TH400 Information Thread

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HotRodPC

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@NickTransmissions , Do you know where I can find these sprags? It's the rear drums with the planets in it. I was told there is a Ford Transmission that used the same size sprag, but I've never been able to find out which transmission model it was from and the 2 Ford Dealer didn't have a clue either. Long been discontinued by GM and must be rare since it was only used in 64 and 65 model Th400's. This is the drums I've had machined out to eliminate the thrust washer and use a torrington bearing. I don't build but my own transmissions anymore since I'm not set up to do them, but in the event I do build one for a rock crawler or off road truck, I'd like to use one of the 2 drums I have left.


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NickTransmissions

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@NickTransmissions , Do you know where I can find these sprags? It's the rear drums with the planets in it. I was told there is a Ford Transmission that used the same size sprag, but I've never been able to find out which transmission model it was from and the 2 Ford Dealer didn't have a clue either. Long been discontinued by GM and must be rare since it was only used in 64 and 65 model Th400's. This is the drums I've had machined out to eliminate the thrust washer and use a torrington bearing. I don't build but my own transmissions anymore since I'm not set up to do them, but in the event I do build one for a rock crawler or off road truck, I'd like to use one of the 2 drums I have left.


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That appears to be a Borg Warner 4R100 45-element heavy duty sprag.
 

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That appears to be a Borg Warner 4R100 45-element heavy duty sprag.
Yes, now that you mention it, I did have a guy tell me long ago, it appeared to him that it was a Ford 4R100 sprag. He's the one that actually told me, that a Ford sprag interchanges and I'd have better luck locating one for a Ford. Now if I can remember that again when I need it... Thanks
 

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Yes, now that you mention it, I did have a guy tell me long ago, it appeared to him that it was a Ford 4R100 sprag. He's the one that actually told me, that a Ford sprag interchanges and I'd have better luck locating one for a Ford. Now if I can remember that again when I need it... Thanks
You're welcome, man...Very interesting, will have to try that out at some point...
 

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I think I need to replace the speedometer driven gear. Is there any way of determining what size (amount of teeth) of speedometer drive gear came factory installed in my THM400? The ID tag says FP, and it's installed to a 454 in an '84 C20 truck. There is also a '40' stamped on the speedometer pinion housing. Does that mean it's 40 teeth, on the driven gear?
 

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I think I need to replace the speedometer driven gear. Is there any way of determining what size (amount of teeth) of speedometer drive gear came factory installed in my THM400? The ID tag says FP, and it's installed to a 454 in an '84 C20 truck. There is also a '40' stamped on the speedometer pinion housing. Does that mean it's 40 teeth, on the driven gear?
Usually you can order the gear by the color. Or count the teeth. Either way will work fine. If you can get it, I'd also go ahead and replace the driven gear. If the drive gear is damaged, you don't know for sure what that did to the driven gear, so be on the safe side, I'd replace both just because.
 

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Usually you can order the gear by the color. Or count the teeth. Either way will work fine. If you can get it, I'd also go ahead and replace the driven gear. If the drive gear is damaged, you don't know for sure what that did to the driven gear, so be on the safe side, I'd replace both just because.
Thank you. Is there any way to determine teeth count without taking it apart to look at the color?
 

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Thank you. Is there any way to determine teeth count without taking it apart to look at the color?
You can use the TCI gear calculator to get a good idea of what you need, drive and driven gear wise when it comes to tooth count. It will get you close but the only fool-proof way is to simply count the teeth on both gears...
 

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You can use the TCI gear calculator to get a good idea of what you need, drive and driven gear wise when it comes to tooth count. It will get you close but the only fool-proof way is to simply count the teeth on both gears...
Thanks. After over-thinking it, as I always do, I remembered I could just unscrew the speedometer cable, and look at the end of the driven gear. Looks yellow to me, huh?
 

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NickTransmissions

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Thanks. After over-thinking it, as I always do, I remembered I could just unscrew the speedometer cable, and look at the end of the driven gear. Looks yellow to me, huh?
Remove the housing retention bracket then speedo housing/driven gear from the case. Then count the teeth on the driven gear if there's enough of the teeth left to count. You should also be able to view the drive gear and count the drive gear teeth by rotating the driveshaft...
 

