TH 700R4 TCC lockup wiring
 

First I have a 1986 GMC that came original with a 700R4, and a previous owner swapped it out for a SM465, I'm going back to a 700R4.
My wiring question is the 700R4 I picked up is from a 1989 Chevy 2500, it has a 3 wire plug for the lockup circuit, one of which goes to the ECM. My 1986 doesn't have an ecm connection for it.
What is this 3rd wire for and is it really necessary, and is there a way to bypass it if it is?
Attached is the schematic for the 1986 circuit, and the 1989 circuit.

Re: TH 700R4 TCC lockup wiring
 

My 85 has a 3 wire hookup, one of the wires went to the egr solinoid for some reason, and my transmission works as it should without that hooked to anything. I'm not sure the wiring on a 85 and 86 is the same though. Is the harness for the 700r4 still in your truck? I'm not sure if it's the same for 86, but the 85 set up is pretty basic with a low vacuum switch, and a brake switch to control the lock up.

Re: TH 700R4 TCC lockup wiring
 

Yes I still have the wiring in my truck minus the low vacuum switch shown in the diagram on the left.
The 1989 diagram on the right shows 2 different wires coming from the ECM and one from the brake switch. This is where I'm confused. I was hoping Hatzie would hop on and provide some input knowing how good he is with wiring problems.

Re: TH 700R4 TCC lockup wiring
 

It should work just fine without terminal D connected. Terminal D is a slightly different diagnostic tap for the ALDL port and the ECM in 89 and an EGR bleed solenoid on some earlier rigs. IIRC the ECM controls EGR bleed on the TBI rigs.

Whoever did those 82-86 wiring diagrams at GM should be hung, drawn, and quartered... They didn't label the transmission side connector with terminal letters and they show wire side on both so it's easy to mis-read the two terminal diagrams. You have to mentally think about where things would be if you plugged the connectors together. If you look at one of the diagrams with EGR purge and three terminals it becomes clearer what's happening.

The 89 transmission should have an additional thermal switch in the internal TCC circuit but that won't affect the external wiring.

You should find and install a Low Vacuum Switch. The Low Vacuum Switch cuts power to the TCC when manifold vacuum drops from the throttle opening. It's an analog lockup control so you don't burn up the TCC by applying torque without cutting out the clutch. You definitely need it to utilize the TCC without damaging it.

Make sure the brake pedal switch has the TCC switch contacts as well... The standard transmission brake pedal switch doesn't have provision for this.

Re: TH 700R4 TCC lockup wiring
 

Lucky for me when the previous owner swapped my rig to a SM465 they were nice enough to tie the wiring up and leave it hanging under the hood by the brake master cylinder. So I have the brake and low vacuum switch, my concern was the internal wiring with it having 2 of the wires going to the ECM. So on the 1989 transmission I just use the A and B terminals and just forget about the C? Here is a close up of the internal wiring. Tell me if I have it right or correct me. Thanks Hatzie

Re: TH 700R4 TCC lockup wiring
 

Plug it all back in and drive it.

C is unused... D is the EGR ECM terminal. Your harness doesn't have a wire in C or D. That doesn't matter.

The picture is not needed. I have the uncompressed scans and a 50" 4K monitor.

Re: TH 700R4 TCC lockup wiring
 

So the brake switch and the low. vac. switch unlock the tcc but what locks it?

Re: TH 700R4 TCC lockup wiring
 

To explain how it works, the brake switch, and low vac switch cut the power to the lock up solenoid inside the pan. Now a detailed description. In the trans there are 2 or 3 pressure switches and a solenoid. Power flows from the fuse panel, through the brake switch, then through the low vac switch and into the trans. If you throttle hard on the motor it's vacuum signal will decrease for for a brief time, causing the low vac switch to break the circuit and cut power to the lock up circuit until one of the switches inside the trans detects pressure in the 4th gear oil circuit. The brake switch works in a similar way, during braking or panic braking it cuts the power circuit and unlocks the torque converter, and remains unlocked until the 4th gear pressure switch detects pressure again. These same connections on the brake light switch also double as a cut off signal for the factory cruise control if equipped.