67 GS 400 switch pitch help

Discussion in 'The Bench' started by Redmanf1, Jun 3, 2022.

  1. DaWildcat

    DaWildcat Platinum Level Contributor

    If everything's working correctly, you get:

    1. high stall whenever throttle is closed (helps with "creep" and improves idle quality at the stop light)
    2. high stall at approx 2/3 throttle
    3. detent at approx 3/4 throttle

    High stall and detent happen at two different throttle angles. Low stall happens as soon as you tip into the throttle and continues until 2/3 throttle.

    Devon
     
    Super Bald Menace likes this.
  2. Redmanf1

    Redmanf1 Gold Level Contributor

    Would you happen to have the factory part number for the transmission modulator?
     
  3. Super Bald Menace

    Super Bald Menace Frame off oil changes

    The microswitch is closed only at idle allowing 12 volts to the converter solenoid to engage high stall at idle. The moment the pedal is pressed to accelerate the switch opens and has no effect on operation again until car is back at idle.
     
  4. LARRY70GS

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

    The modulators currently available are the small brass (red stripe adjustable ones)

    https://www.rockauto.com/en/catalog...nsmission-automatic,governor+/+modulator,8500

    https://www.rockauto.com/en/moreinfo.php?pk=10317260&cc=1434252&pt=8500&jsn=431



    They have no effect on switch pitch operation. The switch pitch valve bodies are unique. Good article on the differences,

    http://www.buickperformanceclub.com/SPTrans.htm
     
    Last edited: Jun 4, 2022
  5. LARRY70GS

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

    If you want to check switch pitch operation, set up a simple test 12 volt test light at the transmission connection. Then turn the ignition to run, engine off. The light should be on at closed throttle, go out off idle, and back on at ~ 60% throttle. Test the kick down similarly. The switch pitch valve body won't allow a kick down except at full throttle, regardless of when it gets the signal.
     
  6. Super Bald Menace

    Super Bald Menace Frame off oil changes

    How does the valve body know what position the throttle is at? It knows vacuum from modulator position and kickdown request due to 12 volts applied. Rpm from governor There isn't anything else is there?
     
  7. bw1339

    bw1339 Well-Known Member

    That only tests the switch. I would put the car in drive (someone stepping on the brakes) and manually actuate the switch. RPMs should slightly increase when the S/P circuit is energized.

    If it's really quiet, one can hear the solenoid click when the engine is off and the throttle is moved through the range.
     
  8. LARRY70GS

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

    Yes, those tests should be done as well. Maybe I'm mistaken, but I thought the OP was more concerned with switch operation, not actual SP function.
     
  9. Redmanf1

    Redmanf1 Gold Level Contributor

    I believe the modulator in the trans is okay but it's a new style. I'm trying to find an old style original looking one.
     
  10. Redmanf1

    Redmanf1 Gold Level Contributor

    I believe the problem lies somewhere in the switches.
    That's why I wanted to learn the function of the switches.
     
  11. LARRY70GS

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

    Have you read Mark De Conti's SP article that I linked? When I was running a SP, I actually swapped in a later valve body to get a part throttle kick down capability, as well as the better filter. The SP valve body makes it harder to tailor your WOT shift points with governor mods. Read the article, it is very informative.

    http://www.buickperformanceclub.com/SPTrans.htm
     
  12. Redmanf1

    Redmanf1 Gold Level Contributor

    I appreciate all the help very much,
    thank you all
     
  13. LARRY70GS

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

    When I had a SP, I used one of Bruce Roe's electronic control boxes. I had him modify it so that it mimicked original SP function. Bruce uses a vacuum switch for the high throttle stator switch to hi stall.
     
  14. TrunkMonkey

    TrunkMonkey Well-Known Member

    [edited: made the reply before Larry did about the valve body...]

    The Switch Pitch valve body's were different and actually are a bit "doggy" especially with a cam that impacts the lower RPM.
    The 2-3/3-2 modulator circuit is different and one spool/spring/bore is absent. So part throttle 3-2 downshift is not there. (relying on the higher stall and torque for "passing". But it is way more fun when you have that 3-2 downshift without having to flatten the pedal.

    I replaced mine with a shiny NOS for a (G.I.) "Hummer" (the actual big Army Jeep Thing™ not that gutter thought you had...). Made a huge difference in dialing in the shifts/downshifts.
     
  15. bw1339

    bw1339 Well-Known Member

    I had the same complaint about the S/P TH425 in my '66 Toronado. It does not do part throttle downshifts based on engine load, unless commanded by the governor or the kickdown switch. Playing with the kickdown switch didn't really work well at all.

    I ended up using two vacuum switches, one for the S/P and another for the kickdown and now it works beautifully. The only problem is the deadband of the mechanical switches, which causes some flutter sometimes, since both the kickdown and S/P affect vacuum levels when energized. Now the S/P energizes at about 10in-Hg and the kickdown at around 5. I'm going to replace them with an electronic pressure switch, which has fully adjustable deadbands and all kinds of electronic filtering.

    [​IMG]
     
    Last edited: Jun 4, 2022
    Max Damage likes this.
  16. Redmanf1

    Redmanf1 Gold Level Contributor

    That's a great post on the transmission by Mark thank you for posting that.
     
  17. Super Bald Menace

    Super Bald Menace Frame off oil changes

    I knew about the part throttle downshift issue and when I had my st400 built I used a later valve body to eliminate that issue.
     
    TrunkMonkey likes this.
  18. Redmanf1

    Redmanf1 Gold Level Contributor

    Lots of great info
     
  19. mrolds69

    mrolds69 "The Cure"

    I can probably hook you up with a stock appearing NOS or NORS modulator Nelson. PM me if interested. But, I think Larry's correct the modulator has nothing to do with the switch pitch set up working or not.
     
  20. Super Bald Menace

    Super Bald Menace Frame off oil changes

    If you're having a shifting issue it's not going to be related to the switch pitch.
     
    TrunkMonkey likes this.

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