First TH350 rebuild, what did I do wrong?

Discussion in 'Transmission & Driveline Topics' started by mtngun, Jul 26, 2019.

  1. mtngun

    mtngun Member

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    My first post, and this was for a 4x4 not a Z28 so remove if inappropriate, but I was drawn to this forum because of the high quality posts on trannies. My first rebuild was a failure and I hoping a 2nd set of eyes will notice whatever it is that I overlooked.

    1976 4x4 got an engine swap and transfer case swap and, even though the 20 year old storebought rebuilt TH350 was still running, I figured I might as well freshen it up while it was out. Never rebuilt an auto before but it's something I wanted to learn, plus I wanted to do some upgrades.

    I followed the Sessions and the Haynes books and online how-to's, including Jakes, as best I could.
    -- red alto clutches
    -- new seals
    -- new bushings
    -- new modulator
    -- new intermediate roller clutch
    -- hardened intermediate outer race
    -- 4 small case-saver inserts plus one large case-saver insert
    -- direct clutch piston machined to 0.695" thick to allow 5th set of plates. I just noticed that Sessions recommends 0.730" thick while Jake suggested 0.675". Don't remember why I chose 0.695" but probably to dial in clearance.
    -- dual feed direct clutch by omitting center seal from direct clutch drum, omitting middle pump stator sealing ring, and plugging the port in the front of the case.
    -- drilled spacer plate 0.125" at 2nd & 3rd feed holes as suggested in Jake's thread.
    -- during the rebuild, I had some uncertainties about the valve body gaskets included in the kit. They were unmarked, no instructions were included, and different than what had been in case when I opened it up (some kind of shift kit), but I verified that all the holes in the spacer plate lined up with holes in the gaskets. One gasket between case and plate, second gasket (with "Z" cutout) between plate and valve body.
    -- upon re-assembly I noted manual valve adjustment was slightly out of spec in "D" position, as per p.207 of Sessions book. I did not take a photo or write anything down, but seem to remember that the valve stuck out a little too much. I was reluctant to bend the S link to make the correction so I left it that way. UPDATE: Ron's book had a typo and my manual valve adjustment was actually fine, see post #2.
    -- old "RV" torque converter replaced with new, unmarked but presumably OEM-spec torque converter that came with the rebuilt tranny 20 years ago and had never been used.
    -- re-adjusted kickdown cable (had to fabricate a new kickdown cable bracket due to a manifold swap).

    After starting the new engine and bringing the tranny fluid up to level, it was immediately obvious that something was not right with the rebuilt tranny.
    -- no clunk when shifting into reverse.
    -- roughly 2000 rpm required to initiate movement when warm, more so when cold.
    -- 1-2 shift worked OK, but there was no 2-3 shift.
    -- several second delay when manually downshifting from 2 to 1.
    -- sometimes shifting to P would engage the parking pawl, but other times tranny would remain in reverse (I never adjusted the external shift linkage).

    Drove 5 miles that way, came home, looked things over, found modulator vacuum line had been knocked loose at manifold, hooked it back up and took another 5 mile test drive but still no 2-3 shift and generally no improvement.

    So pulled tranny and did autopsy.
    -- pan & filter clean but oil looked and smelled burnt.
    -- crud above direct clutch as shown in photo. Clutch material?
    [​IMG]
    -- low-reverse clutches toast after only 10 miles, as shown in photo.
    [​IMG]

    -- forward clutches toast after only 10 miles, as shown in photo.
    [​IMG]

    -- forward clutch assembly clearance was 0.146" with the burnt clutches. I don't recall the as-installed clearance but pretty sure it was not that sloppy.
    -- no other damage or assembly errors detected. Seals all intact. No missing or out-of-order pieces.
    -- governor seems good.
    -- the new modulator that came with the kit did not seem right. It holds vacuum but barely moves when vacuum is applied. Here's a comparison of depth of plunger inside modulator for 3 different modulators, "as-is":
    Kit New, no vac = 0.316" depth, 15 inch vac = 0.333"
    Old, no vac = 0.262" depth, 15 inch vac = 0.415"
    Slightly Used, no vac = 0.347" depth, 15 inch = 0.400" depth

    So the kit modulator may have been inoperative, but still, that would not explain the dramatic failures, would it?

    Gentlemen, what am I missing? I don't want to put the tranny back together until I've identified a smoking gun.
     
