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Overdrive Lockup

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by BettyBurb, Jun 29, 2003.

  1. BettyBurb

    BettyBurb Registered Member

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    I am trying to determine if the converter is locking/unlocking properly in my 700R4. In what gears should the converter be locked/unlocked ???

    Cruising at about 65 in OD if I depress the accelerator a little bit it feels like there is a slight downshift, but it is still in OD. I also have felt something similiar in 3rd. Is the slight feeling of a downshift the converter locking up ??? Should the converter be locked in Drive all the time or some of the time ???

    Also if I apply the brakes lightly in OD, I think I can hear the RPMS go up slightly, but I dont have a tach and am not sure that the RPMs have gone up.
     
  2. MTBLAZER89

    MTBLAZER89 3/4 ton status GMOTM Winner

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    Sounds to me like it's working correctly. Mine locks up in D too.
     
  3. wrathORC

    wrathORC 1/2 ton status

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    Driving around in overdrive you should be able to press on the accelerator at which point the torque converter will unlock because the detent cable (aka throttle valve cable) said to do so. While it is unlocked you can manually put the truck into drive at which time the converter will lock back up. If you hear the RPM increase when you shift into drive that means you only had the torque converter unlocking instead of dropping to third. This is the best way to test to see if the torque converter is actually locking and unlocking.

    You can repeat the same test in third gear (drive).

    The torque converter will lock up in second, third, and fourth gears. There is no provisioning in the stock valvebody to make the torque converter lock up in first gear. Driving around in second gear probably won't make the torque converter lock up unless you're traveling at low speeds without the detent cable pulled. This means if you're going through the gears manually (shifting from first to second to third using the shifter) the torque converter probably won't lock up until you let off the accelerator in third gear.

    I have a pretty sloppy torque converter. If the motor is turning 2000rpm and the torque converter unlocks she slips to about 2550rpm.

    For example, in my truck with airplane gears and 33s, first gear is good for around 50mph but I've had it to 65mph which is enough to scare most ricers. Second gear is good for around 100mph. If the torque converter locked up with the detent cable pulled I could go a little faster because I wouldn't be losing around 750rpm to a slipping torque converter.

    When you put your foot on the brake it tells the torque converter to unlock as well. Think about it, do you want your motor keeping you from stopping as it downshifts through the gears? It'd be like trying to stop a manual transmission without pushing in the clutch.
     
  4. MousePowrd

    MousePowrd 1/2 ton status

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    Your torque converter is working perfectly
     
  5. 84_Chevy_K10

    84_Chevy_K10 Banned

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    [ QUOTE ]
    Driving around in overdrive you should be able to press on the accelerator at which point the torque converter will unlock because the detent cable (aka throttle valve cable) said to do so.

    The torque converter will lock up in second, third, and fourth gears. Driving around in second gear probably won't make the torque converter lock up unless you're traveling at low speeds without the detent cable pulled. This means if you're going through the gears manually (shifting from first to second to third using the shifter) the torque converter probably won't lock up until you let off the accelerator in third gear.

    If the torque converter locked up with the detent cable pulled I could go a little faster because I wouldn't be losing around 750rpm to a slipping torque converter.

    When you put your foot on the brake it tells the torque converter to unlock as well.

    [/ QUOTE ]

    First of all, there is no "detent" cable on a 700R4. It is a Throttle valve cable, AKA "TV" cable. A detent cable is what a TH 350 transmission has that has no function except to kick the tranny down a gear for passing. The TV cable controls all of the shift points and line pressure as well as the kickdown function.

    The fact you stated that the TV cable controls the torque converter clutch is untrue. The torque converter clutch is operated by a vacuum switch. The reason it unlocks when you step on the gas is that it only remains locked under high vacuum conditions.

    Not all 700R4s will lock in 2nd gear, but mine did. If you're going up a hill and you downshift but you operate the engine at part throttle, you will most certainly notice the lockup. I tried it when I still had a 700R4.

    If you want your torque converter to remain locked with your foot in it, you have two options:

    Run +12 volts through a switch and splice it to the cold side of the vacuum switch. With the switch on you'll be able to force the torque converter clutch to lock unless your foot is on the brake regardless of throttle position. This would be my choice. The vacuum switch is by the windshield wiper motor.

    The other option is to acquire a vacuum switch that allows your vacuum to drop a little bit more before it unlocks the TCC. I am not sure what the factory one is set at, but I am fairily sure you could find a lower-rated one.

    Back on the topic of the first post, that TCC is operating perfectly. That's what it's supposed to do.

