Technical Th400 shift problems

Adding some info that might be relevant to the misfire idea.....there is about a tablespoon of oil sitting on the driver side of my intake manifold if oil can get out air can get in correct? Does anyone know how this can happen? Also I had no idea that you don't use the rubber block web gaskets at the front and rear of the intake Edelbrock says just to use rtv so this revelation could show a vacuum leak... I'm pulling it and going to buy another vacuum gauge I'll report back guys thanks again maybe this will be a simpler fix than i had thought

 

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Awesome information thank you so much for the tip I'm going to replace that shifter cable it's cheap insurance and it has been on the car for a few years now

 

The intake will not leak vacuum if the front/rear gaskets are bad but it's a good idea to replace the intake gaskets and use red RTV to stop your oil leak. Also, coat your intake bolts with RTV to stop those pesky intake oil leaks.
The fact your engine shudders a bit tells me you have an intermittent modulator or modulator vacuum line leak. I'd replace that modulator even tho it tested ok....intermittent is the operative word here.

 

Oil up on the intake could be valve cover or oil wicking up thru a manifold bolt. Oil leak at end seal would not cause a vacuum leak... Only oil. And oil flows downhill. If those were leaking the block right below them would be wet with oil. Clean the oil off of the intake and keep an eye on it. Maybe check the intake bolt torque.

 

Update so I replaced the modulator and the car shifted into drive at 4300 rpm but when I went to brake the car was idling at 1700 rpm and would cruise without gas applied at 45-50 mph the engine was over riding the brakes and when I killed it it ran on for a few seconds trans reached 200 degrees I feel like it's majorly messed up now and I'm not sure what else to do!

 

Gentleman I believe the issue is solved I replaced the modulator and the problem presisted however the reason for the insanely high idle was because a 3/8 piece of air cleaner gasket finally made it's way into the secondary throttle plate causing it to idle high I removed it and cleaned up the carb test drove it once the idle equaled out and it shifts perfectly but it shifts a little early between 1st and second how do I turn the canister to get a longer shift between 1st and 2nd? So maybe it was the modulator or maybe a vacuum leak at the carb butterfly but regardless it's solved now thanks to everyone who took time to help me out!

 

What rpm and mph does it shift 1-2 at?
Many modulators are adjustable via small Allen wrench thru vac nipple. They also have different parameters for different applications. Low vacuume (high load an open throttle) increases shift points, high vacuume (low load min cruise throttle) decreases shift points.

Stick your foot into it and it should hold that 1-2 shift. Hook up the down shift switch and you'll love it

 

X 2 ....

 

It's getting the 1-2 shift just shy of 2000 rpm and around 25-30 mph weather I am WOT or not, before this problem it would hold that 1-2 shift until about 3600 and would shift and I'll test to the down shift switch and hook it up thanks for the advice I've heard some say leave it off some say plug it up and I've never tried it so what the hell I'll hook it up thanks again I'll keep y'all posted

 

Which direction should I turn it to give me more time in 1st before the shift?

 

Chevy guy's car had a 326, ST300..No Powerglide..in which case , it had an electric kick down. The switch should be on the gas pedal.

 

Yes. Different springs are part of the shift calibration. Never saw different weights though.

 

I saw he said 2 spd trans assumed PG. I ran a ST300 behind the 396 in a 65 Chevelle... Worked great!

 

Well...... Sounds like your on the right track as far as the diagnoses.......... Nice to see someone who continues to diagnose, and reply with findings/feedback for all to share/learn.

 

Good transmission. Stronger than a PG. Switch pitch converter available in B-O-C cars . Not in Pontiacs.
It was the first multi-pattern GM trans. It was used in 64-66 Tempests behind the 215 six, which was actually a small piston 230 Chevy engine.