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Thanks. After over-thinking it, as I always do, I remembered I could just unscrew the speedometer cable, and look at the end of the driven gear. Looks yellow to me, huh?
Like Nick said, just pull the single bolt at the top of your picture and take the whole housing and gear out. It could be white and just discolored from the heat and grease exposure.
 

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Thanks for posting this thread and the associated links! I'm not inclined to work on transmissions much myself, but want to know what I can so I can have an intelligent conversation with the shop when I have them rebuilt.

Recently I started having issues with a TH-400 in my 78 Chevy C20. After sitting for an extended period it would act as if it was low on fluid, not engaging Drive or Reverse when selected. After what seemed like an eternity (probably 15 to 20 minutes) it would start to react and eventually behave normally. I moved it from the house I'm renovating to the house I'm living in (about 5 miles) and it sat while I replaced the rear springs and shocks (another extended time). While it was off the road, I replaced the transmission filter and installed a "deep" pan with a drain plug. The fluid that came out was not dark or have any burnt smell to it, so I thought that was a good sign. I used the chart in a factory service manual to get me started on the fluid replacement, and when I tried to get the truck to move it did a lot of the same thing. Eventually I added more fluid slowly and kept checking the dipstick until it started to move on its own -- I started to wonder if the dipstick is the wrong one for the truck, as I never saw the level show up on it. Took it out on the street and it seemed to run fine -- because of the broken rear spring I had never put it on the highway, so once I was assured it was OK I took it out and even at highway speeds it seemed OK.

Fast forward to just before I had my knee replacement. The truck was back at the house I'm renovating, and it started to act the same way again. Fluid level seems to be OK, but won't move and acts like it's low on fluid. I'm going to have to have someone work on it as I'm still not 100% from the replacement.

Two questions: 1, are there different dipstick lengths on TH-400s (I don't think this transmission is the original); and 2, what is the difference in depth between the "shallow" and "deep" factory pan? When I sat the replacement pan next to the pan off of the truck they appeared to be the same depth.

Because I'm an oddball my 1985 GMC project is going to have an Olds small-block in it with a switch-pitch TH-400, I bought that transmission and have it in storage along with a redesigned controller for the TC function. 25-plus years ago I scored a couple of finned cast aluminum pans for TH-400s, one of those will be added to the GMC transmission along with an external cooler and fan so I can tow without issues. Thanks again for the post!

Take Care,
KS in KCK
 

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Thanks for posting this thread and the associated links! I'm not inclined to work on transmissions much myself, but want to know what I can so I can have an intelligent conversation with the shop when I have them rebuilt.

Recently I started having issues with a TH-400 in my 78 Chevy C20. After sitting for an extended period it would act as if it was low on fluid, not engaging Drive or Reverse when selected. After what seemed like an eternity (probably 15 to 20 minutes) it would start to react and eventually behave normally. I moved it from the house I'm renovating to the house I'm living in (about 5 miles) and it sat while I replaced the rear springs and shocks (another extended time). While it was off the road, I replaced the transmission filter and installed a "deep" pan with a drain plug. The fluid that came out was not dark or have any burnt smell to it, so I thought that was a good sign. I used the chart in a factory service manual to get me started on the fluid replacement, and when I tried to get the truck to move it did a lot of the same thing. Eventually I added more fluid slowly and kept checking the dipstick until it started to move on its own -- I started to wonder if the dipstick is the wrong one for the truck, as I never saw the level show up on it. Took it out on the street and it seemed to run fine -- because of the broken rear spring I had never put it on the highway, so once I was assured it was OK I took it out and even at highway speeds it seemed OK.

Fast forward to just before I had my knee replacement. The truck was back at the house I'm renovating, and it started to act the same way again. Fluid level seems to be OK, but won't move and acts like it's low on fluid. I'm going to have to have someone work on it as I'm still not 100% from the replacement.

Two questions: 1, are there different dipstick lengths on TH-400s (I don't think this transmission is the original); and 2, what is the difference in depth between the "shallow" and "deep" factory pan? When I sat the replacement pan next to the pan off of the truck they appeared to be the same depth.

