Th400 issues

brandon.

Well-Known Member
Joined
Jun 16, 2013
Location
Raleigh
Lost reverse this weekend in my th400. I run a tci full reverse manual valve body on a semi recent rebuilt th400 with a 34 element sprag and alto red clutches. Got it rebuilt and installed in September. On my test run I accidentally went from the top of 1st to reverse thanks to muscle memory from my np435. Trans seemed fine after that other than reverse didn't engage until 1000-1500 rpm and then it snapped in. All forward gears were 100% fine. Wheeled this way until this weekend at windrock.

It backed off the trailer fine then the next time I went to use reverse just nothing. No movement at all even bouncing off the rev limiter. My first thought was I had damaged the reverse band and it finally let go but I'm not so sure. Against all wisdom I still wheeled the weekend with no reverse. All forward gears seem to work perfectly, doesn't slip at all in 3rd and I still have compression braking in all gears. The only thing different about forward gears is a fluid whine in the trans when it gets over 3500rpm.

I plan to tear the trans apart as soon as I can but that may not be for a couple weeks due to temps and a sprained hand. Any idea what to look for? The compression braking in 1st confuses me, iirc that uses the reverse band.

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It's been a long minute since I've been in a TH400, but I thought I recall the low band controlled engine braking, too. First things first - what does your fluid look like? I'd assume good, since all forward gears and compression braking still seems to be working fine.

Beyond that, I'd drop the pan and inspect the reverse servo. Pop the cover off, take the servo out and look at it inside and out for any cracks. I'd also check your band adjustment at this point. My knee jerk reaction was that either the servo is cracked, or the tang on the band has been busted off. If that doesn't net you any real reason why reverse quit working, I'd sadly say it's time to pull the trans and start yanking all the guts out to see what's going on. My hope would be that the band went out of adjustment, but lets hope nothing happened with the reaction carrier drum.
 
BigTex383 said:
It's been a long minute since I've been in a TH400, but I thought I recall the low band controlled engine braking, too. First things first - what does your fluid look like? I'd assume good, since all forward gears and compression braking still seems to be working fine.

Beyond that, I'd drop the pan and inspect the reverse servo. Pop the cover off, take the servo out and look at it inside and out for any cracks. I'd also check your band adjustment at this point. My knee jerk reaction was that either the servo is cracked, or the tang on the band has been busted off. If that doesn't net you any real reason why reverse quit working, I'd sadly say it's time to pull the trans and start yanking all the guts out to see what's going on. My hope would be that the band went out of adjustment, but lets hope nothing happened with the reaction carrier drum.
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I haven't gotten a chance to drop the pan yet but I spoke to tci. According to them it sounds like the rear band or the servo. They said the rear band is not used in any of the forward gears at all and 1st uses the front band for compression braking. Seems to contradict most of the information online that is online

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I had the same problem with TCI with reverse manual valve body and trans brake. I called them and got a similar answer. I wish they would train their people better. I contacted a guy about building it for me and he told me how to fix it.

Ready for mind blowing answer?

With TCI Reverse is Neutral with transbrake engaged. Yeah? I didn't believe it either. I looked up some TCI literature and it's true. Because I used Reverse as reverse for so long it still slips some in Neutral, but it works now. I need to tighten the servo I think someone said.
 
GubNi said:
I had the same problem with TCI with reverse manual valve body and trans brake. I called them and got a similar answer. I wish they would train their people better. I contacted a guy about building it for me and he told me how to fix it.

Ready for mind blowing answer?

With TCI Reverse is Neutral with transbrake engaged. Yeah? I didn't believe it either. I looked up some TCI literature and it's true. Because I used Reverse as reverse for so long it still slips some in Neutral, but it works now. I need to tighten the servo I think someone said.
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What fixed the issue for you?

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brandon. said:
What fixed the issue for you?

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GubNi said:
Ready for mind blowing answer?

With TCI Reverse is Neutral with transbrake engaged. Yeah? I didn't believe it either. I looked up some TCI literature and it's true. Because I used Reverse as reverse for so long it still slips some in Neutral, but it works now. I need to tighten the servo I think someone said.
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So not repaired yet. Answer seems to be band/servo.
 
