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Old 02-10-2006, 02:43 PM   #1
dremu
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TH400 kickdown / detent cable

For those following my crewcab saga, I've been trying to tune the bigblock in my sixpack to actually pull, as she was bloody anemic, even for a sixpack which I recognize is kinda portly

One thing I noticed is that she doesn't want to downshift, say, onto a highway onramp, when I whomp on the pedal.

Now that I have the timing dialled in and the plugs and wires set up, I figured I'd adjust the kickdown cable.

There isn't one.

Now, not having any other big blocks nor gas TH400's to compare to (my only other TH400 is behind a diesel), I donno if this is supposed to be that way ... but somehow, I think the PO got bored one day and did something stupid. (That or BADLY transplanted the tranny from some other truck, so this confirms my suspicion that the engine/tranny may not be original.)

So, three questions 'fore I toodle off to the boneyard tomorrow.

1. I assume it's supposed to have the kickdown cable, or whatever you wanna call it. Are there any 454's that didn't?

2. Where does the cable go on a TH400? It's a major PITA to get under the truck (short dually tires, 2WD) so any guidance would be appreciated!

I've done the cable on my TH350 when I swapped it, so I know (I think!) how to unhook it on both ends and so forth, once I know where it's supposed to go...

3. When does the kickdown cable do its thing, as opposed to the vacuum modulator? (Explosive X-21 Space Modulator!) I felt her downshift once or twice on a test drive, presumably based on the vacuum, but even that may need a bit of adjustment.

-- A
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Old 02-10-2006, 03:20 PM
Posted in reply to dremu's post "TH400 kickdown / detent cable"
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Russ
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I don't know what year they started using them, but my 88 crewcab has a downshift relay on the firewall. It will shift normally, but definitely will not downshift when you accelerate if that relay goes out.

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Old 02-10-2006, 03:27 PM
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400's don't have a cable. Here is a pic of the gas pedal bracket from an '86. You can see the switch that kicks down the transmission.

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Old 02-10-2006, 03:38 PM
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Yup. TH400 uses a kickdown switch. Some vehicles have it mounted next to the carb. Apparently trucks have it on the pedal bracket (thanks, divorced).
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Old 02-10-2006, 03:42 PM
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huh,


ive never used a kick down switch with any of my t-400's.

but I also run a rachet shifter and just pretty much shift as I drive, same with my t-350 I dont use the cable on them, just shift with the rachet shifter,

anybody else do it this way?
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Old 02-10-2006, 04:21 PM
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The th400 will kick down when the switch is depressed under the pedal, my 86 crew cab does this and every factory setup th400 I have seen has had this. Though that is not to say that all of them was setup like this but all the the ones I have seen do. There are two inputs for the trans for it to understand what the truck is doing. One is being that switch and the second is a vaccum line that goes to the modulator. They wont kick down very easily, usually you know when you are going to do it because the pedal is pressed damn near close to the floor. I have heard someone say though that 2nd to first 1st diwnshift is based on vaccum drop, but I dont know the truth to this. My crew cab is all stock and I have gotten used to the way the big block and 400 act together. Its kind of cool though because the thing has enough power to just push through the gears without really needing a downshift. Sooner or later its coming out though and getting a "freshen up" rebuild to allow for some desired power. Or you can just drop in a full manual valve body like I did in my blazer and then you will KNOW when you downshift, your face is on the windshield then.
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Old 02-10-2006, 05:14 PM
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Okay, thanks all ... saves me a trip to the Pick-my-nose. Now to see if that switch is there (any bets? I found bailing wire, honest to god, holding the parking brake cable in!) ... and maybe tweak the modulator a hair.

-- A
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Sales info, colors, options and more on my Brochures page

The redneck torque wrench: tighten until it breaks, then back off half a turn.
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Old 02-10-2006, 06:06 PM
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go full manual... then you can have any gear you want at any speed and you don't have to worry about kickdown cables etc.

j
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Old 02-10-2006, 07:04 PM
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BMM-20297

$43.95 at Summit Racing

(Just in case you can't find one at the junk yard)
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Old 02-10-2006, 09:40 PM
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Red face yup..no cables..its WIRED!

That switch activates a solenoid in the valve body to downshift the tranny..if the vacuum modulator is hooked up correctly,it will make it downshift into first gear when your in second, when you goose it too,even if the detent switch is disconnected..but it wont force a third to second gear downshift..you either need the detent switch hooked up to do that, or you can yank the shifter into second manually to get "passing gear"..

When I first got my 69 GTO,its TH400's detent switch must have broken..someone put a horn button on the dash,and wrote "PASS" on the button!.. ...first time I pushed it I almost rear ended the car in front of me!.. ..mmm OK..now I know what THAT switch does!..

I found the correct switch on an olds cutlass of the same vintage,and retired the "PASS" button!..I've seen push button starter switches cobbed onto a carb for a detent switch on a few junkers at the boneyards!. ...
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