[pjmoreland]

EDIT: I ended up developing an Arduino-based controller after struggling with the limitations of a vacuum-switch-based setup. Arduino development starts on post #47.

I rebuilt a 700R4 and installed it in my truck recently. I spent a decent amount of time researching how to wire the torque converter clutch (TCC). The methods I found range from installing a manual toggle switch to buying a full blown kit from various companies. I decided to take the middle of the road and use a vacuum switch and a time delay relay. Here are the characteristics I wanted in my system:

1) The TCC locks automatically in 4th gear only

2) A vacuum switch unlocks the TCC when vacuum drops due to acceleration

3) The TCC unlocks when the brake pedal is pushed

4) When all conditions are met for locking the TCC (4th gear, high vacuum, brake not pressed), the TCC will be locked after a delay of 8 seconds

The delay relay is required to prevent hunting, where the TCC is locked and unlocked repeatedly when the vacuum level is on the edge of what is required to lock the TCC. I did experience this issue on my first attempt at wiring the system without a time delay relay. I also tried installing a vacuum delay valve, but the vacuum switch is designed with a small leak which prevents the vacuum delay valve from working properly.

There is a subtle difference between how I wired the relay and how it's usually done. Most examples I found had the relay trigger negative terminal connected directly to ground, and they had a jumper between pins B and D of the transmission connector to hard-wire the 3-4 switch to the TCC. The problem I see with this approach is that it allows the TCC to lock up simultaneously with the shift from 3rd to 4th during mellow acceleration. I wanted the relay to delay TCC lock up until eight seconds after shifting into 4th during mellow acceleration. I achieved this by using the 3-4 switch to provide ground to the relay trigger negative terminal. This approach has two benefits:

1) The shift from 3rd to 4th+TCC during mellow acceleration is too radical of an up-shift, and causes the engine to strain. Delaying TCC in this scenario allows engine RPM to increase a little bit more before locking the TCC

2) Engine strain resulting from 3rd to 4th+TCC is usually responded to by pressing down on the accelerator a bit. Depending on the acceleration rate, this can deactivate the vacuum switch, causing the TCC to unlock immediately after it had just locked. This doesn't feel smooth

Here are the parts I used:

GM 25524845 Brake Switch
TCI CMP3766B Vacuum Switch
HELLA 996152151 Delay On Relay
MUYI 5xSKU20909-MY Relay Socket (ordered on Amazon)
American Autowire 500323 Plug for 700R4
Dorman 18g wire in orange, pink, red, white and black
Packard 56 male & female crimp sockets for bulkhead connector (eBay)

My transmission is a model year 1993, so it already had a normally open 3-4 switch. Some older versions of the 700R4 have a normally closed 3-4 switch. If this is the case, then the switch has to be replaced.

I wanted the wiring to look reasonably stock. Here are some things I did to achieve that goal:

1) I tapped into fused 12V on the orange wire that is on the stock brake switch. I found a stock connector in an early '80s van for the second set of contacts on the brake switch, and connected one of its terminals to the stock orange wire

2) I was able to slip the pink wire from the new brake switch connector through the existing plastic wrap on the harness that contains the stock brake switch wires

3) I used an empty slot in the bulkhead connector to route the pink wire through the firewall

4) I wrapped the pink wire inside the harness that runs over to the distributor

5) I used original style plastic wrap on the wires that run to the transmission

6) I secured the transmission wires in the same firewall clips that hold the brake light wires