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Discussion Starter · #1 ·
I’ve been driving the car a good bit lately since I finally got it tuned right and I’ve noticed it does a weird shifting thing between 30-40mph. So I’m cruising with my foot on the pedal and if I let off a little it will down shift into 2nd and then when I easy back on the gas, just a bit, it will shift into 3rd. As you can imagine this is rather annoying during city driving. I’ve messed with the vacuum module on the trans and turned it all the way in which made the trans shift at higher rpm but still having the issues of up and down shifting around this speed.
Thanks for any help!
 

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1967 400/400
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Did I miss what trans you're running? I assume it's a TH400. Are you watching a tach when it down shifts? Or do you just feel it? I'm going though/ went through some shift issues, myself.
 

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When was the trans rebuilt last? When did you change the filter last? Do you check the fluid regularly? I would make sure that the fluid was full and that the filter/ pickup tube were in place.

Also, have you verified the kickdown switch operation? Maybe unplug it to take it out of the equation.
 

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Discussion Starter · #5 ·
Unsure when it was rebuilt or the filter changed. Got it about a year ago and just recently got it to be able to reliably drive. Checked the fluid and it was low so I add some and the problem still was there.

The car does have a few leaks here and there so I try and keep an eye on all fluid levels.

When I bout the car the lockdown switch was actually unplugged. Just got it hooked back up and it made a huge difference in the power feeling department
 

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Unsure when it was rebuilt or the filter changed. Got it about a year ago and just recently got it to be able to reliably drive. Checked the fluid and it was low so I add some and the problem still was there.

The car does have a few leaks here and there so I try and keep an eye on all fluid levels.

When I bout the car the lockdown switch was actually unplugged. Just got it hooked back up and it made a huge difference in the power feeling department
There are a lot of things to discuss, in your post.
  1. Running an auto trans low, to the point where it only picks up fluid intermittently, could cause a jerking feeling, which could seem like shifting.
  2. The fact that the kickdown switch was unplugged, seems suspect. Yes I'm sure it's helping, but if its operation is erratic, then maybe thats why it was unplugged. I would test it and the kickdown solenoid.
  3. As for leaks, I just put this on. It cost $130 and took an hour to install. I think it adds 2 quarts and it's aluminum, so it dramatically drops temps. More importantly, it's very thick, so if you use Ultra Grey when you install it, you'll cure all off your trans leaks, once and for all.
 

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Discussion Starter · #7 ·
Thanks for the info! The trans was a pint low according to the stick.

How do I check the downshift solenoid? I tested the wire with a test light and when I depress the peddle a good bit the light turns on.
 

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Thanks for the info! The trans was a pint low according to the stick.

How do I check the downshift solenoid? I tested the wire with a test light and when I depress the peddle a good bit the light turns on.
"They say" that if you activate the switch and listen at the pan, that you can hear the solenoid. Personally, I was never able to. But Im curious if in your case, the low fluid was causing a surge, which you interpreted as down shifting. In any event, since you don't know the history of the trans, I think it should be a serious priority for you to drop the pan and change the filter. If it's clogged, missing, has fallen down, etc, it could be a very costly repair. A TH400 may be bullet-proof when right, but they're very unforgiving when they're neglected.

Once that's done, you'll have very cheap peace of mind, and you'll eliminate the fluid level as a culprit. Use Ultra Grey and torq to spec and youll solve your leaks, too.

A shift modulator and solenoid are cheap and easy to swap, so you can always swap them too, but I'd do a search for testing procedures, and see if you have luck there.
 

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Discussion Starter · #10 ·
"They say" that if you activate the switch and listen at the pan, that you can hear the solenoid. Personally, I was never able to. But Im curious if in your case, the low fluid was causing a surge, which you interpreted as down shifting. In any event, since you don't know the history of the trans, I think it should be a serious priority for you to drop the pan and change the filter. If it's clogged, missing, has fallen down, etc, it could be a very costly repair. A TH400 may be bullet-proof when right, but they're very unforgiving when they're neglected.

