Code
P1870
HUMMER
P — Powertrain
Transmission Component Slipping
Views:
UK: 3
EN: 11
RU: 10
AI status
Completed
Completed
100%
Causes
- Low or contaminated transmission fluid (level, aeration, burnt fluid)
- Worn or damaged clutch packs or friction material
- Faulty torque converter or torque converter clutch (TCC) failure
- Faulty shift solenoids or valve body hydraulic faults
- Low hydraulic/line pressure (pump wear or internal leak)
- Damaged wiring or poor electrical connection to TCM/solenoids
Symptoms
- Engine RPM increases without proportional acceleration (slip)
- Delayed engagement from Park/Neutral to Drive/Reverse
- Harsh, soft, or inconsistent gear shifts
- Transmission fluid may smell burnt or appear dark/contaminated
- Transmission overheating or limp-home mode illumination
- Loss of fuel economy or reduced vehicle performance
What to check
- Connect a scan tool and record freeze-frame data and stored transmission codes; note gear commanded vs actual
- Visually inspect transmission fluid level, color, and smell (with engine warm and correct procedure)
- Perform a road/loaded test while logging vehicle speed, engine RPM, turbine speed (if available), and TCM commanded clutch/duty-cycle
- Check for additional DTCs related to solenoids, TCC, speed sensors, or line pressure
- Inspect wiring and connectors to TCM and transmission solenoids for corrosion, damage, or poor pins
- Inspect transmission pan for metal debris and torque converter/solenoid/module-related contamination
Signal parameters
- Engine RPM vs vehicle speed (turbine/ output speed mismatch)
- TCM commanded gear vs actual gear ratio
- Torque converter clutch (TCC) duty cycle or apply signal
- Shift solenoid state (On/Off or PWM duty) and response
- Transmission line/hydraulic pressure (static and under load)
- Transmission fluid temperature
Diagnostic algorithm
- Read and document all stored/ pending codes and freeze-frame data with a capable scan tool; clear codes and re-test to confirm repeatability.
- Check transmission fluid level, color and odor. If low or contaminated, top/replace fluid per service procedure and retest. Note: do not overfill.
- Road test while recording engine RPM, vehicle speed, commanded gear and turbine speed (if available) to confirm slipping behavior and conditions that trigger it.
- Check for related codes (e.g., solenoid faults, speed sensor faults, TCC faults) and address them first; verify wiring/connectors to TCM and solenoids are intact and grounded.
- Perform a hydraulic line pressure test (static and dynamic) to verify pump output and apply pressure. Compare to manufacturer spec; low pressure suggests pump, internal leak, or worn clutches.
- Test shift solenoids and TCC solenoid operation (electrical resistance and apply/actuation tests) and inspect valve body for sticking/contamination if solenoids fail or act inconsistently.
- If pressures and solenoids are within spec but slipping persists, remove pan/ filter for inspection of debris, clutch material, or metal particles and consider a deeper internal inspection (clutch packs, seals, pump, torque converter).
- If internal damage is confirmed or repair is beyond component replacement, recommend transmission rebuild or replacement; update/ reflash TCM if a software update applies.
- After repairs, clear codes and perform full verification road test and recheck for leaks and proper transmission temperatures.
Likely causes
- Low/contaminated fluid or fluid aeration
- Worn clutch pack or TCC slippage
- Faulty/sticking shift solenoid or valve body issue
- Low hydraulic line pressure due to pump or internal leak
Fault status
Status
Transmission slipping detected — possible clutch, TCC, hydraulic pressure, or solenoid/valve body issue. Further diagnostic and fluid/pressure checks required.
Repair difficulty: Hard
Diagnostic time: 3-8 hours
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