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Old 01-01-2018, 08:51 AM   #1 (permalink)
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Th400 reverse issue

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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Old 01-01-2018, 10:15 AM   #2 (permalink)
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No help but let us know what you find, I'm running the same parts.
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Old 01-01-2018, 12:16 PM   #3 (permalink)
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Servo/band issue would be my most basic guess. Can you see the side of the case? Could have pushed the band anchor out of the case.
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Old 01-01-2018, 01:09 PM   #4 (permalink)
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Might have blown the servo seals or a fluid curcuit part in the valvebody.
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Old 01-01-2018, 01:34 PM   #5 (permalink)
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Servo/band issue would be my most basic guess. Can you see the side of the case? Could have pushed the band anchor out of the case.
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Might have blown the servo seals or a fluid curcuit part in the valvebody.
If I'm understanding it correctly doesn't compression braking in 1st use the reverse band? I don't believe the rebuild kit did anything with the servo nor did we check the pin length. I'll check the anchor pins tomorrow, the jeep is at a different location then I'm at today. I know you can pressure test the band but is there a way to check the servo or valve body other than visual? It would be ideal if I can fix this without dropping the trans.

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Old 01-01-2018, 04:08 PM   #6 (permalink)
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At the very least you are gonna have to pull the pan and VB.
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Old 01-01-2018, 05:29 PM   #7 (permalink)
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At the very least you are gonna have to pull the pan and VB.
I figured that much, I just can't wrap my head around what would first cause reverse to have a high engagement and then go out completely but not effect any of the forward gears. If reverse band or servo had an issue it shouldn't retain compression braking in 1st. If the direct drum or clutches had any issues I should have had problems with 3rd. Only thing I can think of is valve body but I can't see why that would go from late engagement to no gear at all.

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Old 01-01-2018, 06:03 PM   #8 (permalink)
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I don't have an answer for your problem, but where did you get a reverse manual valve body with engine braking?
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Old 01-01-2018, 06:29 PM   #9 (permalink)
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I don't have an answer for your problem, but where did you get a reverse manual valve body with engine braking?
Tci221201 but they don't make it at the moment, last time I spoke to tci they were working on a new model

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Old 01-02-2018, 08:42 AM   #10 (permalink)
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Ok thanks. I would really like a billet aluminum one, but thats even harder to find.
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Old 01-02-2018, 09:02 AM   #11 (permalink)
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Reverse band anchors haven't moved. Didn't get a chance to drop the pan yet

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Old 01-02-2018, 01:47 PM   #12 (permalink)
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Reverse band anchors haven't moved. Didn't get a chance to drop the pan yet

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Interested to see what you find. Maybe you grenaded the low roller clutch or the servo isn't applying enough pressure anymore.

I also find it interesting that TCI isn't selling that VB anymore. I wonder if they had issues with them.
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Old 01-09-2018, 11:16 AM   #13 (permalink)
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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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Old 01-10-2018, 09:41 AM   #14 (permalink)
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let us know what you find
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Old 01-10-2018, 06:01 PM   #15 (permalink)
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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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Old 01-10-2018, 06:02 PM   #16 (permalink)
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Any idea what caused the damage to the center support?

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Old 01-11-2018, 06:40 PM   #17 (permalink)
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Pic is from internet of what broke off. No damage to the id of the direct drum or the center support bushing

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