Kickdown cable?

[PLavelle]

I have a TH400 in my Vortech supercharged '67 Chevy C10. The truck pulls real hard when it is manually shifted, but if you just put it in drive and stomp on it, it shifts way too quick and doesn't feel at all the same. Is this a problem with the kickdown cable? Could it be anything else?

[JimKshortstep4x4]

The TH-400 does not use a "kick down cable" but instead uses an electric detent switch to down shift the transmission. The switch is located on the left side of the TH-400 and is activated by a carb mounted kickdown switch similiar to the one pictured. In 1971, the kickdown switch for the TH-400 was moved inside the cab, unlike the 70's back.

The upshift can be adjusted some by adjusting the vacuum modulator but the shift points should be checked prior to adjusting the modulator.

Jim

[oldiron]

PLavele, Hope you don't mind that I moved this but I think this will get more response in the pickup forum.
Oldiron suburban moderator

[PLavelle]

Thanks. That's where I meant to put it.

[PLavelle]

Thank you for the help, Jim.

[Rollie396]

I'm going with a t400 as well and I've been asking around about kickdown options too.I been hearing really good things about the kickdown unit that Lokar sells and I think that is the route I'm going.
Just another option for you.

[PLavelle]

Rollie, do you have a link to the kickdown unit?

[Rollie396]

here's a pic from lokar's site.http://lokar.com/interior_pages/prod...400kd_coil.jpg
Here is what Summit offers it for.It's a bit spendy but apparently it's worth the cash.http://store.summitracing.com/partde...889+4294904018
I've been told that Summit's price is hard to beat though I still have some homework to do and I haven't talked to Lokar yet.
P.S. the summit link includes a pdf file with install instructions from Lokar.Maybe it'll give you clearer idea of whether it's what you're looking for.
Hope it helps.

[69RAT]

I went the lokar route when I changed carbs, electric won't work with an edelbrock. Once you get it adjusted properly, (takes time) it's great. There's even holes that line up on the frame for the box so no drilling!

[Rollie396]

I didn't know that 69RAT.What's the reason?

[KFreddy]

Getting back to the original question on why the TH400 seems to shift early. The Th400 is factory calibrated to shift between approx 3000 and 4800 RPMs dependent on the original factory application/ rear end gear.

Three things determine shift points, the transmission vacuum modulator, the transmission governor and load/speed.

The vacuum modulator is located on the passenger side just next to the tranny. It has a set screw on it, this is good for minor shift point/ speed adjustments. High vacuum: soft and early shifts, lower vacuum firm and later shifts. I say minor adjustments because this is all in relation to available vacuum. Low throttle = lots of vacuum, high throttle = little or no vacuum, hence the tranny shifts at higher RPM when vacuum is low. You really can't adjust the vacuum when there is none. Ultimately the vacuum modulator is only good for adjusting part throttle upshifts, 200 to 400 RPMs at best, it will not affect WOT upshifts.

More importantly is the transmission governor, it overrides the vacuum modulator at about 3/4 to WOT (again because at high RPM there is little or no vacuum.) The governor is located at the rear of the main Trans case on passenger side on a Th400. The transmission governor is controlled by the detent switch that is located on the gas peddle of a Th400 equipped car or truck, unfortunately factory detent switches are non-adjustable. Lokar makes adjustable detent switches, also know as kick down kits; however this kit needs to be relocated from being gas pedal controlled to carburetor controlled. A better and more direct solution is that the transmission governor is adjustable. These adjustments can be made because the transmission governor is composed of weights and springs that are geared to the Trans output shaft. There are two outer (primary) and two inner (secondary) weights, and two springs. The inner, secondary, weights mainly affect full throttle 1-2 and 2-3 shifts. The outer, primary, weights mainly affect the part-throttle 1-2-3 shifts. If your shifts occur too soon you need lighter springs and/or smaller weights. If the shifts are too late, you need stiffer springs. B&M and TCI make governor recalibration kits. Transmission governor adjustments however will take a bit of trial and error to dial in.

(As a side note there are usually ink stamp numbers on transmission governors ranging from 11,13,15,17 etc., up to 23, 25 and 27. I do not know the correlation of these #s i.e. line pressure, spring tension, weights, rear gear ratio; but I do know that the higher the # the later automatic up-shifts will occur. A 25 or 27 stamp most likely had a 2.73 or 3.08 factory gear and an 11 or 13 stamp most likely came with a factory 4.56 or 4.11 gear.)

The third factor is load and speed and power, taking the above into account the valve body is internally calibrated to up-shift in a predetermined range based on a combination of engine load and speed. They are also designed to downshift when conditions require additional RPM to maintain vehicle speed or more commonly when we nail the fun pedal.

To gain total control of a Th400, installation of both a governor recalibration kit, a shift kit and some very light tinkering with the transmission vacuum modulator should do the trick.

[69RAT]

Rollie,
68-70 used a plunger actuated by the throttle linkage to activate the electric switch. The stock unit doesn't line up with an Edelbrock linkage. To my knowledge it only worked with the quadrajet linkages. 71-72 moved it to the firewall with the pedal linkage. I'd post a pic but I don't know which box the 3-4 of them I have are in.

[72402]

With a supercharged engine, you have pressure in the intake instead of vacuum. The modulator valve cannot function without vacuum. Sounds like it's working right.
Frank

[KFreddy]

Quote form Holly carburetor web site:

"There are Carburetors that are intended for use with superchargers, and have a unique capability that allows their power valves to reference the intake Manifold vacuum below the supercharger. This allows the power valve to operate as it should, based off intake Manifold vacuum. A power valve provides further enrichment to the main metering system under load (low vacuum) conditions. Without this external referencing, the power valve would be "reading" the Supercharger boost pressure, which has no bearing on the engine load."

Based on this information and if Lavelle is not using said carburetor his set up infact is not working correctly. Again whether or not his set up is working correctly it wold only affect the transmission vacuum modulator; not the transmission governor that truly affects the transmissions WOT shift points.

[Rollie396]

Sorry 69RAT I misunderstood your original post.I somehow thought you were saying that the Lokar kd wouldn't work with an edelbrock carb.....my bad.