Makes no sense to me why the tranny would be wired to that unless its just a common ground.
Easy there cowboy. Now in this scenario I don't have a clue what your talking about and I'm not going to back up in the thread to figure it out, but just to educate you a bit, about how things don't make sense as far as transmission connections having to do with smog devices, intakes, TCV's (Temp Control Vacuum) switches, and other electronics, especially in the older NON COMPUTER ECM controlled vehicles, I'll tell you how it makes perfect sense and how it's a benefit to you. OK, now onto the edgumahcation.
Had you even noticed in some late mid to late 70's and into the mid 80's cars and trucks, you'd go get a part and the parts guy might ask, "Is this manual transmission or Auto Transmission and you're thinking WTF does that have to do with my flux capacitor or my mechanical choke or whatever the case may be. Some of the guys familiar with smog devices will also sometimes wonder, why does an Auto Trans require or have this device, but a manual trans vehicle does not? Why does a manual trans vehicle get to legally run a bit more dirtier than an Auto Trans vehicle according to State Smog Laws? The reason has to do with probably what you're saying doesn't make a bit of sense.
An auto trans can have a pressure switch installed into a port that's only getting pressure once the vehicle has hit it's top gear. As in 3rd for a Th350 which is the final drive gear and what gear you're most likely in once you've hit your cruising speed. So what this switch does is enables some devices to be deactivated or activated once you're in final drive gear. Let's say for example, and EGR Valve can be disabled at idle, and while in R, N, 1st or 2nd gear, but once you get into your cruise speed and the pressure switch from the trans activates then it can signal for an OK to turn the EGR valve on, either elecrtonically, or an electronically controlled vac switch. Now that's just an example, I'm not saying it is true. So because a manual or 3 speed trans didn't have the provision for that in that year model, it may not have the EGR valve and not required. It could be that 3 speeds manuals didn't have it, but the 4 speed manuals did too. And it could be the manuals didn't have the provision in this year, but they came out with it the next year. Cuz some manual transmissions have it too, it's just not pressure activated, it's activated by location of a shift fork. This is also similar to Lock Up converter technology, or some cruise control systems, to where cruise control can only be set when you're in top gear, or your converter be able to lock up in top gear. Now that changed some in 700r4's since you can in some models lock up in 3rd or 4th gear, and some just lock up in 4th gear. Anyway, different variables for different reasons, but now you know when things don't make sense like that, they actually do make sense.
You ever worked a vehile where it was not getting any fuel and the guys replaced EVERYTHING to do with the fuel system, pumps, TB's, Filters, senders, hoses, tank switches, done it all and still no fuel. Totally stumped !!! Change an Oil Pressure switch and BAM, now it's up and running. WTF??? How does that make sense? Because it's an engine protection device, if you lose oil pressure for no oil, or oil pump gone bad, rather than let you seize your motor or put a rod thru the block, it disables the fuel pump and shuts the motor down so you don't hurt the motor. Lots of things on vehicles that don't make sense, make great sense once you find out the purpose.