TH400 kickdown cable connection (Buick to Pontiac)

Discussion in 'The "Juice Box"' started by Utah455, Mar 20, 2020.

  1. Utah455

    Utah455 Platinum Level Contributor

    So, I bought a new engine harness for my 70 GS 455. Question is, the electrical connection that connects down to the transmission for the TH400 kick down. The Buick harness had 2 pins in an L format, but my transmission only has one pin.

    Now let me say, my transmission (per the serial number) is from a 1973 Pontiac. Number is 2P3xxxxxx. 2 for Pontiac, P for assembly in Pontiac Michigan, and 3 for 1973. Then the six digit serial number from what it came from.

    Anyone have any information on connecting a 2pin Buick harness to a 1 pin Pontiac unit?

    Before frame off restoration, it never did down shift when I slammed the gas. I always had to manual drop it down a gear.

    Last edited: Mar 20, 2020
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The 2 prong connection was originally for the switch pitch, that was 65-67 Buick and some other makes. In later years, that second connection was part of the TCS (Transmission Controlled Spark emissions system). That second connection went to an oil pressure switch on the valve body. It provided a signal for the TCS system when the transmission up shifted to direct drive (3rd).


    The lower vertical spade is the kick down, the horizontal is for the SP, or TCS. If your car doesn't have either, than it doesn't make any difference. You should trace the second wire back, and see where it goes. You can cut them back and tape them off, or do nothing. Just plug it in. The 70GS455 did have a version of the TCS. Unless you are doing a concourse restoration, eliminate it, it's a performance and gas mileage killer.
    Last edited: Mar 21, 2020
  3. Utah455

    Utah455 Platinum Level Contributor

    Looks like my pictures never uploaded yesterday. Here they are.

    I’ll check out the wiring diagram and see what they are for and check out the book Larry is referencing.
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    That second connection in the plug is inconsequential. Plug it in, the kick down will work just fine.
  5. Utah455

    Utah455 Platinum Level Contributor

    Thanks Larry! Guess the last question is, do I make sure the Pink (w/black stripe) wire with the inline fuse is connected? Or the Black (w/white stripe) wire is connected to the trans? Or just plug it in since it looks keyed (L shape) and the appropriate wire will connect.

  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The kick down also depends on the gas pedal switch, the wiring, and the downshift solenoid inside the pan. All of it is easy to test. You can start with your transmission in the interest of being thorough. Run a jumper wire from the positive terminal of the battery, touch the spade for the kick down, and pull the connection away. Every time you touch the spade and pull it away, you should hear a click emanating from the pan. you need to be in a relatively quiet environment. Hear the click? The down shift solenoid is working.

    Next, go inside the car. You'll need a test light, or multi meter. With the key in the run position, pull the connection off the gas pedal switch. Probe the connector, there should be power on one side. Yes?, plug it back in. Now probe the connector at the trans. Operate the gas pedal switch by hand, and look for power when you close the switch. Alternately, you could just operate the switch by hand and listen for the click.

    That's it, if you find no power at any point, the problem is wiring or connections.

    A properly operating kick down is essential for full throttle shift RPM. The 400 will short shift at full throttle without the kick down operational.
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Tape that wiring off, it won't work anyway, you don't have the second connector at the trans, or the pressure switch inside on the valve body. None of it will work. You probably don't have the thermovacuum switch on the intake or the vacuum solenoid at the carburetor. It's all TCS.

  8. Utah455

    Utah455 Platinum Level Contributor

    My old wire harness had those wires missing. I don’t think My old Edlebrock carb had those connections.

    No switch on the intake. If it’s that one towards the front with three vacuum connections.

    Don’t know about the carb solenoid. I do have a new Q Jet (72 year) on order from Quadrajet power.

    (76 motor with 72 heads bolted to 73 Pontiac trans in a 70 Stage 1 body). It’s what was given to me. Had no idea the drivetrain was that jacked up when I bought it in 2001 until this restoration. It ran and I just drove it.

    The switch on the gas pedal works. I ohmed it out with a meter (continuity check) to verify the switch actuall worked.

    No power to the car yet. Still in process of putting back together. Tackling wire harness with all the Covid going on.



  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    You don't need any of that stuff. Carry on.
  10. Utah455

    Utah455 Platinum Level Contributor

    Sounds good. Thank you!

    BRUCE ROE Well-Known Member

    A 65-67 TH400 with 2 prongs is for kickdown and switch pitch as
    described above. But a 70s TH400 with 2 prongs uses lower for
    kickdown and upper to control emissions stuff. The upper wire connects
    to a pressure operated switch on the valve body. Bruce Roe
  12. stump puller455

    stump puller455 1970 GS 455

    Perfect Timing I have the exact same issue my transmission has a two prong connector and my original wiring harness is a single connector it’s the factory harness in my 1970 GS 455 …
    I’m going use the info above Thanks to everybody for taking the time and explain in great detail…. that’s what I love about the v8 Buick board
    Utah455 likes this.

    BUICKRAT Torque Rules!

    I'ma scared to hook up that switch in my car...I'll be in the puckers

    Got all the parts, too
  14. BrunoD

    BrunoD Looking for Fast Eddie

    All the 70-72 GS 455 came with the 2 prong and the wiring harness had 2 wires for the connection.So you may have a 350 GS or Slylark,or somebody put a 350 wiring harness in the car before you got it.Bruno.
  15. stump puller455

    stump puller455 1970 GS 455

    Correction my 1970 gs 455 has 2 prong you are correct Bruno thanks for your input i’m checking it out right now I did the jumper wire and I hear the click ! Now for the rest of it

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