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Home >> GMC >> 1987 >> Forward Control 7.4 W, Standard >> Repair and Diagnosis >> Transmission >> Automatic Trans >> Overhaul - THM 400 >> Testing >> Vacuum Modulator Check >> Vacuum Diaphragm Leak Check
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Vacuum Diaphragm Leak Check

Insert pipe cleaner into vacuum connector pipe as far as possible and check for transmission fluid. If fluid is found, replace modulator. Gasoline or water vapor may settle in vacuum side of modulator. If this is found without presence of fluid, modulator should not be changed. If vehicle is to be exposed to temperatures 10°F or below, modulator MUST be replaced.