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.

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• 1991 Gmc Sierra Repair Manual

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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. It's time to speak out for your right to repair We have a chance to guarantee our right to repair electronic equipment—like smartphones, computers, and even farm equipment. This is a once-in-a-generation chance to protect local repair jobs—the corner mom-and-pop repair shops that keep getting squeezed out by manufacturers. Join the cause and tell your state representative to support Right to Repair.

1991 Gmc Sierra Repair Manual

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