Th400 Kickdown

[Jebadiah]

I'm having trouble figuring out what energizes the kickdown solenoid on these trucks. Mine is an 88 c3500 with a 454 and th400. I recently replaced the tbi system with a carburetor. Since I have done this swap, I have no passing gear. I figured it might be controlled by the tps sensor but I'm not sure of this. Is there a switch somewhere or will I have to install one? Or can I make the factory setup work?

 

[Erik the Awful]

I believe there should be a switch where the TBI throttle arm goes.

 

[Schurkey]

I believe there should be a switch where the TBI throttle arm goes.

Far as I know, it's controlled by the ECM via the sensor inputs--TPS, MAP, etc.

No computer, no kickdown.

Not impossible to rig-up a switch to do it the old way. Gonna be a pain it the ass 'cause I don't think they ever made a kickdown switch bracket by the gas pedal for GMT400. Remember, the trans kicks-down BEFORE you hit WFO.

This thread should probably be moved to the LSX + Carb Swaps sub-forum.

 

[any4xx]

I’m apt to be wrong here, but I don’t think the TH400s were ever fed a kick down signal by anything other than a 12v switch. Some vehicles had the switch above the throttle pedal and others had them near the bell crank on the carburetor/throttle body. I’ve wired both home-designed and manufacturer specific switch systems into various vehicles over the years. Easy to do Even if it once WAS fed by a computer.

 

[L31MaxExpress]

I’m apt to be wrong here, but I don’t think the TH400s were ever fed a kick down signal by anything other than a 12v switch. Some vehicles had the switch above the throttle pedal and others had them near the bell crank on the carburetor/throttle body. I’ve wired both home-designed and manufacturer specific switch systems into various vehicles over the years. Easy to do Even if it once WAS fed by a computer.

Relay off the ECM in a TBI. Used the TCC output pin to control the relay via MPH and TPS table.

I added the older switch on the 87 G20. It had the mounting points already on the pedal bracket. Slide a piece of rubber fuel line over the switch stem when you mount it. It will save a ton of frustration getting it to work correctly.

 

[Schurkey]

Slide a piece of rubber fuel line over the switch stem when you mount it. It will save a ton of frustration getting it to work correctly.

No.

The mistake folks make that leads 'em to poke some rubber hose over the switch stem is that the switch is self-adjustng AFTER you take the time to set the initial position of the switch. Ideally, this is done after you pull the switch apart to polish corrosion off the conductors, and clean-out all the hardened, decades-old grease and reapply fresh lube. (Plain white grease is fine.)

Once the switch has been over-extended, they add thickness to the stem so that it still works right--but the better, proper solution is to merely push the spring-tab, and return the switch to it's starting point. From there, you whack the throttle WFO, and it "ratchets" to the correct position.

Adding to the problem is that it's really common for the gas pedal "rod" that pulls on the throttle cable to get bent from overly-stiff throttle springs, or pushing the pedal beyond what it takes to fully-open the throttle. This results in less movement of the switch stem. So make sure the gas pedal rod is able to pull the throttle completely open without the pedal having to be forced hard into the carpet pad.

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The actual "ratchet" mechanism. It's POSSIBLE but unlikely that the ridges in the plastic would be worn by the points on the copper conductor that ride on the plastic shoe.

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[Jebadiah]

Thank you all for the information. If the kickdown is controlled via relay from the ecm, does anyone know which relay that would be, and couldn't I just energize the relay with a switch to operate it?

 

[L31MaxExpress]

Thank you all for the information. If the kickdown is controlled via relay from the ecm, does anyone know which relay that would be, and couldn't I just energize the relay with a switch to operate it?

Mine was mounted next to the fuel pump relay on the same bracket on the firewall of the van. Same exact weatherproof relay as the fuel pump relay as well.

All I did is use the switch to ground the relay instead of the ECM.

 

[Jebadiah]

Mine was mounted next to the fuel pump relay on the same bracket on the firewall of the van. Same exact weatherproof relay as the fuel pump relay as well.

All I did is use the switch to ground the relay instead of the ECM.

Perfect thank you. That is what I will do as mine has that extra relay.

 

[Jebadiah]

Mine was mounted next to the fuel pump relay on the same bracket on the firewall of the van. Same exact weatherproof relay as the fuel pump relay as well.

All I did is use the switch to ground the relay instead of the ECM.

Which wire did you ground on the relay? I can't seem to find a diagram for it. TIA.