Whazziss?

[capturedlive]

I recently purchased a 1972 GMC 2500 Sierra from the original owner. It's got the 402 cu. in. engine with 400 turbo transmission, and it's in pretty great shape.

Here's my question: Under the dash, behind the heater hose, there's a button that pushes up. The former owner told me it does something to add oomph to the throttle and that it came stock with the truck. I later looked through the manual, did Google searches and even talked to a couple of mechanics, and I'm not able to find anything on that gizmo. Does this sound familiar to anyone?

Thanks ahead of time.

[capturedlive]

My new toy:

[1972RedNeck]

Does it have A/C? I think it is the switch to turn the A/C compressor on. I don't exactly know what your talking about. A picture would help a lot.

[prostreetC-10]

Well first of all welcome. Second is your a show off (Awesome looking truck lol). Lastly are you talking about the vent hoses? Can it be said turbo 400 kickdown? Can't remember what they look like or where they are located on these trucks even though I've had a couple with the th400.
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[too much stuff]

If you are looking at the switch above the gas pedal, that is the kickdown for a turbo 400. I think if you push the gas pedal down while you are looking at it it should make contact. 71-72 had this setup i believe.

Nice truck!

[leddzepp]

You are describing a kick down switch....yet the location you are describing is the A/C compressor switch. Kickdown switch should be on the gas pedal on a 71-72 truck.

[capturedlive]

Thank you all for the replies.

Well, I just looked at some kickdown switch pics on Google Images and, sure enough, that's what it is. I just gave a poor description of it, the vent hose partially obscures it and it feels like it's off to the right of the gas pedal a little. I do have A/C, but I don't recall him telling me to hit any buttons to get that going. It just gets cool whenever I turn it on. Sorry I didn't provide a pic myself, but it's hard for me to get down in there and get a decent shot.

I guess my next question would be, how do I use it? Do I engage it when the truck is moving, or do I have to push it when it's stationary? Does it disengage on its own when it reaches a certain speed? I just don't want to do something stupid and hurt the truck.

[too much stuff]

You shouldn't have to do anything except drive it. When you go to WOT (wide open throttle) it should down shift automatically (providing it is all hooked up and working)

[jim-bob]

Originally Posted by too much stuff

You shouldn't have to do anything except drive it. When you go to WOT (wide open throttle) it should down shift automatically (providing it is all hooked up and working)

..so is this like hitting passing gear to go around someone ??

[jerry moss]

Originally Posted by jim-bob

..so is this like hitting passing gear to go around someone ??

thats exactly what it is-kickdown switch. welcome from nor cal

[truckster]

That is your passing gear. The THM 350 had a mechanical kickdown cable and the 400 was electrically operated. They do the same thing.

[prostreetC-10]

correct

[ChevLoRay]

Earlier versions, like my '69, had the kick-down switch at the carburetor. When I got mine, the switch had been eliminated, due to a non-stock (AFB) carburetor installation. You'd think the P.O. would have been able to figure out how to mount it and also learn he still needed it, but then you wouldn't believe some of the other stuff that he did to it.....

[capturedlive]

Thanks again for all the valuable input. I haven't taken the truck out on the freeway since I brought it home, but next time I do, I'll have a go with the kickdown switch and see if it turns into the Batmobile.

On a related note, a mechanic suggested that I might want to replace the 400 with a 700(R4?) because I wasn't planning on using the truck as a hauler. Personally, I'm usually in the camp of "If it ain't broke, don't fix it," but I'm always up for illumination from the masters.

[red71cheyenne]

Good call on the "if it ain't broke, don't fix it". Drive it til the wheels fall off or the 400 gives up. The 400 used to be all there was for towing unless you had a manual transmission. I'm betting it will tow just about anything you want and do fine at it.

[too much stuff]

Originally Posted by capturedlive

Thanks again for all the valuable input. I haven't taken the truck out on the freeway since I brought it home, but next time I do, I'll have a go with the kickdown switch and see if it turns into the Batmobile.

On a related note, a mechanic suggested that I might want to replace the 400 with a 700(R4?) because I wasn't planning on using the truck as a hauler. Personally, I'm usually in the camp of "If it ain't broke, don't fix it," but I'm always up for illumination from the masters.

And i suppose your mechanic has a good deal on a trans also???
If the trans is working fine, i wouldn't change it..

[Boog]

The Turbo 400 trans is one of the best transmissions ever made. They will stay behind a big block for years, no problem.
Now the suggestion you might want to switch to an OD tranny would be for fuel mileage purposes. 700R4 installs are all over these forums if you're interested. If mpg isn't a great concern of yours just drive it and enjoy it.

[Sillyoldman]

Originally Posted by Boog

The Turbo 400 trans is one of the best transmissions ever made. They will stay behind a big block for years, no problem.
Now the suggestion you might want to switch to an OD tranny would be for fuel mileage purposes. 700R4 installs are all over these forums if you're interested. If mpg isn't a great concern of yours just drive it and enjoy it.

I tend to agree with you on that. My 401/400 combo are original to my '71. I had the tranny rebuilt in the late 90's. The motor has never been opened beyond valve covers.

[Longhorn Man]

if you have the original 3.07 gears, you'll be fine on the freeway.

[too much stuff]

Originally Posted by Longhorn Man

if you have the original 3.07 gears, you'll be fine on the freeway.

3/4 ton with big block and auto... Wouldn't that have 3:54 or maybe 4:10 gears?