 

I actually still have my st300 trans with original TC on it some day I'll rebuild it but it was working great when I made the swap only downside is I can't swap back and forth due to the case legnth difference so I had to have a new drive line fabed up. I adjusted my th400 at the canister but it's still shifting 1-2 a little early and soft at part throttle but at WOT it will shift around 3200 rpm and it's livable I'm going to do a little more fine tuning before I get on the road this evening and I'll be looking at the kickdown plug and weather it has 12v to it.. thanks again everyone!!!

 

All the way to 75 in 1st lol it was my first old school ride and it sure was odd to drive a 2 speed but on the same token it was reliable as the sun coming up

 

Unless there's a shift kit in it, (there's different levels too)
It should shift easy and early at cruise power.
It's not going to shift like a 4 speed , it might get both shifts and you not even feel them if your not paying attention.
With a shift kit and turned way up you can get the thing to bark the tires and kick you square in the ass without even trying every time it shifts. It's fun for a while but it will get old eventually if you drive it everyday. Plus It's hard on parts.

With the electric kick down hooked up it will be a different car at WOT.
Research it. Sure they work without it, but not like they are supposed to. Guys who say you don't need it are the same type that by-pass a NSS or do some other stupid shit too.

Jeff Smith: The TH400 transmission is unlike all the other typical three-speed automatic transmissions from the late 1960s and in through the ‘70s and ‘80s. Most of the other transmissions used a mechanical linkage or cable to indicate wide-open-throttle (WOT). This WOT trigger achieves two things: it ensures the transmission line pressure is at maximum for strongest application of the clutches and then downshifts the transmission to the next lowest gear, depending upon vehicle speed. Above a certain speed, as determined by the governor, the transmission will not downshift from third to second or second to first gear. This is to protect the transmission from damage. If you want to see what this input is worth, attach a pressure gauge to the transmission (it’s an 1/8-inch pipe thread plug just above and behind the shifter shaft).

 

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That's what concerned me was that it shifted pretty hard with the fti shiftkit (firm and positive)prior to having this issue with the modulator but now at partial throttle I can hardly notice the shifts lol but if this is better on the parts in my trans that is the most important thing to me. And thank you for the information on the kickdown I have to admit I was ignorant on how or why it works haha I'll be hooking this up in just a little while I dont see where i have the switch on my gas pedal so I'll try to find it and see if it's functioning and report back

 

Am I missing it? The switch isn't where I thought it was

 

That pedal doesn't resemble the two I have TH400's in, one pedal set up was out of a '70 Vette, the other is in my OT PU. I'll see if I can get a clear pic for you.

 

IIRC, earlier model TH400s had the kickdown switch mounted under the hood on the throttle linkage. Later models had a kickdown switch mounted under the dash, actuated by the pedal lever. Below is the under dash style.

 

Thank you and thanks claymart I'll look under the hood and see if I can't find it I have a loose hanging plug that went into the st300 as it was never hooked up so when I do find it it may need to be replaced seen some aftermarket ones for the pedal also...but my question is now if I don't have one or if mine is shot will it hurt anything to drive 800 miles without it or should I order one up and wait? Thanks!

 

It won't hurt anything, you'll just have to manually drop it in to a lower gear if you need it. Back in a bit.

 

Transmissions definitely aren't my thing, but have you checked with FTI? Maybe its a common problem they know about?

 

Top pic is from an '82 Chevy PU, bottom came from a '70 Vette.

 

Thanks for the photos it looks like one could be installed on my assembly fairly easy

 

I think easiest thing is to just hit a wrecking yard, probably save a bundle over buying something new.

 

No harm in driving it without the switch. All you'll be missing is "passing gear" downshift at wide open throttle only. You should still have a part-throttle downshift under normal driving conditions as long as the throttle pressure and governor pressure are within specs. What kind of vehicle are we dealing with here anyways?

 

Okay that's good to know and in Odessa there is a nice old car junk yard I'll pick a used one up at thanks for the tip and the car is a 65' tempest....I'm really glad you clarified that about only loosing passing gear at WOT because I couldn't figure out why the trans would down shift just driving around town(part throttle) I was under the impression the kick down switch would always be working so thank you for elaborating on that!