Operation CHARM
: Car repair manuals for everyone.
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GMC Truck
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1989
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K 3500 Truck 4WD V8-454 7.4L
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Repair and Diagnosis
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Transmission and Drivetrain
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Automatic Transmission/Transaxle
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Description and Operation
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3L80/3L80HD (400/475) Automatic Transmission
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Hydraulic System, Brakes
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Functions of Valves & Hydraulic Control Units
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Vacuum Modulator Valve
Vacuum Modulator Valve
Provides modulator pressure that senses engine torque and vehicle speed. The vacuum modulator speed is used to vary the shift points according to throttle opening and to raise line pressure proportional to engine torque.