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How to Identify a Switch Pitch Tranny

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  • ja70cad
    This was posted on the MTS message board some time ago by Dave Brode. I don t mean to come through like a know it all, but here are some corrections to prior
    Message 1 of 1 , Jul 9 1:32 AM
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      This was posted on the MTS message board some time ago by Dave Brode.

      I don't mean to come through like a know it all, but here are some
      corrections to prior responses and some facts about the switch pitch.
      Fwiw - I was building and using these trannys 20years ago when most
      guys didn't have a clue about them; 1. - the electric hook up is not
      for the back up lights. 2. '66 Buicks with 401" and 425" engines had
      a round bellhousing pattern. The '67 Buicks and all the switch pitch
      equipped Olds' had the same pattern as the 472/500. I'm not sure
      about '67 and older Caddy bell pattern [anyone feel free to say if
      they are same/different].3. Two wires does not mean it is a switch
      pitch for sure.-- Here's how to id the switch pitch trans and
      convertors; 1. '67 or older date on tag [unless case has been
      swapped] 2. Must have two wires. However, some 70s era TH400s hada
      second wire that closed a elec circuit through a pressure sensing
      switch on the valve body to control emmisions control stuff. If it is
      still in the car and has two wires, pull the pan. If one wire goes to
      a solenoid on the back of the pump it is SP. If both wires go to
      gizmos on the valve body it is not. Note - The wire that all TH400s
      have is the detent wiring ["passing gear"]. You will see the solenoid
      for detent. It actually does not bolt to the valve body, but the case
      right beside the valve body. The bolts pass through the seperator
      plate. The "smog" switch is a screw thread deal, if my memory serves
      me correctly. 3. Out of car: Trans - The splines on the pump are 1/2
      as long as splines on "normal" pumps. The splines inside the
      convertor are also different length. Because of this, you can't feel
      splines inside a SP convertor like you can a normal convertor. Note -
      if your stall speed is apx 1000, you may indeed have a switch pitch.
      The 13" TH400 SP convertors stall at apx 1000 in low and 2500 in high
      [with good traction]. Fyi - you can use a 12" convertor from a '64-
      '67 Buick or Olds 2 speed fora higher stall speed. The Buick/Olds
      [but not Pontiac or chev] 2 speeds were also switch pitch. They stall
      apx 2000/3000 rpm. Note - although the convertor will swap, no other
      2 speed parts work on the TH400 SP. A quick check without pulling
      your pan would be to [using a fused circuit] put 12 volts on the
      terminal. See if the stall speed changes.Beware - if your trans is a
      smog control 2nd wire version, do not allow trans to reach high gear
      with 12 volts to the circuit. The pressure switch closes in high, and
      your 12 volts will see direct ground. Dave
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