Can anyone tell me what this vacuum switch (?) is ??
 

This appears to be a vacuum switch with wires on one side an a vacuum port on the other... Mounted on the firewall btween the brake booster and the power junction block.

anyone know what it does??

Re: Can anyone tell me what this vacuum switch (?) is ??
 

I think that's the tip in switch for the lock up trans convertor be it a 700r4 or a th350c.

The 84 dad has had that one and one on the PS firewall forget what the other was for but one for sure was the tip in switch.

Re: Can anyone tell me what this vacuum switch (?) is ??
 

yes, it is the lock up torque converter switch for a 700r4,th350c, etc.
if you are running either of those transmissions you should hook it back up to prolong the life of your trans.

Re: Can anyone tell me what this vacuum switch (?) is ??
 

If the truck was or is a diesel, I think it is a special "LOW Vaccum" switch special to the diesel, to possibly the tranny or something else.

Re: Can anyone tell me what this vacuum switch (?) is ??
 

TH-350C...!!!!!

I didnt know there was such a beast!!!
After you guys suggested it, I did a little research, and you are right...!
It didnt dawn on me when I saw the electrical connection on the side of it.

I normally play with much older cars!!

Thanks guys!

Re: Can anyone tell me what this vacuum switch (?) is ??
 

Yes it is the lockup for the TH350C. You do not have to hook it up, if you hook it up it will allow you to run 300 - 500 RPM less when you are running highway speeds. It will not hurt the transmission if you do not run it, it will just act as a normal TH350.

Re: Can anyone tell me what this vacuum switch (?) is ??
 

Might as well use it...! Save a little on gas....

Re: Can anyone tell me what this vacuum switch (?) is ??
 

not having it hooked up does let it function like a regular th350 but it creates more heat in the transmission. heat and auto trannys dont work well together. it is worth it to hook it back up for gas mileage and the life of your trans. it is just a vacuum line from the switch to the manifold vacuum supply.

Re: Can anyone tell me what this vacuum switch (?) is ??
 

i have the TH350C gas mileage is better with it hooked up

Re: Can anyone tell me what this vacuum switch (?) is ??
 

Since you already have that there, it would work to control the TCC in a 700R4 also. A 700 would also give you a lower first and 2nd gear, as well as overdrive. I mention this since you are on the fence for a trans.