Code
P1787
DAEWOO
P — Powertrain
Hydro system mal
Views:
UK: 6
EN: 8
RU: 7
AI status
Completed
Completed
100%
Causes
- Low or contaminated transmission fluid
- Clogged transmission filter or passages
- Faulty hydraulic pump or torque converter pump
- Worn or damaged valve body or internal hydraulic seals
- Stuck or failed shift/control solenoids
- Faulty pressure switch or pressure sensor
Symptoms
- Transmission warning lamp (MIL) or TCM fault lamp illuminated
- Harsh, delayed or no shifts; gear hunting or slipping
- Transmission operating in limp/reduced-gear mode
- Unusual whining or grinding noises from transmission
- Visible transmission fluid leak or burnt-smelling fluid
- Erratic line pressure or loss of drive under load
What to check
- Read and record all freeze-frame data and pending/confirmed codes with a capable scan tool
- Visual inspection for fluid leaks, damaged connectors and wiring at the transmission and pressure sensor/solenoids
- Check transmission fluid level, color and smell (burnt/dark indicates contamination)
- Inspect/replace transmission filter and screen if serviceable
- Perform electrical checks on solenoids and pressure sensor (voltage, resistance, connector continuity, grounds)
- Perform hydraulic line pressure test at the manufacturer test port and compare to spec
Signal parameters
- Main/line hydraulic pressure: varies by model — typically in the range of ~80–300 psi (550–2000 kPa) depending on gear and load (consult service manual for exact spec)
- Pressure regulator/control pressure: varies with gear and throttle — compare to TCM commanded pressure
- Shift/control solenoid: 0–12 V supply; PWM duty cycle 0–100% when commanded (frequency often 50–300 Hz depending on design)
- Pressure switch: on/off switching or analog voltage/ohm values as specified in service data
- Solenoid coil resistance: typically a few ohms to low tens of ohms (check exact spec in manual)
Diagnostic algorithm
- Connect a full-function scan tool. Read and record all transmission codes and freeze-frame data. Clear codes and attempt to re-create the fault under controlled conditions.
- Check transmission fluid level, condition and smell. If fluid is low, inspect for leaks before topping to proper level.
- Visually inspect wiring, connectors and grounds for the pressure sensor, solenoids and TCM. Repair any damaged wiring or poor connections.
- Replace or clean transmission filter/screen if serviceable and the fluid is contaminated; then retest.
- Perform electrical tests: measure solenoid coil resistance and continuity; check sensor voltages and grounds with key on. Compare to service specs.
- Install a transmission hydraulic pressure gauge at the factory test port. With the engine idling and under specified load/gear, record line and regulator pressures and compare to spec.
- Use the scan tool to command each solenoid and observe pressure response and current draw. Identify sticking or non-responsive solenoids or valves.
- If pressure is low or fluctuating and electrical checks pass, suspect pump, torque converter, or internal leakage; consider further disassembly (valve body inspection) or bench-testing the pump/TCU.
- If valve body or solenoids are faulty, replace or rebuild as required. If pump or internal mechanical damage is found, plan for overhaul or replacement of transmission components.
- After repairs, refill with correct fluid, clear codes, perform calibration/learn procedures if required, and verify proper operation via road test and scan tool monitoring.
- Safety note: use proper supports and follow safe lifting procedures; hydraulic pressure tests must be done with appropriate adapters and PPE.
Likely causes
- Low fluid level or severe fluid contamination
- Clogged filter/strainer reducing flow
- Failed hydraulic pump or internal leakage in torque converter
- Sticking/failed shift solenoid or valve body problem
- Pressure sensor/switch failure or wiring fault
Fault status
Status
Hydraulic system malfunction detected — transmission hydraulic pressure/control out of range.
Repair difficulty: Medium
Diagnostic time: 2.0 - 5.0 hours
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