Code
P0749
MERCEDES-BENZ
P — Powertrain
Pressure Control Solenoid Intermittent
Views:
UK: 18
EN: 18
RU: 15
AI status
Completed
Completed
100%
Causes
- Damaged, corroded or loose solenoid electrical connector
- Broken, chafed or intermittent wiring (open/short/intermittent contact)
- Faulty pressure control solenoid (intermittent internal fault)
- Poor or intermittent power supply or ground to TCM/solenoid
- Contaminated or low transmission fluid affecting solenoid operation
- Faulty Transmission Control Module (TCM) or engine control module interface
Symptoms
- Transmission harsh/erratic shifts or slip
- Delayed shifting or stuck in a gear (limp mode)
- Intermittent or recurring transmission-related warning lamp
- Occasional loss of drive or reduced performance under load
- Transmission fluid leaks or abnormal fluid smell/colour if contamination
What to check
- Retrieve freeze frame and full code list with a capable scan tool; note any related transmission codes
- Monitor live data and run active tests for pressure control solenoid using dealer-level scan tool if available
- Visually inspect solenoid connector and wiring for corrosion, looseness, chafing or heat damage
- Wiggle test harness/connectors while monitoring live signal to reproduce intermittent behaviour
- Check transmission fluid level and condition (contamination, burnt smell)
- Measure supply voltage and ground at solenoid connector with ignition ON and during activation
Signal parameters
- Coil resistance: typically low single-digit to tens of ohms (manufacturer spec required) — measure cold and hot
- Supply voltage: approx. battery voltage on supply pin with ignition ON (verify 11–14 V, per spec)
- Control signal: PWM duty cycle from TCM; 0–100% depending on commanded pressure
- Command switching frequency: varies by vehicle/TCM (refer to Mercedes technical data)
- Expected behaviour: stable, repeatable PWM and solenoid current when commanded — intermittent drops or spikes indicate wiring/connector issues
Diagnostic algorithm
- Connect a full-function scan tool, read and save codes and freeze-frame data. Note any related transmission or electrical codes.
- Clear codes, perform a controlled road test while monitoring pressure control solenoid status and related parameters to try to re-create the fault.
- Visually inspect the solenoid connector and harness for damage, contamination, bent pins or corrosion. Disconnect, inspect and reseat connector.
- Wiggle the wiring and connectors while monitoring live data to identify intermittent changes. Also check connectors at the TCM.
- With ignition OFF, measure solenoid coil resistance between the solenoid pins. Compare to Mercedes specification. Re-measure after several cycles (hot) to detect intermittent coil faults.
- With ignition ON and engine off (or per safe shop procedure), backprobe the connector: verify constant supply voltage and a good ground. Command the solenoid on with scan tool and observe voltage/duty cycle and current draw.
- If supply/ground are good and the TCM commands appear correct but the solenoid behaves intermittently, swap or bench‑test the solenoid (or install a known-good solenoid) to confirm.
- If wiring or connector faults are found, repair with proper crimp/solder joints, dielectric grease and secure routing. Replace damaged harness sections as required.
- If wiring/solenoid checks good, consider TCM software updates or TCM replacement only after confirming external circuitry and solenoid operation.
- After repair or replacement, clear codes and perform transmission adaptation/relearn procedures per manufacturer. Road test to confirm repair and ensure no recurrence.
Likely causes
- Corroded connector pins at solenoid or TCM causing intermittent contact
- Wiring harness damage where it flexes (harness rubs or connector movement)
- Solenoid internal intermittent failure (opens/closes intermittently)
- Weak/poor ground at transmission or vehicle ground strap
- Contaminated fluid or debris preventing reliable solenoid movement
Fault status
Status
P0749 — Pressure Control Solenoid Intermittent: Transmission control module has detected an intermittent electrical/signal fault on the pressure control solenoid circuit. Possible wiring, connector, solenoid or control module issue; may cause harsh or incorrect shifting.
Repair difficulty: Medium
Diagnostic time: 1.0 - 3.0 hours
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