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Lockup TCC Wiring

Time:2018-01-13 06:00Turbochargers information Click:

Wiring Lockup

Lockup TCC Wiring

This page is all about what I learned about the wiring needed to control the lockup torque convert clutch (TCC) after I decided to swap the original TH400 transmission in my 1975 GMC Suburban for a TH700R4 transmission to gain the benefits of a modern overdrive transmission and help tame my "great for towing" 4.11 rear axle gears on the highway. As part of this project, I also had to build a custom Throttle Valve Cable Bracket so I could hookup the TV cable to the carburetor. This information applies in large measure to the TH200R4 transmissions, but not to any of the computer-controlled transmissions that came later - they are all controlled by the computer and the wiring is more or less just to all the sensors involved and is specific to the computer you have to use.



The only wiring work you need to do on these transmission is wire up the controls for the lockup feature on the torque converter. This can be accomplished with simple kits from several places or with custom wiring of your choice. The kits typically have two wires going into the transmission - one is power to the torque converter clutch and another is a ground connection. When you have power and a ground, the converter is "locked". When one or both are disconnected, the converter is unlocked. The kits typically have a vacuum switch, some basic wiring, perhaps a pressure switch to know what gear the transmission is in, and sometimes even a simple speed sensor to prevent lockup below a certain MPH. These kits are all "plug and play" simple, so you can just buy one and be done with it. Or you can figure it out and create your own kit - it's your choice and your money. Being the sort of "gotta do it my way" kind of guy that I am, I decided to do my wiring from scratch and figure out all the details along the way.

One thing of note is that some of the wiring is inside the transmission pan, so when you drain any remaining fluid by removing the pan to change the filter, that's the time to get the wiring done the way you want it. No sense doing that more than once.



Here are some pictures of the wiring on my donor transmission as I received it. The first photo shows the wiring connector on the driver's side of the transmission with the three clipped wires coming out of it - look closely, it is there in the grime and dirt. It shows the three wires coming out of the wiring plug on the transmission and the basic location of the wiring plug in relation to the shift linkage. The next four photos show the internal wiring inside the transmission pan - there was quite a bit inside here on my donor transmission! The final photo shows the underside of the transmission case connector - the white plastic piece is clipped into the transmission housing and there are plus that go into it from the top and bottom - it's quite a complicated little piece!

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It's important to know what all that wiring does and why it's there - because I'm going to change it to suit the needs I have after swapping this transmission into a non-computer-controlled vehicle. There are a total of four possible pressure switches, three possible wires coming into the transmission, and a solenoid to do the actual work of locking up the torque converter. Note that I said "possible" - not all transmissions used all of the pressure switches and wiring. In addition, there are two different style solenoids that were used in various applications.

For reference, I'm including a pretty detailed list of the wiring and switches inside my transmission. It had pretty much every possible wiring option and folks can use this to help ID what goes where for doing retrofit or swap applications. I had a heck of a time figuring this out from available information on the internet and eventually had to buy a factory manual for the year and model vehicle my donor transmission came from to confirm all the details. I'm posting it here for others to learn from my experience.

  • The solenoid has the single red wire going to it and is next to the pressure switch with the single green connector. This solenoid is the single wire "self-grounding" style solenoid. Most swap applications I've seen (including kits from TCI, etc.) require using the two wire style solenoid with a separate ground wire that can be controlled - it's easier to do the wiring on the ground side of things in many situations.
  • The pressure switch with the single green connector is installed in the "TCC Signal" location and is a normally open unit that is self-grounding. I believe the contact is connected to ground whenever the TCC is engaged.
  • The pressure switch with the two blue connectors on it is installed into the "4th Clutch" location and is a normally open unit that is not grounded. The contacts are connected whenever the transmission is in 4th gear.
  • The pressure switch with the two green connectors on it is installed into the "4-3 Downshift" location and is a normally closed unit that is not grounded. I believe that the contacts are disconnected during the 4-3 downshift and I would assume this prevents problems with the TCC being locked up during the downshift.
  • The pressure switch with the two red connectors on it is installed into the "3rd Clutch" location and is a normally open unit that is not grounded. The contacts are connected whenever the transmission is in 3rd gear.
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