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TH400 2nd to 3rd shift Question

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by KRAZIE87K5, Sep 14, 2001.

  1. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    My TH400 can't decide wether to stay in 3rd at highway speeds. I can be cruising along nicely at 65mph, and then all the sudden, the trans starts to "slide" back into 2nd gear. Nothing happens immediatly, but it slowly shifts to 2nd. As you can imagine - my RPM shoots up once I try to accelerate, and it will refuse to shift back to 3rd until around 4500RPM in extreme cases.

    Fluid levels are fine (not overfilled - but on the full-hot line), and vaccumm is present. Ideas?

    Thanks guys and gals!

    <font color=red>Just give me a tank, and point me in the right direction!</font color=red>
    <font color=blue>DONT MESS WITH AMERICA!</font color=blue>
    454/TH400/NP205 - 14BFF/D60/w/ 4.10s
     
  2. mnstr_fx

    mnstr_fx Registered Member

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    What gear ratio and tires are you running? Trying to figure out if you are back down on a shift point, leaking internal seal, short in kick down switch etc.... Also, is this a stock 400 or do you have some mod's done to it?




    76-82 Chevy Pickup / Blazer - K5-K10-K30
    90' TBI
    TH 400
    203 / 205 Doubler
    Dana 60, Corp 14 - 4.88'd - Detroit Locked
     
  3. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    It is a stock tranny. I have 35" BFG AT tires. Gear ratio in the rear axle (right now) is 3.73's.

    The wierd thing about it is that some days it drives great - like today. I made it all the way to work at 70mph, and no problems. It didn't slip once. Yesterday, it slipped so much, I used an extra 1/4 tank of gas on the way home from the exceedingly high RPMs I have to crank to keep my speed in traffic.

    I am going to drop it at a trans shop on Monday to see what they can tell me - that is unless the folks here at CK5.com get me the answer first!

    <font color=red>Just give me a tank, and point me in the right direction!</font color=red>
    <font color=blue>DONT MESS WITH AMERICA!</font color=blue>
    454/TH400/NP205 - 14BFF/D60/w/ 4.10s
     
  4. mnstr_fx

    mnstr_fx Registered Member

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    That puts you at a little less than 2300 RPM for ~60 MPH. That's high enough to stay away from the kick down stuff. There are two main things that I am thinking of. 1) The tranny is probably slipping internally. My old 400 had this annoying leak internally. This caused me to BURP it every morning. Once it burped, problem fixed. The problem was when it got cold, it no longer burped. SMOKED!! 2) The second problem may be in the kick down switch. TH400 use a kick down switch not a kick down lever. Maybe there is a short / ground in the switch. Long shot but maybe happening.

    Does there appear to be any slipping in any other gears? Slight tint to tranny oil?

    76-82 Chevy Pickup / Blazer - K5-K10-K30
    90' TBI
    TH 400
    203 / 205 Doubler
    Dana 60, Corp 14 - 4.88'd - Detroit Locked
     
  5. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    Absolutely no slipping in 1 and 2, but reverse also sometimes slips. The trans fluid seems to be OK to me, I did not notice any metalic tinge to the oil...

    I am not using the kickdown switch though... And I had bent the little electronic connection over to one side (for whatever reason). Could this happen with too much vaccumm? I wonder if I disconnect the vac. if the problem will go away???

    Still looking for more ideas... I will take a peek at the switch and at the fluids again today. Its about time to change the fluid anyhow. I put new fluids into it when I got it... but the pan was covered in some chunky stuff (nasty oil) before I purchased it. I cleaned the trans out and hoped for the best. It has really worked great until recently. Maybe taking it in for a tranny fluid flush would be a good idea! I will try that.

    <font color=red>Just give me a tank, and point me in the right direction!</font color=red>
    <font color=blue>DONT MESS WITH AMERICA!</font color=blue>
    454/TH400/NP205 - 14BFF/D60/w/ 4.10s
     

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