Locate vacuum hose by transmission
Locate vacuum line to transmission
Repair guides and support for C/K pickup trucks sold by General Motors under the GMC brand, including the GMC Sierra.
The year range is pretty non specific.
If the truck has a vacuum hose to the transmission (to the vacuum modulator), the line will be a metal tube attached to the large tube that the transmission dipstick is in. It will come up and connect to a hose that will run to the base of the carburetor.
The function of this line and the modulator it goes to, is to give the transmission some sense of how much throttle is being applied during a shift to make the shifts softer or harder depending on load. The transmission will work without this line, but the shifts will be hard as if the throttle was all the way to the floor.
If the hose is broken or missing, there will also be a vacuum leak at the carburetor, causing a rough/high idle and possible an intermittent backfire.
If the truck is new enough, the transmission will be electronic and not have a vacuum modulator at all, and will have a plug with several wires connected to it on the drivers side.
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