TH400 issues

Matt69olds

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95% of the 400 won’t do a 3-1 downshift above 30ish. There are very few valve bodies with a Calibration that allow it. If yours does, and you don’t like it, start with unplugging the kickdown solenoid connector. If that solves the problem, try replacing the kickdown solenoid.
 

jfrancom101

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So update on my truck. I went out there today, I loosened the bleeders on the front brakes and there was still resistance but with a little bit of rocking a was able to get it to start moving, and after a bit it seemed to have almost no resistance. (Maybe as more fluid leaked out?) So I pulled it to where I can work on it, and out of curiosity I put it in reverse...and it works. I tried multiple times before moving up and down on the shift box thinking that maybe the stick was positioned well and nothing, but this time out of nowhere it's working.
I'm planning on pulling the plow of, seeing if I can figure out the braking issue and then doing the test you you said, but any ideas about what might be going on?
 

AuroraGirl

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So update on my truck. I went out there today, I loosened the bleeders on the front brakes and there was still resistance but with a little bit of rocking a was able to get it to start moving, and after a bit it seemed to have almost no resistance. (Maybe as more fluid leaked out?) So I pulled it to where I can work on it, and out of curiosity I put it in reverse...and it works. I tried multiple times before moving up and down on the shift box thinking that maybe the stick was positioned well and nothing, but this time out of nowhere it's working.
I'm planning on pulling the plow of, seeing if I can figure out the braking issue and then doing the test you you said, but any ideas about what might be going on?
id say your truck had sticking brakes and perhaps rusty or something.. or the trans slipped a lot trying but once free it wasnt bad. could be wear, fluid issue, something else.
 

jfrancom101

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id say your truck had sticking brakes and perhaps rusty or something.. or the trans slipped a lot trying but once free it wasnt bad. could be wear, fluid issue, something else.
Yeah I'm wondering about rusty brakes. I did replace the calipers, shoes, and master cylinder for the front and the back ones don't seem too bad because they still turn no problem.
I'll want to check the transmission fluid for sure, I saw where it leaked a little bit so I probably need to pull the pan and replace the gasket and filter I'm guessing?
 

AuroraGirl

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Yeah I'm wondering about rusty brakes. I did replace the calipers, shoes, and master cylinder for the front and the back ones don't seem too bad because they still turn no problem.
I'll want to check the transmission fluid for sure, I saw where it leaked a little bit so I probably need to pull the pan and replace the gasket and filter I'm guessing?
A leak could be many things but that is where I would start since you can see whats in the filter/is it seated(replace it ofc), what the fluid is like, fix anything immediate area like modulator, vacuum line to it from intake, etc.
 

jfrancom101

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So I'm changing the fluid and everything today. I found this sensor and the wire connected to it runs towards the torque converter and then is broken. I can't see where it should be going. I also found what looks like a vacuum sensor that doesn't have anything connected to it. Anyone know where they should be going?
 

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Bextreme04

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So I'm changing the fluid and everything today. I found this sensor and the wire connected to it runs towards the torque converter and then is broken. I can't see where it should be going. I also found what looks like a vacuum sensor that doesn't have anything connected to it. Anyone know where they should be going?
The vacuum can is your modulator. It should be a hardline to a vacuum source on the intake manifold between the carb and distributor. It controls how hard the shifts are and when they happen(within a window determined by the governor)

The plug with a wire goes to the trans kickdown switch. It should go up the drivers side of the bell housing and then up to the firewall and through and then connects to a switch on the accelerator pedal.
 

jfrancom101

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The vacuum can is your modulator. It should be a hardline to a vacuum source on the intake manifold between the carb and distributor. It controls how hard the shifts are and when they happen(within a window determined by the governor)

The plug with a wire goes to the trans kickdown switch. It should go up the drivers side of the bell housing and then up to the firewall and through and then connects to a switch on the accelerator pedal.
Sweet! thanks! do you by chance have a picture of what the vacuum port plugs into? I have an edelbrock carb.
 

jfrancom101

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Also other question, The fluid looks great except a small film of darker sludge at the bottom of the pan, is that ok Also while dropping the filter these two pieces fell down, one piece looks like part of an old dipstick, but any idea for the other? also the filter was kinda loose on the bolt, Should it be tight up closer?
Last thing, should the doughnut thing in the first picture have grease on it? or what is it? I can't tell if its just the sludge or or a place for grease intentionally there, but I kinda doubt it.
 

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Matt69olds

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The donut thing is a magnet. It collects the debris and keeps it out of the filter. There will always be some stuff stuck to the magnet, that’s normal thrust washer and clutch wear. The stuff that collects in the bottom of the pan is the non-magnetic stuff. You have typical wear, nothing to be concerned with.

If the modulator was disconnected, it’s going to shift very late and very hard. Look around the carb or manifold, I bet you find a vacuum port that’s open. If not, look at the tuneup sticker under the hood, it will probably show where the modulator is suppose to be connected. If not, any manifold vacuum source should work. Don’t use a length of vaccum hose, use some 3/16 or 1/4 brake line. Use rubber hose only on the connections. Vaccum hose can collapse when it gets hot, that will close off the vacuum to the modulator and cause weird shift problems.

As for the filter, the factory used a fiber filter media. I’m not sure what has happened over the last few years, but the fiber used in those filters is much more restrictive than it use to be. Ma y people complain of transmission whine, especially when cold. Use the filter with a brass screen, sometimes it’s listed as the Allison filter. While it doesn’t filter quite as well, the Allison filter will catch any debris big enough to cause problems.
 

jfrancom101

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The donut thing is a magnet. It collects the debris and keeps it out of the filter. There will always be some stuff stuck to the magnet, that’s normal thrust washer and clutch wear. The stuff that collects in the bottom of the pan is the non-magnetic stuff. You have typical wear, nothing to be concerned with.

If the modulator was disconnected, it’s going to shift very late and very hard. Look around the carb or manifold, I bet you find a vacuum port that’s open. If not, look at the tuneup sticker under the hood, it will probably show where the modulator is suppose to be connected. If not, any manifold vacuum source should work. Don’t use a length of vaccum hose, use some 3/16 or 1/4 brake line. Use rubber hose only on the connections. Vaccum hose can collapse when it gets hot, that will close off the vacuum to the modulator and cause weird shift problems.

As for the filter, the factory used a fiber filter media. I’m not sure what has happened over the last few years, but the fiber used in those filters is much more restrictive than it use to be. Ma y people complain of transmission whine, especially when cold. Use the filter with a brass screen, sometimes it’s listed as the Allison filter. While it doesn’t filter quite as well, the Allison filter will catch any debris big enough to cause problems.
Perfect Thanks for the info! I guess the modulator is the only more urgent one, I think the kickdown isn't as urgent because I never really need to floor it.
Do I need to find where to put the thrust washer back?
And I have a port right into the manifold that i can use for the modular do that should be fairly easy
 

Bextreme04

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The filter you need is an ACDelco TF231. If you can see filter media in the small hole on the bottom, it is the wrong filter and can cause issues. When I changed mine out with the recommended one from the parts store with paper media, it would whine like crazy on a cold start because the filter was too restrictive.
 

jfrancom101

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The filter you need is an ACDelco TF231. If you can see filter media in the small hole on the bottom, it is the wrong filter and can cause issues. When I changed mine out with the recommended one from the parts store with paper media, it would whine like crazy on a cold start because the filter was too restrictive.
Awesome! That happens to be the filter I already got :D
 

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