Th400 kickdown issues

[1987_chevy_]

I have a freshly rebuilt 383stroker with a freshly rebuilt th400. I got the truck dyno tuned and when I got it back the kickdown worked fairly well! I'm a young kid so full throttle is a necessity! Now the tranny does not want to kickdown! Here's what I found

1st : the kickdown switch that sits on the carb is getting 12 volts of power when there is no throttle!

2nd : when I do wide open throttle the switch is only sending 2volts to the solenoid in the tranny!

Is 2 volts enough to power the solenoid? Could the switch be bad?? Any help would be awesome! If someone could check the voltages on there kickdown that would be awesome!!

 

[Honky Kong jr]

I have a freshly rebuilt 383stroker with a freshly rebuilt th400. I got the truck dyno tuned and when I got it back the kickdown worked fairly well! I'm a young kid so full throttle is a necessity! Now the tranny does not want to kickdown! Here's what I found

1st : the kickdown switch that sits on the carb is getting 12 volts of power when there is no throttle!

2nd : when I do wide open throttle the switch is only sending 2volts to the solenoid in the tranny!

Is 2 volts enough to power the solenoid? Could the switch be bad?? Any help would be awesome! If someone could check the voltages on there kickdown that would be awesome!!

I’d say that switch is bad. I used to put the kick down for my 400s on a push button on the shifter threw a relay.

 

[1987_chevy_]

I’d say that switch is bad. I used to put the kick down for my 400s on a push button on the shifter threw a relay.

That's what I was thinking, I was really hoping someone could measure voltage on their th400 to give me an exact answer

 

[Honky Kong jr]

That's what I was thinking, I was really hoping someone could measure voltage on their th400 to give me an exact answer

I pulsed the ground from my shifter to a relay and fed it 12volts it liked it

 

[RustyPile]

That's what I was thinking, I was really hoping someone could measure voltage on their th400 to give me an exact answer

Plain and simple, the switch is bad.. Change it..

 

[HotRodPC]

Wait, Wait Wait !!! What do you mean the switch that sits on the carb? Did you Jerry Rig the full throttle kickdown switch to work off the Carb under the hood?

I realize it's an 87, probably a half ton and didn't have the gas pedal switch. But, I do believe you still have the same gas pedal bracket to where you can add in a factory OEM Th400 switch installed, and then just run the orange wire to the plug on the Th400 and call it a day. You really don't want that kickdown switch on the carb do you? That's the old 60's to early 70's way of doing things. That also limits you to what aftermarket carbs you can use and ******** making sure it's getting hit at the right time to activate kickdown.

Take a look at your gas pedal bracket under the dash and see if you have provisions for the OEM switch.

 

[RustyPile]

Wait, Wait Wait !!! What do you mean the switch that sits on the carb? Did you Jerry Rig the full throttle kickdown switch to work off the Carb under the hood? .

Has to be a "jerry rig"... He's running this carburetor on a 383.. https://www.holley.com/products/fuel_systems/carburetors/quick_fuel/hot_rod_series/parts/HR-650

 

[GMH78]

Hi from Australia legends!
HotRodPC, I was researching some stuff about TH400’s just this evening and you seem to know your way around them better than most.
I’ve got a TH400 in my HZ Holden (not a square body I know, I signed up to the page just to ask this question).
It kicks down from top to second as long as not going faster than 50mph but will not kick down from 2nd to 1st regardless of how fast (or slow) I’m going.
Also. With a worked motor when should it shift up if I’m getting stuck right into it? I kinda feel like it doesn’t hold them long enough but it’s probably normal. Changes into 2nd at about 40 and into top at around 60.
Thanks man, I’d really appreciate your opinion.

 

[chengny]

I'm no A/T expert but just my thoughts:

I’ve got a TH400
It kicks down from top to second as long as not going faster than 50mph but will not kick down from 2nd to 1st regardless of how fast (or slow) I’m going.

Is your detent switch/solenoid working? A TH400 will do a 3-2 downshift even without an operational detent solenoid, but it won't drop down into 1st unless it (the solenoid) is pulling the valve open.


Also. With a worked motor when should it shift up if I’m getting stuck right into it? I kinda feel like it doesn’t hold them long enough but it’s probably normal. Changes into 2nd at about 40 and into top at around 60.

IDK, but a 1-2 upshift that doesn't happen until 40 MPH seems wicked high to me. I think my TH400's usually shift into 2nd at about 10 MPH - maybe 15 tops.

 

[74 Shortbed]

I think he's saying full throttle shifts??, playing with governor weights and springs will change your shift points at full throttle, adjust modulator valve for part throttle.

 

[chengny]

In 1987 the operation of the detent solenoid - in a TH400 - was controlled by the ECM via the "downshift control relay".

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I would imagine the OP has stripped all the ECM/downshift related wiring and has run a hot wire directly to the switch that is mounted on the carburetor - and connected the other side of the switch to the detent solenoid with the ORN 55 lead . If that hasn't been done, that might be part of the problem.

 

[74 Shortbed]

Didn't think about that ECM crap, yes, that could very well be the problem.

 

[HotRodPC]

There's only 2 things that determine shift points on a Th400 for normal driving. The governor and the vacuum modulator. Then the switch is only used for full throttle shifts and downshifts if and when needed.

This 40mph you're talking about, is that full throttle or normal driving? First, I'd get my normal driving, light and part throttle shift points where I wanted them by adjusting the vacuum modulator and or weights in the governor. Is it possible you have a governor for a diesel engine Th400? Diesel Governors are set up to shift at lower rpms. But, get the normal driving shift points where you want them first. The kickdown switch on a Th400 is for full thorttle only, it's not a way to tune your shift points like the detent cable is on a Th350, and one of the reasons I like a Th350 better. I can tune that shift exactly how I want it.

 

[GMH78]

Hi from Australia legends!
HotRodPC, I was researching some stuff about TH400’s just this evening and you seem to know your way around them better than most.
I’ve got a TH400 in my HZ Holden (not a square body I know, I signed up to the page just to ask this question).
It kicks down from top to second as long as not going faster than 50mph but will not kick down from 2nd to 1st regardless of how fast (or slow) I’m going.
Also. With a worked motor when should it shift up if I’m getting stuck right into it? I kinda feel like it doesn’t hold them long enough but it’s probably normal. Changes into 2nd at about
Thanks man, I’d appreciate your opinion and then I’ll let ya get back to Chev stuff.

 

[GMH78]

Thanks for the info boys.
Shifting under normal conditions is perfect. It’s only on WOT where I think it could hold gears a little longer.

It is a fully rebuilt box with a 2500 stall. I believe it was out of a Chev truck of some sort. It is apparently longer than a normal one.
Car is a 78 Holden but it does have the kickdown switch mounted on the throttle and it does have power.