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!
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 diagram below is what I believe to be correct for my transmission, though the wiring terminal colors they list don't match what I found inside my transmission. That could be due to my transmission being rebuilt at some time, or due to the manual not matching what actually happened on the assembly line. Either way, the details are handy - especially the small wiring schematic in the upper right hand corner of the page and the listing of what wires are A, B, C, and D in the transmission case connector. Note that terminal C is not used in any application that I can find and my transmission electric plug had a solid plug in that position so it was not usable. It might be nice if it was there to wire a simple "TCC is engaged" light on the dash that was not a "I think it should be engaged", but rather "it really is engaged" switch...
If you choose to not use any of the switches, they can be left in place, or removed and replaced with 1/8" NPT plugs. All four holes must have a switch or plug in them for the transmission to function, so double check stuff or you'll be removing the transmission pan and fixing it once the transmission is in the car - quite the messy experience. I opted to remove all but the "4th Clutch" switch and replace the others with 1/8" NPT plugs - the switches seemed to work and were nifty enough to save for other possible projects. Plus, removing the "TCC Signal" switch makes it easier to get at one of the bolts for the solenoid, which I had to remove to see about changing to the two-wire style solenoid from my original one-wire self-grounding style solenoid.
Here's the TCI internal wiring diagram for reference. Note that is shows pipe plugs in the three unused pressure switch locations. The labeling is done in a somewhat confusing way, but it does contain useful information, so I'm including it here for reference purposes.
Here was my first cut at a wiring diagram that should work similar to the TCI kit, plus this will have an indicator light and the ability to force the torque converter to lock up in any gear. Like most other kits, it controls everything on the ground side of the system, though the control switch does toggle things completely off when in the center OFF position. I like having that control, but it's not strictly needed. It does offer a nice safety feature in case of electrical system problems down inside the transmission, so I would prefer to do it this way. If this added niftiness isn't important to you, it's possible to substitute a SPDT center-off switch and run the power wire straight out to the solenoid with a wire heading up to the LED indicator as shown here. Since SPDT center-off switches might be easier to find than DPDT center-off switches this could be a viable option for some folks.
After creating that basic diagram and pouring over the manuals, I found a few interesting details that kept my idea hamster going for a while. This is more stuff to think about before deciding on what wiring to use for your transmission.
Sticking with the ground side controls, here's a wiring diagram the moves the vacuum switch into the main power line. It's a bit odd because the wire coming off the 4th Gear Pressure Switch simply exits the transmission case and goes to ground, but it should work fine, and doing it this way allows the indicator to work the same way it does in the previous diagram. You could in theory substitute a self-grounding switch here as well and only have two wires coming out of the transmission case, but I'm not sure at what pressures the various switches trigger at. Because of that, I would tend to use them in their original locations to be sure all was working as expected/desired.
Taking things a step further, I created this diagram to move the control completely to the power side, switch to a one-wire self-grounding solenoid, and try to use the self-grounding switch in the "TCC Signal" location to control the indicator light. If this works, I think the indicator light would be more useful - assuming, of course, that my understanding of the "TCC Signal" switch is correct. If not, the previous diagram is probably the best so far. I have also included the required changes to have the vacuum switch control the power feed to the dash switch.
This is a small update on the previous diagram to add a "very high vacuum" switch to allow the TCC to stay locked whenever manifold vacuum is very high - such as when doing engine braking on a long downgrade. Other than a bit of shuffling on the diagram to make everything fit neatly, the only addition is the vacuum switch connected between the power side of the brake switch and the power feed to the SPDT Center-Off switch. This switch is hooked up to manifold vacuum (the other is hooked to ported vacuum) and can be tapped off the same vacuum line that goes to your vacuum gauge - you do have one of those, don't you? - so that you don't have to run more wires out into the engine compartment. If you decided to wire this new switch in the engine compartment, you would need a third wire heading out to the new vacuum switch and the two vacuum switches could share a common return wire back to the switch. This addition can easily be done after the above wiring has been done and tested.
Finally, a bit of re-arranging and cleaning things up netted me this diagram. Before creating this, I roughed in the wiring on the vehicle and made this diagram follow the rough harness layout to make it easier to conceptualize what goes where while I was staring at the diagram and doing the actual wiring. On the actual harness I added connectors just inside of the firewall and just before the switch so the harness could be built in pieces outside the vehicle. If I had been able to size it better beforehand, these extra connectors would not have been needed, but they made my life easier and will come in handy if I ever have to remove or disconnect the wiring for service or troubleshooting.
Also, after playing with the wiring a bit on the vehicle while driving around, I could not get the indicator light to work the way I originally thought it should. To make the light work more sensibly, I added a ground wire to ground the indicator light and ran the "power" side of the light from the "lockup in any gear" lead coming off the switch. This lead is connected to the solenoid power lead inside the transmission, and gets power when the solenoid has power applied to it. That makes the indicator light go on whenever the solenoid should be locking up the converter. I'll have to research the pressure switch I was trying to use a bit more.
Here are the photos of the wiring work I did as I put things back together.
Here are close-up shots of the various wire connectors used on the TH700R4 transmission. The connectors on the pressure switches were standard fare - I had a box of crimp-on connectors already, so I just pulled the old wires out of the plastic connectors and put in new wires with new crimp-on connectors. I had a heck of a time finding the right connectors for the internal wiring plug, but I did manage to get a few that seem to work fine at my local GM dealer. He did have to scrounge through his wiring terminal kit to find them, though. The external wiring connector I got from the dealership has wiring pigtails attached and came with crimp-on butt connectors, but you can get the bare connector with terminals from PATC under part #71XXX. The case connector is still available from GM if needed, complete with a new o-ring. Check out my Automatic Overdrive Transmission Swap page for a complete list of all of the relevant parts, part numbers, and sources.
TODO: Update this page once I have done some road testing of my transmission and lockup wiring!
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Page last updated 12/27/2011 10:23:21 AM