78 Chevy C20 TH400 Kickdown

[78 Chevyrado]

I don;t know why it was like this but when I got the 78 C20 it would never kickdown ever...

I tried to set it by the way the manuals say to do it.

Still didn't work.  So I did this mod to my switch under the dash.   The way to set it is to pull it one way then floor the gas and it will set that switch.  well on mine if you floored the gas it never even touched the switch.  no idea if that was from factory or grandad did something wrong but here was my solution.  wrap ever larger pieces of hose until it acts right.  took this much of layers of rubber hoses to make my truck act right.  It's been perfect ever since, 20 years and growing.

[JohnnyPopper]

Clever!

So it's electric? I thought that year range was still a mechanical cable.

[zieg85]

all TH400's where electrical kickdown.  Location of the switch on the CK's is on the go pedal. 

[JohnnyPopper]

Makes sense, I feel smarter now. Thanks!

[78 Chevyrado]

I've been looking through my old pics and such of the truck and things I did to make it work.  This is one of them.  This is likely from 2006 or 2007.

That truck never downshifted and irritated me greatly on the freeway.  after this fix, 20 years later still works like a charm.

[bd]

I've been looking through my old pics and such of the truck and things I did to make it work.  This is one of them.  This is likely from 2006 or 2007.

That truck never downshifted and irritated me greatly on the freeway.  after this fix, 20 years later still works like a charm.

Have you verified that the carburetor throttle plates open 100% whenever the accelerator pedal is depressed to the floor? 

In many instances in which the accelerator pedal lever arm didn't contact or sufficiently depress the detent switch, I found that the lever arm was distorted from its original configuration, preventing WOT.  Sometimes, the problem was introduced when a truck that was originally delivered from the factory with rubber floor mats was subsequently fitted with carpet and pad, effectively decreasing accelerator pedal clearance with the floorboard, hence pedal travel before contacting the floorboard.  The decreased pedal travel limited both throttle deflection and detent function.  In both scenarios, bending the arm into its original geometry restored both THM 400 detent and WOT.  It could be worth a look.

[78 Chevyrado]

I've been looking through my old pics and such of the truck and things I did to make it work.  This is one of them.  This is likely from 2006 or 2007.

That truck never downshifted and irritated me greatly on the freeway.  after this fix, 20 years later still works like a charm.

Have you verified that the carburetor throttle plates open 100% whenever the accelerator pedal is depressed to the floor? 

In many instances in which the accelerator pedal lever arm didn't contact or sufficiently depress the detent switch, I found that the lever arm was distorted from its original configuration, preventing WOT.  Sometimes, the problem was introduced when a truck that was originally delivered from the factory with rubber floor mats was subsequently fitted with carpet and pad, effectively decreasing accelerator pedal clearance with the floorboard, hence pedal travel before contacting the floorboard.  The decreased pedal travel limited both throttle deflection and detent function.  In both scenarios, bending the arm into its original geometry restored both THM 400 detent and WOT.  It could be worth a look.

Yes I had verified that previously.  There was house carpet in the truck and i had to cut out a section to let the gas pedal do further down to the floor.  even then, it still would never downshift.