Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1991JIMMY 5.7 K, AUTOMATIC, 4L60/MD8REPAIR AND DIAGNOSIS (SINGLE PAGE)TRANSMISSIONAUTOMATIC TRANSAUTOMATIC TRANSMISSION - OVERHAUL - 4L80-ETESTINGHYDRAULIC PRESSURE TESTSNOTES
1991 GMC Jimmy 5.7 K, Automatic, 4L60/MD8
Hydraulic Pressure Tests: Notes
1991 GMC Jimmy 5.7 K, Automatic, 4L60/MD8SECTION Notes
CAUTION:
Parking and service brakes must be applied throughout hydraulic pressure test. Factory recommends using TECH 1 scan tool to test line pressure. Total time for testing should not exceed 2 minutes.
- Before performing hydraulic pressure tests, check fluid level and condition. Check manual control linkages for correct adjustment. See appropriate TRANSMISSION SERVICING article. Ensure engine is properly tuned.
- Connect a tachometer to engine and an oil pressure gauge to line pressure test port. See Figure. Compare line pressure to force motor current draw. See FORCE MOTOR VS. LINE PRESSURE .
NOTE:
Hydraulic pressure is controlled by pump output and regulated by computer-controlled force motor. Line pressure is boosted (pressure is doubled) by force motor in the "R" selector position.
RENDER: 1.0x
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.