TH 700R4 TCC lockup wiring

[Dannyboy1966]

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.