1992 K1500 Blazer-700r4 torque converter not locking up at all.

Discussion in 'Transmissions' started by Kelcey, Nov 15, 2019.

  1. Kelcey

    Kelcey Newbie

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    Basically as the title says. Truck runs and drives and shifts normally otherwise, but the torque converter never locks, all the way to 80 mph. Cruise works, no check engine light, ABS and Brake light are on. Where should I start on diagnosis? Fluid is clean and at the proper level. What are some common issues? Snapchat-908524281[1].jpg
     
  2. JavaMan

    JavaMan Newbie

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    I know that the TCC will disengage when the brake is pressed. By brake light, do you mean on the dash or on the back of the truck? If the rear brake lights are on, you have a problem with brake switch, which would cause the no lockup issue.

    Sent from my SM-G965U using Tapatalk
     
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  3. Mark Gilbert

    Mark Gilbert Newbie

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    I imagine that it is just the dash lights because the cruise control works, and the brakes should kill that.

    I have not worked on a 700-r4, but a little quick reading and I see some sort of vacuum switch being mentioned that enables the lock-up on the torque converter. I guess it provides a signal for lock-up and the brake switch will interrupt it to disengage.
     
  4. Kelcey

    Kelcey Newbie

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    The brake light on the dash is on. The rear brake lights go off and on with the brake pedal like normal.
     
  5. Philhammerjr

    Philhammerjr Newbie

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    I have a 92 k1500 blazer as well. There is no vacuum switch. The tcc is completely controlled by the ECU with the brake switch acting as an override. You should be able to use a jump wire in the aldl to rule out the ecm(pin f to a). If it does engage then the ecm or ecm connector is the issue. If it does not the issue is on the positive side (brake switch or power to the brake switch)or the solenoid itself or the converter clutch. My money would be on the brake switch. The contacts are probably worn out. It's possible for the brake lights to still work even if the switch is worn since the contacts for the brake lights and for the lockup are separate.
     
    Last edited: Nov 17, 2019
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  6. OleGreen93

    OleGreen93 Newbie

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    Is the engine getting up to operating temp? The ECU needs to see a certain coolant temperature before it locks the torque converter. When the weather started to cool down my 92 c1500 torque converter would not lock unless i drove it around for a long time to get it up to operating temp. I found out my thermostat was opening too early. Put a new 195 degree in and the torque converter locks when it should and my temp gauge is accurate now. Not sure if this would be your issue but wanted to throw that out there
     
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  7. Kelcey

    Kelcey Newbie

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    According to my dash gauge and a heat gun, the truck is getting up to temp. I believe the truck needs to see 149 degrees to lock and it's above that and still not locking.
     
  8. PlayingWithTBI

    PlayingWithTBI Desert Old Guy

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    You need to data log this while driving to see if all of the TCC lock up conditions are met (besides the brake switch once you eliminate that) You need inputs from your CTS, TPS, Speedometer and Tachometer for it to lock up. My minimum CTS is set at 16,25*C (61.25*F) mandatory lock up is 100MPH.
     
  9. someotherguy

    someotherguy I'm Awesome

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    There are multiple (3 IIRC) sets of contacts in the brake switch; you can have functioning brake lights and still have cruise/TCC lockup problems. Check the switch.

    Richard
     

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