Because I'm an oddball my 1985 GMC project is going to have an Olds small-block in it with a switch-pitch TH-400, I bought that transmission and have it in storage along with a redesigned controller for the TC function. 25-plus years ago I scored a couple of finned cast aluminum pans for TH-400s, one of those will be added to the GMC transmission along with an external cooler and fan so I can tow without issues. Thanks again for the post!

Take Care,
KS in KCK
i would think to find the right dipstick you could stab it in and see where it sticks out the bottom and know based on that.. but id imagine different dipsticks were for different lenghts outside of the tranbs for placing the dipstick somewhere else. I cant see why they would literally have it be different in the trans? even a deeper pan you would want it in the same spot
 

NickTransmissions

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Thanks for posting this thread and the associated links! I'm not inclined to work on transmissions much myself, but want to know what I can so I can have an intelligent conversation with the shop when I have them rebuilt.

Recently I started having issues with a TH-400 in my 78 Chevy C20. After sitting for an extended period it would act as if it was low on fluid, not engaging Drive or Reverse when selected. After what seemed like an eternity (probably 15 to 20 minutes) it would start to react and eventually behave normally. I moved it from the house I'm renovating to the house I'm living in (about 5 miles) and it sat while I replaced the rear springs and shocks (another extended time). While it was off the road, I replaced the transmission filter and installed a "deep" pan with a drain plug. The fluid that came out was not dark or have any burnt smell to it, so I thought that was a good sign. I used the chart in a factory service manual to get me started on the fluid replacement, and when I tried to get the truck to move it did a lot of the same thing. Eventually I added more fluid slowly and kept checking the dipstick until it started to move on its own -- I started to wonder if the dipstick is the wrong one for the truck, as I never saw the level show up on it. Took it out on the street and it seemed to run fine -- because of the broken rear spring I had never put it on the highway, so once I was assured it was OK I took it out and even at highway speeds it seemed OK.

Fast forward to just before I had my knee replacement. The truck was back at the house I'm renovating, and it started to act the same way again. Fluid level seems to be OK, but won't move and acts like it's low on fluid. I'm going to have to have someone work on it as I'm still not 100% from the replacement.

Two questions: 1, are there different dipstick lengths on TH-400s (I don't think this transmission is the original); and 2, what is the difference in depth between the "shallow" and "deep" factory pan? When I sat the replacement pan next to the pan off of the truck they appeared to be the same depth.

Because I'm an oddball my 1985 GMC project is going to have an Olds small-block in it with a switch-pitch TH-400, I bought that transmission and have it in storage along with a redesigned controller for the TC function. 25-plus years ago I scored a couple of finned cast aluminum pans for TH-400s, one of those will be added to the GMC transmission along with an external cooler and fan so I can tow without issues. Thanks again for the post!

Take Care,
KS in KCK
You're welcome, Ken...Symptoms can be an indicator of worn applied elements, particularly the forward and direct clutch packs...There are two applied elements working when in reverse: The low reverse band and direct clutch. If the direct clutch is stressed, it won't grab until there's sufficient fluid pressure in the circuit to keep the pack clamped and holding. That's one possibility. The others include a weak pump, issues in the modulator system or valv body.

First thing I'd do is perform a line pressure test at start up to see what your pressure readings indicate upon first start up after sitting overnight or for a while. If line pressures are very low then you'd have to do some trouble shooting to determine why (most common reason is worn PR valve or worn crescent in the pump body or sticking/stuck modulator valve). If pressures are normal (55-70 PSI in P, N, Drive; 85-120 PSI in Reverse), I would then check your modulator and modulator valve as well as engine vacuum. Stick a vacuum gauge on the engine or on the hose leading to the modulator...In a stock application, the trans needs between 12-15hg to function normally so if the engine has a vacuum leak, that may explain your issue. IF vacuum is healthy, then the next step is to drop the pan, look at it's contents and see if anything concerning is present (dark, burnt clutch material, lots of bronze or metal shavings, etc) and go from there.

As far as pan capacities, see the first post in this thread - the capacity chart is the first or second chart shown.