    Last edited: Jul 26, 2019
  2. mtngun

    mtngun Member

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    Update: revisiting Ron Sessions instructions for manual-valve adjustment, p.206-207.

    Ron's photo has an arrow pointing to the place on the manual valve that is supposed to be flush with the valve body when the valve is in "Drive." I had assumed Ron meant flush with a STEPPED SECTION on the manual valve. My manual valve was not flush with that stepped section, so I was worried about that.
    [​IMG]

    Pic of my manual valve in "D." Not flush with stepped section. (for this photo I slapped parts together with only a few bolts, that's why some bolts are missing).
    [​IMG]

    But the text on p.206 says "With the rooster comb in this [Drive] position, the REAR of the manual valve must be flush-to-0.030" in from the REAR edge of the valve body."

    When Ron says "rear," I think he is talking about the output end of the valve body, not the end with the S-clip ?????

    Here is the REAR (output end of the tranny) with the rooster comb in "Drive." The REAR end of my manual valve is flush with the REAR end of the valve body. I think that is what Ron meant and his photo mistakenly had the arrow pointing at the FRONT end, not the REAR end.
    [​IMG]

    So I now believe that my manual valve adjustment was perfect. Agree?

    So why did my rebuild fail ?
     
    Last edited: Jul 26, 2019
  3. 1972_Z-28

    1972_Z-28 Veteran Member Lifetime Gold Member

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    Welcome aboard. I know it not a Camaro but maybe someone will help you out. You certainly gave alot details
    I don't have any answers for just wanted to welcome you. Now go buy a 2nd gen Camaro HAHA.
    GL
     
  4. camaro71/holland

    camaro71/holland Veteran Member

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    FiFir of all, I feel for ya.... it sucks when you put so much time and energy to end up like this...

    It's been a while since I've slammed a 350 together, but let's see where we end up.

    You are correct on the manual valve > I've always checked the rear to be flush without a problem.

    The vacuum modulator isn't the problem: if it was, then you would have late and very hard upshifts without burning everything up or no 2-3 shifts.

    Right now I'm thinking in the direction of the pump. You've burnt up clutches that shouldn't 'go' together, especially not in such a short distance.... It looks like you didn't have enough line pressure from the pump (you mentioned high RPMs to even move the car) what caused the clutches to burn that fast (and forwards don't go that often anyway).

    What is the direction of the 'tangs' in the pump? Are they pointing up when the pump half is on the table?
     
  5. mtngun

    mtngun Member

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    Thanks for taking the time to read my post, Camaro71/holland.

    Tangs pointing toward the inside of the case, not toward the torque converter side.

    The converter was able to seat into the case completely with a "clunk" as it seated. When the case was bolted to the engine, there was a little bit of play between the converter and the flexplate, and I used two thin washers (total 0.090" - 0.100") between the converter and the flexplate to take up most, but not quite all of the slack.
    [​IMG]

    If I set the pump half on the converter it drops in until the converter rests against the tangs like so.
    [​IMG]

    I'm glad to take photos of any particular part upon request. No one is going to hurt my feelings with their suggestions. Obviously I screwed something up or else it would not have failed.

    It'll take me a week or two to order replacement parts. If I still haven't found a smoking gun by then I'll post step by step photos of the re-rebuild with the hope that someone will catch me if I do something wrong.
     
  6. Bandit723

    Bandit723 Veteran Member

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    well not a trans expert but when i had mine built it burnt the clutches on the dyno. builder did figure out there was a line pressure blockage.
     
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  7. mtngun

    mtngun Member

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    Bandit723, what caused the blockage? Just gunk?
     
  8. budro6968

    budro6968 Veteran Member

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    Did you do air checks with the pistons installed? A nick in a lip seal can spell early death.
     
  9. mtngun

    mtngun Member

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    Yes, I did air checks. The only anomaly -- and these days I struggle to remember details unless I wrote them down -- but I want to say that the forward / reverse check worked on forward but hissed on reverse. Again, my memory is not clear, and the air-check instructions that I was following (from the various books and tutorials) were not as clear as I would have liked them to be.

    When I do the re-rebuild I'll post step by step photos of things like that, and ask for clarification as needed.
     
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  10. budro6968

    budro6968 Veteran Member

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    Has been 30 years since I was in Any Auto trans. I went to school back in the early 80's to learn. I always liked working on Automatic trans. You took some great pics I like the detail. Look forward to more.
     

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