TH400 shifting too early

Fightin Rooster

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Hello,

New 74 C20 to me 3 weeks ago. When in Drive, my TH400 shifts out of first at about the right time based on how heavy the foot is, goes thru second like nothing if at all, then right into third. This is a problem at 20-30 MPH, so I'll shift it into second until I'm ready for third. I'll often downshift to second as I'm approaching a stop so it goes thru 1st & 2nd when I get going. If I don't come to a complete stop, it seems to stay in 2nd. I feel like by 5 mph and below it should have dropped to 1st, so maybe another symptom.

My understanding is the kick-down on the TH400 is not like the TH350 with a cable attached to the throttle cable on the carb, but something in the cab on the gas pedal. Mentioning this truck doesn't have that or any kick-down I see.

I have another car with a TH350 that was doing the same thing and I found the vacuum line was not fully attached (I got it that way). I fastened it and it's been great ever since.

Based on that, I inspected the vacuum line and replaced the modulator. No change. The modulator I removed already looked new.

I'm pretty sure I checked for the presence of vacuum at the source, but I'll double check that.

Fluid is right level (checked with engine running after going thru the gears manually and driving 1 mile +) and bright clean red.

Any suggestions?
 

Frankenchevy

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Check the governor weights on the side of the trans. May be something awry there. Is the trans stock to the truck? If you have a th400 from a diesel truck, the gov weights will be all wrong. Make sure you don’t have any vacuum leaks or obstructions from the manifold vacuum source to the trans modulator.

Try capping the trans modulator and the vacuum source. The absence of vacuum should replicate WOT and therefore should shift late. If there’s no change, something is wrong in the vacuum circuit, modulator adjustment/function, etc if there is a WOT type shift, it’s the governor or something internal I’d suspect...

The lockdown circuit is electronic on a th400. It’s just a switch on the gas pedal that closes at WOT. It’s presence should not change upshift points. Only whether it’ll pull 2nd from 3rd above 30mph or so.
 

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Mine did something similar. My vacuum advance cannister on the distributor went bad. I had that line T'd to the trans.
 

Fightin Rooster

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Thanks Franken! I had no idea there was such a thing. Went to old youtube-U and there's a decent vid on the topic.

I'm going to double-check vacuum first as you suggest.

Like yours, mine is likely a bit of a frankentruck. Per seller, engine replaced in 2015 and trans rebuilt at the same time. Sticker from local shop and look of the motor indicate engine swap is true. I should decode engine serial # to get a better idea. Obviously not sure about the trans. Finding a serial # there may help for clues. Per build sheet sticker, it was born with a 350 & TH400. Sticker in the gas door says "diesel only" but I was guessing that the bed was swapped.

Thanks for the help with the kickdown explanation. That's what I thought but of course didn't know. I felt it was unrelated but thought it was worth mentioning. Glad I did because now I know more about it.

I checked page 1 of your build thread yesterday after your other advice. Looks cool. I laughed when you sprung for a crate motor but then wanted to throw the carpet kit into the washer. Seems like we have the same budget priorities. I'm gonna read it some more.

Thanks again!
 

Fightin Rooster

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Thanks square. Funny (in my mind) how the one affected the other. So your trans vacuum line is attached to a ported vacuum? Pretty sure the other car TH350 is connected to manifold vac and pretty sure the truck is too. Several things to double check.
 

Matt69olds

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You can raise part throttle shift points s couple mph by turning the adjustment screw inside the vacuum neck of the vacuum modulator. Unfortunately, it will also raise the 1-2 as well as the 2-3 shift. Maybe you can find a happy medium?


Some shift kits include a small spring that goes on the modulator valve behind the vacuum modulator. It’s purpose is to lower shift points. If the trans has ever been rebuilt it might have one installed. It’s easy to check.

You may need to play around with govenor weights and springs to get the shift points where you want. Lucky got you the governor is easy to get to on a 400. It’s behind a cover on the passenger side held in place with 4 small bolts. You will loose very little fluid, but the gasket may tear. Have a replacement on hand just in case.

The electric kickdown switch/solenoid is only used at WOT. Zero influence on part throttle shifts.
 

Fightin Rooster

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Thank you Matt. Sweet Olds? btw. Makes me think there's a damn cougar in the car ;)

I don't think a spring is present as none was observed when I changed the modulator.

I believe you gentlemen are on to something with the governor weights & springs. If I opened it and found something obvious like a spring missing, then I could fix it. If it was not obvious, then I'd be guessing. I think I will leave that to a trans mechanic.

