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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: TexasRed on May 20, 2008, 10:14:36 pm
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ok, I have a question about the 700r4. My truck originally came with a th350c. It's got a 700r4 in it, they just used the lockup connector from the th350c. My question is I believe that lockup disconnect is controlled on this pickup by 2 things, a brake switch that's connected to the pedal along with the cruise (I got that), but the second thing is a vacuum switch that is either connected to port or manifold vacuum and when the vacuum changes it breaks the circuit and the transmission comes out of lockup. It's also supposed to protect it from entering lockup at idle and stalling the engine. My question is what does that look like and does my truck come with it? I have an 1984 GMC Sierra SWB. If anyone has a picture of this that would be cool too. I found something on the firewall I thought was the switch but it looks like a connector switch for the distributor delay which I since kind of done away with when I started making the underhood routing a little cleaner. Thanks for any help.
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The low vacuum switch wasn't used on all models. It's designed to unlock the converter by overriding the pressure switch. The pressure switch controls the lock up function. There are several ways you can wire up the lock up function.
I drew up a schematic and posted it recently (try searching) and you can use several variations depending on when you want it to lock up.