Code
P1850
HUMMER
P — Powertrain
Brake Band Apply Solenoid Circuit
AI status
Completed
Completed
100%
Causes
- Open or short circuit in solenoid wiring
- Poor or corroded connector or pin at solenoid or TCM
- Faulty Brake Band Apply Solenoid
- Faulty transmission control module (TCM) or driver output
- Low or contaminated transmission fluid causing mechanical sticking
- Blown fuse or faulty relay in the transmission power feed
Symptoms
- Malfunction indicator lamp (MIL) or transmission warning lamp illuminated
- Abnormal shifting (harsh, delayed, or slipping) or stuck in a gear
- Reduced transmission performance or limp-home mode
- Unusual transmission noises when selecting gears
- Possible drivability issues under load
What to check
- Retrieve freeze frame and permanent codes with a scan tool; note transmission state when code set
- Visually inspect wiring and connectors at the brake band apply solenoid and TCM for damage, corrosion, or loose pins
- Check transmission fluid level and condition (color, smell, contamination)
- Check related fuses and relays that feed the transmission solenoid circuits
- Measure solenoid coil resistance with an ohmmeter at the solenoid connector (engine off, harness disconnected)
- Backprobe solenoid connector and monitor voltage/ground while commanding the solenoid ON/OFF with a scan tool
Signal parameters
- Solenoid coil resistance: typically a low-ohm value (commonly single-digit to a few tens of ohms). Compare to factory spec for exact value
- Command voltage: when commanded ON the control circuit should show battery voltage (approx. 11–14 V) or a switched ground depending on circuit design
- Control signal type: many transmission solenoids are driven by switched ground or PWM from the TCM (duty cycle variable)
- Expected PWM frequency: often in the tens to hundreds of Hz (refer to factory service data for exact frequency)
- No voltage or infinite resistance indicates open circuit; near-zero resistance indicates short to ground
Diagnostic algorithm
- Connect a professional scan tool, read all codes and freeze frame data. Note gear, speed, engine rpm when code set.
- Clear codes, perform a road test or reproduction attempt to confirm the code returns and capture live data while setting condition.
- Visually inspect solenoid connector, pins and wiring from solenoid to TCM. Repair any obvious damage before further testing.
- Check transmission fluid level and condition; top or service fluid if low/contaminated following manufacturer procedure.
- With ignition OFF, disconnect solenoid harness and measure coil resistance across solenoid terminals. Compare to factory spec. Replace solenoid if out of range.
- With harness connected, backprobe the solenoid connector. Command the solenoid ON with a scan tool and measure voltage at the power pin and control pin. Verify proper supply voltage and switching to ground (or appropriate control signal).
- If no supply voltage is present, trace power feed back to fuse/relay and repair as needed. If supply is present but control signal absent or incorrect, test continuity from TCM to solenoid and inspect TCM connector pins.
- Check continuity/insulation to ground to detect shorts. Repair wiring harness faults (splice, replace damaged section, or replace connector) as required.
- If wiring and solenoid test good but code persists, consider TCM driver failure—verify with manufacturer procedures and replace TCM only after wiring and solenoid proven good.
- After repairs, clear codes and perform a thorough road test and verify no reoccurrence. Recheck for additional stored transmission codes.
Likely causes
- Damaged harness or chafed wire to the band apply solenoid
- Corroded connector terminals at the solenoid or TCM
- Solenoid coil out of specified resistance range (open or shorted)
- TCM driver transistor failed or internal circuit fault
- Low transmission fluid level or internal hydraulic leakage preventing band application
Fault status
Status
Transmission Brake Band Apply Solenoid Circuit Fault — Electrical fault in the band apply solenoid circuit detected. Inspect solenoid, wiring, connectors, fuses/relays, and TCM.
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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