Torque Converter Clutch Hot Wiring

Well  to answer a few questions i have gottenon a 700R4( and 2004R) , they obviously have what is called an internal torque converter clutch, or TCC for short). as i have gone over before, it is basically a wet clutch applied by the transmission to lock up a torque converter for mileage. it is usually engaged by the computer control from the factory. now, there 700R4’s that had lock up internally in the transmission without any external computer control. gm actually originally designed the valve body for this, then quickly converted it to computer control primarily for emissions/mileage reasons. most 700r4’s have aluminum plugs in the valve body where the original tcc valve would have gone. you can retro-fit the valve body with aftermarket parts to do this as well. i have never done one or ran across one yet, but i know it can be done.

BUT,  we are all cheap bastards, so usually what is going on now-a-days is swapping these older overdrive trannys into street rods and plow trucks for mileage…..or we simply had a working tranny laying around from an old derby car we gutted and th350/400’s are getting hard to find.

So, when doing a swap into another vehicle, the only real critical thing you need to worry about is adjusting and installing the throttle valve cable correctly. the electrical connector for the TCC can be left unhooked.  OR if having one rebuilt, you can simply install a cheap kit into the pump and omit the clutch all together…..which is what i did. however this causes you to lose some efficiency and mileage due to the converter slippage.

to be honest even though it has a 4 prong connector hanging out of the case, the actual engagement of the TCC solenoid is a 2 wire solenoid with one grounded and one 12v power source. GM used several different wiring formats throughout the years some using as many as 4 pressure switches …….some only one. this is because gm was trying to lock up the converter in more than one gear…..which probably led to pre-mature tranny failure.

the way i wire it up it to basically drop the pan, rip the 4 pin connector out of the case and install as single wire kickdown plug from a th400, then run a single power wire from the 1 pin th400 connector to one side of the TCC solenoid. now, the other wire off the solenoid is ground. i then use a pressure switch in the 4th gear passage  on the valve body as a ground. there is usually one there anyway, but you should make sure you got the right one.

this way no matter what you do for power, the TCC will only engage in overdrive. you can run the ground wire off the TCC solenoid straight to ground but if you forget to shut off the switch, it will stall out the engine and scare the shit outta ya. i would either use the ground switch or leave the tcc unhooked all together.

now, if you have any idea what i am talking about by now, you can figure out that you could just wire the damn TCC to a toggle switch  on the dash and lock it up whenever you want. by using a psi switch in the 4th gear port, you basically can flip the switch for highway driving and as soon as it hits 4th gear you got lock-up. for in-town/city driving you can leave the switch off pretty much cause a TCC doesn’t doo much good on these trannys in heavy traffic anyways other than tear up the tranny faster.

the pressure psi switchswitch you want to wire to is the one marked letter “A”. this is the 4th gear pressure port. GM used both normally open and normally closed switches….both single and 2 pronged. i like to use a single prong normally open/psi closed switch. i believe the GM part number is GM#8627332 . now some valve bodies didn’t use and psi switches at all and simply have 1/8″ pipe plugs. if you decide to do this, you can omit the rest of the psi switches and replace em with pipe plugs to save confusion later on. you can also modify your wiring and make whatever switch you have on your valve body work to engage the TCC only in 4th gear.


here is a rather wiring tcc terrible drawing of how the wiring goes. the red obviously is 12v and the blue is ground to the 4th gear psi switch.






now, if you do not like the idea of just using a toggle switch , or the driveability using a toggle switch isn’t to your liking,  you can then take the 12 wire and instead of wiring it to a toggle switch you run it to a low manifold vacuum switch….then a brake pedal switch… series….before you get to the transmission. the part numbers on these components i believe are a Low-vacuum switch – GM #14014519 and Normally open brake cancel switch – GM #25524845 .

 the low-vac switch so the converter will unlock in low vacuum situations, such as going up hill and heavy throttle.  The brake cancel switch unlocks the TCC when the brakes are applied.  Trucks that came stock with 700R4 trannys came with a low-vacuum switch and a brake cancel switch.  The stock low-vac switch didn’t let the TCC unlock soon enough with this mod. the vacuum on the stock switch kicked in at 3.5″, meaning the vacuum has to get real low before it will unlock.  The vacuum switch part number above  kicks in at 7.5″, which will let it unlock sooner giving you a bit better performance. The brake cancel switch on 700R4 equipped trucks also disconnects the cruise control (if equipped) when the brakes are applied

Now to the outside wiring. run the power wire from 12v key on hot through a normally open brake cancel switch.  This switch will allow current flow, only when it is depressed.  The brake pedal depresses the switch, normally, and releases the switch when the brake is applied.  This switch works exactly opposite the brake light switch, whereas the brake light switch allows current, only when the brake pedal is depressed, the cancel switch interrupts current when the brake pedal is depressed, thereby unlocking the torque converter clutch, such as in a panic stop.  The cancel switch is mounted on a bracket under the dash.  The brake pedal arm makes contact with the switch when it is all the way out.  Just so you will know, most of the time the brake cancel switch is also a cancel switch for the cruise control, if you have cruise.  This is why it is called a cancel switch, it cancels power when the pedal is pressed.  You need to make sure that when the brake pedal is all the way out there is power going through, press the pedal and the power is interrupted.

From the brake cancel switch, power is routed through the low-vacuum switch.  use a junkyard donor for the plug if you can as the prongs are close together and can be a pain in the ass. you can probably find most of these parts in a junkyard if you look in the right spot.  If you can’t find a plug, you can either use some slim female spade connectors or solder the wires on.  Mount the low-vac switch on the firewall or at the very least away fro mthe engine a bit.  Then connect to the new single-pin connector at the transmission .


you have pretty much just made yourself 120$ lock up wiring kit for a fraction of the cost. enjoy!

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