    And before you guys comment on that I don't own a 700R4 and therefore don't know what I'm talking about, my '84 used to be a 700R4/208 and I swapped it out last year after driving my truck for over 3 years with the 700R4.
     
  6. Thunder

    Thunder 3/4 ton status

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    FYI....... Lockup on TBI trucks is controled by the ECM... There is no vacume switch.
     
  7. MousePowrd

    MousePowrd 1/2 ton status

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    And if you hard wire the switch as listed above, shifting with the convertor locked will destroy the TCC clutch eventually, and could snap off the input shaft with enough power /forums/images/graemlins/shocked.gif
     
  8. 84_Chevy_K10

    84_Chevy_K10 Banned

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    [ QUOTE ]
    FYI....... Lockup on TBI trucks is controled by the ECM... There is no vacume switch.

    [/ QUOTE ]

    Bullshit. The 700R4 (aka 4L60) is not computer controlled. Only the 4L60E is. Lockup continued to be controlled by the TV cable until the computer took over with the change to the 4L60E.

    If the switch was wired as I instructed, it would be used only when you're already in 4th gear and want to force the TCC to remain locked. You wouldn't shift with the switch on, either. If wired the way I was trying to explain, when switched off, the circuit would still work as the factory intended it to.
     
  9. Thunder

    Thunder 3/4 ton status

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    Tim
    Dont tell me BULLSHIT /forums/images/graemlins/angryfire.gif
    As usual, you have no clue what you are talking about.
    The lockup in a truck TBI 700R4(which is what the orginal poster is asking questions about) is controlled by input from the MAP sensor,Esc module(RPM),VSS, and TV position. Based on this info The ECM decides when to lock up the TC. Or if the TV valve is open too far, it overides the ECMs commands and unlockes it. There is no vacume switch controling the lockup on TBI trucks. And I fully encourage you to post info that proves otherwise.
    GM quit using a vacume switch in 1986 when they switched from carb to TBI.
    The only trans function the ECM controls, is the TC lockup in Truck TBI 700r4/4l60 trans.


    By the way didn't you replace the 700 in your truck because you blew it up???????
     
  10. 84_Chevy_K10

    84_Chevy_K10 Banned

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    [ QUOTE ]
    By the way didn't you replace the 700 in your truck because you blew it up???????

    [/ QUOTE ]

    Nope. I replaced it for a number of reasons. One was that I didn't have enough gear for the tires I wanted to run and I didn't want to pay to re-gear. Another was that I had always wanted a manual anyway so when I found out how easy it was, I went ahead and did it.

    As for my old 700R4, it is still going strong over a year later in my friend's '86 K10.

    I see no more arguments on my lockup idea with the toggle switch. You can wire it however you want. I have seen this done sucessfully MANY times.

    As for your TBI comment, I will research it further simply because of your comment.

    Honestly, I don't care one way or another, but I'll do my homework and retract my statement if I should turn up incorrect, as I have at times done in the past when I was wrong.

    I'm so glad that this place is such a wealth of knowledge that when you call someone out on something, all they can do is tell you that you don't know what you're talking about. There are quite a few people on this site like that but I am definitely not one of them.

    Obviously nobody wishes to challenge me as to whether the TV cable has anything to do with the TCC disengaging. Apparently I must have some clue what I'm talking about there as nobody wishes to call me out on that. That's one thing that I'm definitely sure of anyway as I still have my TCC harness in my garage at my mom's house.
     
  11. 84_Chevy_K10

    84_Chevy_K10 Banned

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    Further, if what you say is true about the ECM taking an input from the MAP sensor and using that, among other things, to decide whether the TCC will lock, you yourself have further proved my first point that it was not simply TV position that detirmines whether it locks, but engine vacuum as well, no matter what system your truck may or may not have.

    It is not important as to whether or not you have a vacuum switch or the input is taken from the MAP sensor. The main point I was initially trying to make is that TV pressure is not what detirmines if the TCC locked. If that was the case, disconnecting the plug for the TCC would make it lock all the time (even when cold, your foot was on the brake, heavy throttle, etc.) which I am sure that we all know is not the case. The TV cable has little to no effect on whether the TCC locks. Engine vacuum is where this decision is made....vacuum switch, MAP sensor, EFI, carb, whatever...TV is not what detirmines lockup, period.

    VSS? Doesn't an 88 still have a mechanical speedo? Does any mechanical speedo truck have a VSS?
     
  12. dyeager535

    dyeager535 1 ton status Premium Member

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    All trucks with a locking converter have VSS. Checked my '86 manual, even Th350C's have VSS.

    VSS was what determined the minimum speed of lockup, which is why it was located in the same module as cruise control when there was no ECM in the truck.
     

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