal. I know there is not one at the carb. I included some pics to reference the 2 prong plug.
Also, where does the switch at the pedal connect into the system? How does it operate?

[ranger danger]

If I have this right, the power runs from the fuse panel to the switch at the pedal then runs from the pedal to the trans vertical connector...right??

[ranger danger]

Ok so there is no switch at the pedal or carb. I guess that means I need one of these to get things working.

or

[john]

Here's a pic of the sw and bracket, and a link to a bracket.
http://www.rickscamaros.com/camaro-k...1967-1969.html
I posted wtb on the parts board and bought the switch from a member.

[ranger danger]

Originally Posted by john

Here's a pic of the sw and bracket, and a link to a bracket.
http://www.rickscamaros.com/camaro-k...1967-1969.html
I posted wtb on the parts board and bought the switch from a member.

Thanks for the help John! I'm assuming the bracket bolts to the carb mounting stud on the left rear. Then the throttle linkage contacts the switch when at or near WOT. There are 2 wire contacts on the switch. I assume one is hot from key-on source and the other goes to the trans. What does the other little part do? Where can I get the switch?

[ranger danger]

What my brain hears when people start talking about Automatic Transmissions! https://youtu.be/6kAeN5knZMQ

So my question is simple yet complex What does a kickdown switch actually do? Does it change the shift parameters so the trans will shift only at a higher RPM when energized? Does it force the trans to shift to the next lower gear until de-energized?

[clinebarger]

Originally Posted by ranger danger

What my brain hears when people start talking about Automatic Transmissions! https://youtu.be/6kAeN5knZMQ

So my question is simple yet complex What does a kickdown switch actually do? Does it change the shift parameters so the trans will shift only at a higher RPM when energized? Does it force the trans to shift to the next lower gear until de-energized?

That was a funny video!

When the Detent Solenoid is activated, It dumps line pressure off the Detent Valve allowing it to shift, Then Drive Oil (Forward Clutch Oil) is allowed into the Detent Oil passages (Becoming Detent Oil).
Detent Oil is sent to the following....

1. Modulator Passage
2. 1-2 Valve
3. 2-3 Valve
4. 3-2 Valve
5. 1-2 Accumulator Valve
6. Vacuum Modulator Valve

2-1 Detent....
Below 20 mph detent oil directed to the 1-2 valve exhaust port will close the 1-2 shift valve, shifting the transmission to First Gear.

3-2 Detent....
Detent oil at the 3-2 "Downshift" valve will close the 2-3 shift valve below approximately 70 mph shifting the transmission to Second Gear.

Oversimplified version....Without getting into Modulated Pressures & Governor Pressures & how they interact with Detent Oil.

[ranger danger]

Originally Posted by clinebarger

That was a funny video!

When the Detent Solenoid is activated, It dumps line pressure off the Detent Valve allowing it to shift, Then Drive Oil (Forward Clutch Oil) is allowed into the Detent Oil passages (Becoming Detent Oil).
Detent Oil is sent to the following....

1. Modulator Passage
2. 1-2 Valve
3. 2-3 Valve
4. 3-2 Valve
5. 1-2 Accumulator Valve
6. Vacuum Modulator Valve

2-1 Detent....
Below 20 mph detent oil directed to the 1-2 valve exhaust port will close the 1-2 shift valve, shifting the transmission to First Gear.

3-2 Detent....
Detent oil at the 3-2 "Downshift" valve will close the 2-3 shift valve below approximately 70 mph shifting the transmission to Second Gear.

Oversimplified version....Without getting into Modulated Pressures & Governor Pressures & how they interact with Detent Oil.

Ok so when activated, it reprograms the shift point to a higher RPM for each gear, right?! If I wanted to, I could hook up a toggle switch to the trans and, when turned on, it would effectively reprogram the shift points?

[Captainfab]

A little more simplified......when the kickdown is energized, the trans shifts down to the next lower gear to aid in acceleration. This typically happens when the when the gas pedal moves to WOT.

[cadillac_al]

I would look for a gas pedal from a th400 truck first. 2nd choice I would look for the carb mounted switch. It's just something extra to deal with when removing the carb. FWIW I have driven th400's a few years without a kickdown switch with no ill effect. I was driving so slow I didn't realize I had no kickdown.

[ranger danger]

Thanks everybody, I think I got a handle on it now!

[Boog]

You can drive it normally and it shifts automatically without that switch. When a down shift is needed for passing, etc. you can pull the shift lever down one notch to 2nd and accelerate as wanted then simply put it back in drive to resume cruise. Sort of a semi-automatic for those who like to shift.

[ranger danger]

Originally Posted by cadillac_al

I would look for a gas pedal from a th400 truck first. 2nd choice I would look for the carb mounted switch. It's just something extra to deal with when removing the carb. FWIW I have driven th400's a few years without a kickdown switch with no ill effect. I was driving so slow I didn't realize I had no kickdown.

I like the idea of a pedal meant for the purpose but what I'm finding is they are cable operated and mine is rod operated. I assume changing to cable is no big deal. I found several for sale complete.

Boog
Re: TH400 Kickdown operation
You can drive it normally and it shifts automatically without that switch. When a down shift is needed for passing, etc. you can pull the shift lever down one notch to 2nd and accelerate as wanted then simply put it back in drive to resume cruise. Sort of a semi-automatic for those who like to shift.

I drive truck for a living, I don't feel the need to shift any more than necessary!