Looks like I'll be dropping the trans. Reverse servo seems fine and nothing out if the ordinary that I can see other than debris and lots of metal flakes on the valve body gasket. I'm assuming reverse band lining is gone
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Glitter! ...brings the suck when it's in your trans pan...Good luck, and hope no other parts got shredded. I had a 700r4 that the 4th gear drum and input shaft would not separate after the trans letting go. Granted, I was in the throttle (like wide the fuq open) and it let go fast...sucked for me because my ignition box shit the bed that day, I dropped in a backup dizzy that was stand along thinking "it would be fine..." even though it didn't have a rev limiter...so I ended up replacing a transmission, two bent pushrods, and a whole slew of valves. Oops... I think @Croatan_Kid remembers that day, too...lol.

Anyway, I don't know what rebuild kits cost these days, but it could be a fun winter project. I actually like building transmissions. I'd make sure you wash out that servo and look it over inside and out really well make sure its not cracked...but you're right -- it looks like the band is toast. Just make sure it didn't eat your reaction carrier drum.
 
BigTex383 said:
Glitter! ...brings the suck when it's in your trans pan...Good luck, and hope no other parts got shredded. I had a 700r4 that the 4th gear drum and input shaft would not separate after the trans letting go. Granted, I was in the throttle (like wide the fuq open) and it let go fast...sucked for me because my ignition box shit the bed that day, I dropped in a backup dizzy that was stand along thinking "it would be fine..." even though it didn't have a rev limiter...so I ended up replacing a transmission, two bent pushrods, and a whole slew of valves. Oops... I think @Croatan_Kid remembers that day, too...lol.

Anyway, I don't know what rebuild kits cost these days, but it could be a fun winter project. I actually like building transmissions. I'd make sure you wash out that servo and look it over inside and out really well make sure its not cracked...but you're right -- it looks like the band is toast. Just make sure it didn't eat your reaction carrier drum.
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Iirc I paid under $250 for the rebuild. Think I can get away with just replacing whatever is damaged or do I need to replace all the clutches due to contamination? Trans only had 4 runs on it since rebuild. I'm assuming my 1st limiter to reverse mistake tore apart the reverse band and it just took a bit to finally let go

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brandon. said:
Iirc I paid under $250 for the rebuild. Think I can get away with just replacing whatever is damaged or do I need to replace all the clutches due to contamination? Trans only had 4 runs on it since rebuild. I'm assuming my 1st limiter to reverse mistake tore apart the reverse band and it just took a bit to finally let go

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That's really your call with replacing only damaged parts. I'd inspect all the other clutches and soft parts very carefully. They could be rinsed with fresh trans fluid and possibly be okay, but there is the possibility of having some flakes embedded in the other clutches. If you source your own kit, they'll come with all the clutches anyway. I believe I have seen some kits that have clutches only with no replacement steels, but that's been a while. The valve body and case will need a good flush too to get all the clutch material and metal fragments out. Might even have to pull all the springs/pistons throughout the valve body to give it a good flush. Or, if you don't...Not that I'm recommending it, but I've done it this way -- replace the bad parts, put in fresh fluid, run it through it's paces, and do another fluid/filter swap. But that's potentially running a risk of leaving contamination in the trans. Don't forget about draining all the fluid from your torque converter as well.

Did a shop do the build for you for $250? If so, that's not a bad deal. A complete rebuild kit sometimes runs close to that.
 
BigTex383 said:
That's really your call with replacing only damaged parts. I'd inspect all the other clutches and soft parts very carefully. They could be rinsed with fresh trans fluid and possibly be okay, but there is the possibility of having some flakes embedded in the other clutches. If you source your own kit, they'll come with all the clutches anyway. I believe I have seen some kits that have clutches only with no replacement steels, but that's been a while. The valve body and case will need a good flush too to get all the clutch material and metal fragments out. Might even have to pull all the springs/pistons throughout the valve body to give it a good flush. Or, if you don't...Not that I'm recommending it, but I've done it this way -- replace the bad parts, put in fresh fluid, run it through it's paces, and do another fluid/filter swap. But that's potentially running a risk of leaving contamination in the trans. Don't forget about draining all the fluid from your torque converter as well.

Did a shop do the build for you for $250? If so, that's not a bad deal. A complete rebuild kit sometimes runs close to that.
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That was just the kit, I have a buddy who rebuilds trannys that helped me

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Figured it out, main issue and debris seems to be from the center support sealing rings. I'm still confused as to why I had no issues in 3rd. Reverse band looks good as do all the clutches other than direct drum. They seem to have material but are discolored. Last 2 pics are from the first direct drum clutch disc, the outside material looks like it did originally.
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Any idea what may have caused the damage to the center support?