Once that's done, you'll have very cheap peace of mind, and you'll eliminate the fluid level as a culprit. Use Ultra Grey and torq to spec and youll solve your leaks, too.

A shift modulator and solenoid are cheap and easy to swap, so you can always swap them too, but I'd do a search for testing procedures, and see if you have luck there.
Should I use the gasket that comes with most filters plus the ultra grey? Or just ultra grey
 

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Should I use the gasket that comes with most filters plus the ultra grey? Or just ultra grey
I've done both and have had 100% success, both ways. Clean both surfaces free of gasket material, wipe down with acetone, lacquer thinner, or whatever... apply evenly and torq to 11 foot lbs.

If you do splurge for the Summit pan, it comes with the Delco filter, extension tube, and gasket, plus it has a drain plug and will run cooler. With that pan, I did not use any gasket.
 

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Discussion Starter · #14 ·
Got the new filter and gasket in, still doing the weird shift thing. Tried to adjust the vac and if I turn it all the way in it will make a pop sound and keep turning. I adjusted it out half way, well after the popping sound, and it didn’t seem to change the shift point. I have a video I will upload and if you watch the oil pressure gauge you can see when I let off the throttle and get back on it right in 2 and shift to 3 instantly.
Shifting Video
 

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Sucks that it didn't fix your issue, but it's good insurance, cheap and quick, glad that's out of the way. Sounds like your modulator isn't doing its thing. I hate to tell you to buy one because it might not be the issue, but theyre cheap and easy to replace, too. I think $35

I'm no trans expert, but I do know what blows them up and a few other basics.
 

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Discussion Starter · #16 ·
Yeah I was thinking the same thing. At least my trans leak is fixed as well! I took some pics when I had the pan off because it had some writing on it that didn’t look factory. Also, I degreased the trans and found the red strip so I believe it to be the upgraded one.
 

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Discussion Starter · #18 ·
I went with the gasket since it came with the filter. Cant beat the price, $10 for both
Just order a new vacuum modulator. Found one for $15 on Amazon
 

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I suggest purchasing a new stock vacuum modulator. Verify the vacuum line to the modulator has no leaks, isn't shared with any other device, and is on a manifold vacuum source. If the stock one you purchase is adjustable (it may or may not be), do not adjust it at this time, just install it as is out of the box. Assure your trans fluid level is correct. Check it warm, car running.

It sounds to me (36 year master tech) like your vacuum modulator is malfunctioning. I'm not surprised by what you say of how you've been adjusting it.
The course of action I've laid out should be cheap and easy and give a baseline to diagnose what the issue is should a new mod not cure it as there's a host of other issues that could be causing your symptoms.
Vacuum modulators really rarely need adjusting unless you're operating your transmission well outside of typical parameters.
Turning an adjustable one's screw all the way in and all the way out will likely puncture the diaphragm.
 

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Discussion Starter · #20 ·
I suggest purchasing a new stock vacuum modulator. Verify the vacuum line to the modulator has no leaks, isn't shared with any other device, and is on a manifold vacuum source. If the stock one you purchase is adjustable (it may or may not be), do not adjust it at this time, just install it as is out of the box. Assure your trans fluid level is correct. Check it warm, car running.

It sounds to me (36 year master tech) like your vacuum modulator is malfunctioning. I'm not surprised by what you say of how you've been adjusting it.
The course of action I've laid out should be cheap and easy and give a baseline to diagnose what the issue is should a new mod not cure it as there's a host of other issues that could be causing your symptoms.
Vacuum modulators really rarely need adjusting unless you're operating your transmission well outside of typical parameters.
Turning an adjustable one's screw all the way in and all the way out will likely puncture the diaphragm.
Thanks for the advice. From what you mentioned I bet the diaphragm is busted just from the sound it makes. I have one coming in next week and will give an update. It is an adjustable one so I’m glad you said to not adjust it haha
 
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