[cadillac_al]

Dang throttle rods! I didn't catch the year of your truck. If it's an oldie then there may not be a gas pedal switch. I guess carb mount it is. Cadillac used to use a carb mount switch too (qjet) for a while in the 70's. I have never took one off to see how they mount but it could be a cheap option if you find one in a junkyard.

[ranger danger]

Originally Posted by cadillac_al

Dang throttle rods! I didn't catch the year of your truck. If it's an oldie then there may not be a gas pedal switch. I guess carb mount it is. Cadillac used to use a carb mount switch too (qjet) for a while in the 70's. I have never took one off to see how they mount but it could be a cheap option if you find one in a junkyard.

I'm wondering if a cable pedal assembly from a early 70's would bolt on to mine? I looked at pics and it appears to be the same 3 bolt pattern.

[Captainfab]

What year of truck are you working on? Cable throttle conversions have been done many times.

Originally Posted by ranger danger

I'm wondering if a cable pedal assembly from a early 70's would bolt on to mine? I looked at pics and it appears to be the same 3 bolt pattern.

[ranger danger]

Originally Posted by Captainfab

What year of truck are you working on? Cable throttle conversions have been done many times.

Its a 69' Suburban with a TH400 and a crate 454 running a edelbrock performer intake, qjet carb and MSD pro billit dizzy.

[crazy longhorn]

Originally Posted by Boog

You can drive it normally and it shifts automatically without that switch. When a down shift is needed for passing, etc. you can pull the shift lever down one notch to 2nd and accelerate as wanted then simply put it back in drive to resume cruise. Sort of a semi-automatic for those who like to shift.

I did the same deal on the 400 t that I ran...(4.10 gears/eaton unit)....I did not want a shift from 3 to 1 @ 30 mph! In a slight bit of curve, it would spin my truck in circles As far as the 3 to 2 downshift, you don't need the switch for that. I did not see any issues, set up that way either? I would invite Cline back in, for his expert thoughts on affect on line pressures, & or any detrimental troubles with the trans, set up without the elect kick down ? I trust your thoughts Cline...Longhorn

[Captainfab]

I was thinking that the '69's had a cable throttle. I guess maybe it wasn't until '71? I'm more familiar with the '60-'66's. It should be a fairly simple upgrade for your '69 Burb.

Originally Posted by ranger danger

Its a 69' Suburban with a TH400 and a crate 454 running a edelbrock performer intake, qjet carb and MSD pro billit dizzy.

[roger97338]

Originally Posted by Boog

You can drive it normally and it shifts automatically without that switch. When a down shift is needed for passing, etc. you can pull the shift lever down one notch to 2nd and accelerate as wanted then simply put it back in drive to resume cruise. Sort of a semi-automatic for those who like to shift.

Recently I was reading about how the TH-400 functions, in the 1974 Chevy Pickup factory service manual, and it turns out that manually downshifting and having the electric kickdown do the downshifting are actually different.

The difference is that if you manually downshift from drive to second, the front band is engaged, which provides engine braking. Using the electric kickdown doesn't engage that band, and does not provide engine braking. And when you're downshifting to pass, you definitely don't want engine braking.

That's how I understood it anyway.

Actually, here are the pages I'm talking about. Maybe I misinterpreted what I read.

[clinebarger]

Originally Posted by roger97338

Recently I was reading about how the TH-400 functions, in the 1974 Chevy Pickup factory service manual, and it turns out that manually downshifting and having the electric kickdown do the downshifting are actually different.

The difference is that if you manually downshift from drive to second, the front band is engaged, which provides engine braking. Using the electric kickdown doesn't engage that band, and does not provide engine braking. And when you're downshifting to pass, you definitely don't want engine braking.

That's how I understood it anyway.

Actually, here are the pages I'm talking about. Maybe I misinterpreted what I read.

If your accelerating while in D2 there is no engine braking, It's only when you let off the accelerator. No issues with kicking it down manually. But having a working detent system is the correct way.

[roger97338]

Originally Posted by clinebarger

If your accelerating while in D2 there is no engine braking, It's only when you let off the accelerator. No issues with kicking it down manually. But having a working detent system is the correct way.

Thank you very much for the clarification. I've read around here that you're the man when it comes to transmissions.

Did I miss something when I was reading about the TH-400 in the FSM? Did I misinterpret what I read? Or are you one of the people used as a reference when they write textbooks about transmissions? (I'm not being a smart-***. That knowledge started in people's heads before it was ever put to paper.)

[clinebarger]

Originally Posted by roger97338

Thank you very much for the clarification. I've read around here that you're the man when it comes to transmissions.

Did I miss something when I was reading about the TH-400 in the FSM? Did I misinterpret what I read? Or are you one of the people used as a reference when they write textbooks about transmissions? (I'm not being a smart-***. That knowledge started in people's heads before it was ever put to paper.)

You didn't miss anything, I think it mislead you on the Engine Braking part,
It states the Front Band applies....So the Sun Gear & Direct Drum cannot Overrun on the Intermediate Sprag or Roller Clutch (Depending on year).

If your accelerating in 2nd, The Sun Gear & Direct Drum are held stationary (By the Intermediate Sprag & Clutches)....Whether the Front Band is applied or not.

The engine braking only applies when you let off the throttle, Instead of the engine returning to idle (Direct Drum & Sun Gear Overrunning Clockwise), The Band holds the Direct Drum stationary basically locking the Sun Gear to the engine......Like a Manual Transmission.