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Is that just a chewed up seal on the center support, or is something cracked? Can't really tell due to the angle of the picture and some glare. If all your clutches and steels look good and aren't burnt, you can mic their thicknesses to make sure they're all in the same ballpark. Honestly, I'm thinking at this point you had some debris blocking some passages in your valve body. I'd carefully reassemble everything you've got after cleaning the parts in fresh fluid and check the gaps in the drums with a dial indicator. That's the only way to make for sure that you've got the correct clearances in your drums. You might be able to get away with feeler gauges in multiple spots around the diameter if you're careful. I don't recall all the specifications off hand, but I'm sure you have (or could source) a TH400 build book. I've got one in my basement somewhere, but I don't know when I'll have time to dig it up. After that, I'd inspect that band good and hard and make sure it's truly G2G, then clean the living crap out of the valve body. Might just have gotten lucky and gained some experience points on your own reassembly.

I'm thinking those center support seals were chewed from the beginning upon installation. All likelihood points to they got pinched/rolled on installation. Those things are a bitch to get in there right the first time without dorking them up.
 
One more note - make sure you inspect the ID of the direct drum from where those seals where chewed up. It needs to be smooth. Any nicks/burrs will cause you nightmares.
 
That's not the seal that's chewed up, it's the ridge on the center support that the seals ride in. One seal is just gone and the rear most is over the oil port. The direct drum looks fine, perfectly smooth and no marked inside it at all. Only thing I can think of is a pre-existing issue with the center support. With the center support as I pulled it there isn't any way any pressure could be made for reverse to work. One of my friends had a similar issue but it was just a nick in the seal not damage on the center support.

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brandon. said:
That's not the seal that's chewed up, it's the ridge on the center support that the seals ride in. One seal is just gone and the rear most is over the oil port. The direct drum looks fine, perfectly smooth and no marked inside it at all. Only thing I can think of is a pre-existing issue with the center support. With the center support as I pulled it there isn't any way any pressure could be made for reverse to work. One of my friends had a similar issue but it was just a nick in the seal not damage on the center support.

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There's your problem! Hate to play captain obvious there...but I'd bet a hefty bounty that this happened on installation. Spit on it some before you ram that thing home (that's what she said). In all seriousness, when you put new seals on the center support, use a dab of vasoline or some other petroleum-based lube when you put the direct drum back on. It'll break down in the transmission fluid and you wouldn't even know it was there. Works great for installing the check balls and keeps them in place.
 
BigTex383 said:
There's your problem! Hate to play captain obvious there...but I'd bet a hefty bounty that this happened on installation. Spit on it some before you ram that thing home (that's what she said). In all seriousness, when you put new seals on the center support, use a dab of vasoline or some other petroleum-based lube when you put the direct drum back on. It'll break down in the transmission fluid and you wouldn't even know it was there. Works great for installing the check balls and keeps them in place.
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Not because it was moved on installation, the groove it sat in is gone. Back half of it is broken off. The seal was still clipped even, if you look at the picture the damaged portion isn't the seal. It's the journal the seal rides in that sits below the outer edge of the seal. I'll get a better pic tomorrow but I circled on the internet pic what is broken off
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Above explains it all...lol. Damn good thing I didn't say I'd bet a paycheck...that might hurt. Could have been caused from a stress riser in the center support if the ridge didn't have much of a radius in the base of the groove when it was machined....surge in line pressure when you did the 1-R shift might have done it in, and it took some time to wear out. That's speculation, but I think it's plausible. Seems like you'll need a new center support?
 
BigTex383 said:
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Above explains it all...lol. Damn good thing I didn't say I'd bet a paycheck...that might hurt. Could have been caused from a stress riser in the center support if the ridge didn't have much of a radius in the base of the groove when it was machined....surge in line pressure when you did the 1-R shift might have done it in, and it took some time to wear out. That's speculation, but I think it's plausible. Seems like you'll need a new center support?
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Yeah, my buddy has 2 good core transmissions so I'll switch to the newer style that has a anti fretting ring. Time allowing we will finish tearing it apart this weekend and rebuild it

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