what is the sensor on the firewall, passenger side

what is the sensor on the firewall, passenger side that accepts a vaccuum line on an 85 chevy c10 with a 305. right now it's not hooked up and runs ok, when i plug it in, it doesn't seem to make a difference.

only thing that i could think of is the map sensor.

i don't think there is a map sensor in an 85

i don't know then...it was just a guess.

Definately wouldn't run right without a MAP hooked up. Can you get a picture of it or find where exactly the other end goes?

Do 85 trucks even have a VCM? I had an 1984 it didn't have notta 1 sensor. The chevy cars had them in 82 , but i'm not sure when the truck line got computerized.