I did discover something: The other car (305/TH350) trans vacuum is connected to ported vacuum. The truck (350/TH400) is connected to manifold vacuum. ( I said I would check for the presence of vacuum at the source and there is ... plenty). I only have the practical experience of knowing what is working, not the knowledge of what it should be, so I will connect to a ported and see if that makes a difference...and maybe read stuff to see what is correct.
 

Matt69olds

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Cougar? What am I missing?

The modulator needs to be on manifold vacuum. Another thing to look at is the line itself, make sure it’s not full of carbon or crimped/bent. You may have the correct amount of vacuum at the modulator, but if the line is restricted the actual vacuum the modulator “sees” might be slow to change. With the engine idling, if you unhook the modulator you should hear a definite change in the way the engine idles.

If the trans works fine as is, no need to pay a shop to fix this. Go to your nearest transmission shop and get a spare governor If the have a bucket full of them(most busy transmission shops will) look for one with light weights. Get a couple governor gaskets, just in case. Swap in the governor, then go for a drive. If the replacement governor shifts later, you know your on the right track.

TransGo, B&M, TCI, all sell governor calibration kits. They are easy to work with, but can be time consuming, depending on how picky you are with shift points. I’d suggest using a donor governor, that way if you get way out of the ballpark with changes you have a know good part to reinstall. Only change one weight/spring at a time. The weights control the shift points, the springs tend to control the “spread” between the shifts. The heavy outer weights control the WOT shifts, the inner weight influence the part throttle.

If your really cheap, you can get things pretty close without buying the calibration kit. The governor pivots are just 6 penny nails. Cut off the heads, grind your inner eights down a little, install new nails, and either crimp or bend the ends to keep the nail from backing out. Once again, use a different governor, that way you have a know good part.
 

Matt69olds

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Here is the instructions from the B&M calibration kit. As you can see, there are hundreds of different possible combinations so it will take a few tries to get it where you want it.

https://documents.holley.com/20248.pdf
 

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Check the governor weights on the side of the trans. May be something awry there. Is the trans stock to the truck? If you have a th400 from a diesel truck, the gov weights will be all wrong. Make sure you don’t have any vacuum leaks or obstructions from the manifold vacuum source to the trans modulator.

Try capping the trans modulator and the vacuum source. The absence of vacuum should replicate WOT and therefore should shift late. If there’s no change, something is wrong in the vacuum circuit, modulator adjustment/function, etc if there is a WOT type shift, it’s the governor or something internal I’d suspect...

The lockdown circuit is electronic on a th400. It’s just a switch on the gas pedal that closes at WOT. It’s presence should not change upshift points. Only whether it’ll pull 2nd from 3rd above 30mph or so.
I. Should have read everything before posting, my bad. I hope this is still relevant, but I'm pretty sure that a stuck kickdown switch will cause a late upshift. Check it with an ohmmeter make sure it opens and closes.

Sorry as I said should have read everything before posting. My comment is irrelavant in this situation.Even if it was shorted closed it would be late shifts not early..
 
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Matt69olds

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Your correct. It’s easy enough to check. The trans should downshift into second below 60ish when floored. The solenoid inside the trans can also stick, if that happens it won’t matter what the switch or wiring does.

In this case, I don’t think that’s his problem. He is complaining about a early shift.
 

Ricko1966

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Your correct. It’s easy enough to check. The trans should downshift into second below 60ish when floored. The solenoid inside the trans can also stick, if that happens it won’t matter what the switch or wiring does.

In this case, I don’t think that’s his problem. He is complaining about a early shift.

Yea like I said should have read a bunch more before hitting the reply button. I'm bad about that.
 

Fightin Rooster

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Matt, your car somehow reminded me of Talladega Nights Ricky-Bobby, therefore the cougar reference.

Thank you for the wealth of info and saving me $$$. I'm working on a few other things right now but will get back to this soon.
 

Matt69olds

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Matt, your car somehow reminded me of Talladega Nights Ricky-Bobby, therefore the cougar reference.

Thank you for the wealth of info and saving me $$$. I'm working on a few other things right now but will get back to this soon.


It’s been awhile since I last saw that movie. Will Ferrell characters dad drive a 69 Chevelle. That’s the only car I can think of from the movie.
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This is my usual smart ass reply to “what year Chevelle” question. I had the almost exact meme for my 87 442, except it said “call my Olds a Monte Carlo....”
 

Fightin Rooster

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That is hilarious! I was pretty sure I didn't have the right model car from the movie. There's a meme for everything and this one is spot on.
 

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