There are most likely different lengths dipsticks across all of the TH400s that have been made since they went into such a diverse arrary of vehicles across multiple manufacturers but any dipstick from a C or K series truck should work for your application as they are pan-agnostic.
 

HotRodPC

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Thanks for posting this thread and the associated links! I'm not inclined to work on transmissions much myself, but want to know what I can so I can have an intelligent conversation with the shop when I have them rebuilt.

Recently I started having issues with a TH-400 in my 78 Chevy C20. After sitting for an extended period it would act as if it was low on fluid, not engaging Drive or Reverse when selected. After what seemed like an eternity (probably 15 to 20 minutes) it would start to react and eventually behave normally. I moved it from the house I'm renovating to the house I'm living in (about 5 miles) and it sat while I replaced the rear springs and shocks (another extended time). While it was off the road, I replaced the transmission filter and installed a "deep" pan with a drain plug. The fluid that came out was not dark or have any burnt smell to it, so I thought that was a good sign. I used the chart in a factory service manual to get me started on the fluid replacement, and when I tried to get the truck to move it did a lot of the same thing. Eventually I added more fluid slowly and kept checking the dipstick until it started to move on its own -- I started to wonder if the dipstick is the wrong one for the truck, as I never saw the level show up on it. Took it out on the street and it seemed to run fine -- because of the broken rear spring I had never put it on the highway, so once I was assured it was OK I took it out and even at highway speeds it seemed OK.

Fast forward to just before I had my knee replacement. The truck was back at the house I'm renovating, and it started to act the same way again. Fluid level seems to be OK, but won't move and acts like it's low on fluid. I'm going to have to have someone work on it as I'm still not 100% from the replacement.

Two questions: 1, are there different dipstick lengths on TH-400s (I don't think this transmission is the original); and 2, what is the difference in depth between the "shallow" and "deep" factory pan? When I sat the replacement pan next to the pan off of the truck they appeared to be the same depth.

Because I'm an oddball my 1985 GMC project is going to have an Olds small-block in it with a switch-pitch TH-400, I bought that transmission and have it in storage along with a redesigned controller for the TC function. 25-plus years ago I scored a couple of finned cast aluminum pans for TH-400s, one of those will be added to the GMC transmission along with an external cooler and fan so I can tow without issues. Thanks again for the post!

Take Care,
KS in KCK
Yes, there are for sure different dipsticks. Not sure how many, there are many though since they're intended for many different model cars, trucks, vans, RV's etc

There is a deep pan and a shallow pan, and the difference is very obvious. One is I'd say an inch deepl the other usually found on trucks is close to 3 inches deep. So if yours that you compared looked the same, then they are the same.

Awesome for you and the variable pitch Th400. I had a VP Th400 out of a 67 Olds that was behind an Olds 425. I wanted to use it too just for the cool factor of having 2 different stall speeds. I wanted to make a street/strip truck and use a toggle switch for close to stock stall, and hit the switch and go into about a 3000 stall. Well, as many awesome awesome projects and opportunities I've had with cool parts, that I had to sell to make ends meet, those parts got sold on eBay for about 5 times what I paid for them, not to mention all the other spare parts I had too. But yeah, get that done and let us know how it goes. I had trouble finding someone that build my converter the way I wanted it built and I knew it would be expensive if I did find someone, so that was part of the decision in selling the parts to someone who may have already had the converter they wanted.

What is sittting for an extended period? You know an older transmission that sits for several months or years will tend to harden the seals in the clutch drums and such and it won't move. After warming up a bit, you might get a little action out of it as the seals soften just a bit. If that's the case, I've had some real good luck in the past by adding a trans additive. I know I know, I too had additives to anything, engine, trans, anything, but I have to say, it's been more than once or twice where I've seen additives help an auto trans that sat for quite some time. The additives seem to soften the seals and make them a bit more supple and pliable to where the transmission will work again. I'll also add, if it does work out, don't have high expectation for a long lived transmission. But hopefully, it'll be patch job to get you by for a couple to a few months to when it's a better time to get a rebuild done. You can add 1 bottle and see what happens. If it's needs 1 more, go for it. If 2 bottles don't do it, then give it up, it's not going to work this time.
 

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