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• In a Mercedes-Benz Vito (W639 and derivatives) the condenser / engine-cooling electric fans do not start when the A/C is selected if the control unit (Front-SAM/ECU) receives no “enable” signal, does not find battery power, or cannot drive the fan motors.
• The usual root-causes are:
– Blown high-amperage fan fuse F87 (60 A) or F57 (40 A) or a defective fan relay / internal SAM driver.
– Open-circuit or shorted fan motors / fan-control module (PWM pack).
– Faulty refrigerant high-pressure sensor (trinary transducer) or coolant-temperature sensor sending implausible data, so the ECU never commands the fans.
– Low or zero refrigerant charge (compressor inhibited, therefore fans never get the request).
– Corroded grounds, damaged harness near radiator support, or water-ingress into the Front-SAM.
– Climate-control / ECU software or stored DTC that suppresses fan activation until cleared.
System architecture
• W639 uses two brush-type electric fans bolted to the radiator shroud. Early builds switch the fans in two stages through relays and ballast resistors; later builds drive the fans steplessly via an on-board PWM control module supplied with a permanent 12 V feed (F87, F57).
• The decision logic sits in the Front-SAM; it receives:
– A/C pressure (three-wire analogue transducer, 0.5…4.5 V ≈ 0…30 bar).
– Engine coolant temperature (from ECU via CAN).
– Vehicle speed and A/C request (CAN from climate panel).
• Typical engagement thresholds:
– Stage 1 ≈ 15–16 bar or ≈ 100 °C coolant.
– Stage 2 ≈ 18–19 bar or ≈ 105 °C coolant.
• If any input is implausible, the SAM sets a DTC and inhibits the driver FET/relay → fans stay off, and the ECU may inhibit the compressor clutch as a secondary protection.
Supply-side faults (won’t spin even with direct 12 V command)
• High-current feed: Check F87 (60 A, engine-bay, right pre-fuse box) and F57 (40 A) with a multimeter, not by eye.
• Relay / power stage: Early relays (K9/1, K9/2) can burn contacts; PWM modules suffer cracked MOSFETs or overheated PCB.
• Grounds: G102 (behind left headlamp) and G108 (front right) must show < 0.1 Ω to battery negative during 30 A load.
Load-side faults (fans electrically dead)
• Fan motors seize when brushes wear – resistance either open (> 10 kΩ) or short (< 0.2 Ω). A 12 V battery jumper directly on the two fan pins is the quickest test (keep fingers clear).
• Harness fatigue: the twin 6 mm² +12 V cable sometimes chafes on the radiator support; verify continuity under wiggle.
Control-side faults (fans healthy but never commanded)
• Pressure sensor: compare scanner live data with mechanical manifold gauges; 0 bar or ≥ 32 bar reading with static engine OFF indicates sensor or wiring. Replacement part no. A000 905 29 00 (green three-pin Bosch type).
• Coolant sensor: if ECU reports 40 °C with hot engine the fans stay idle; test with 2.2 kΩ resistor (~90 °C).
• Refrigerant charge: static ≈ ambient °C in psi rule (e.g. 25 °C ≈ 25 bar absolute ≈ 3.6 bar gauge). Underfill or empty system keeps pressure < 2 bar → compressor and fans locked out.
• Software / DTC: Common codes—B1004 (fan motor open circuit), B10B7 (pressure sensor, value implausible). Clear codes after repair, or SAM continues to inhibit output.
Cabin blower vs. condenser fans
• The interior blower (under dash) is a different circuit (fuse F22 15 A and resistor pack/hedgehog). Make sure you are troubleshooting the correct fan set.
• Beginning with late-2012 production Mercedes replaced the discrete two-relay layout by an integrated PWM fan control which fails more often from thermal cycling.
• SAM water-ingress recalls: TSB LI83.30-P-051702 (seal upgrade) – symptoms include inoperative fans and random A/C shutdown.
• Aftermarket diagnostic tools (Launch X-431, Autel MaxiSys) now allow active “Fan Actuation Test” identical to STAR/Xentry, simplifying DIY verification.
• Why the fans may appear “dead” but are normal: at 10 °C ambient and low refrigerant pressure the logic never reaches the 15 bar threshold, so fans are legitimately off. Always read live pressure before assuming a fault.
• Analogy: The pressure sensor is like a thermostat in a home HVAC—if it never reports “too hot,” the external condenser fan never starts.
• Releasing R-134a or R-1234yf into atmosphere violates EU Regulation 517/2014; recovery must be done with certified equipment.
• High-amperage circuits (40–60 A) pose fire risk; disconnect battery ground before probing PWM module.
• Fuse numbering may vary on very early 2003 builds—always cross-check with VIN-specific WIS diagram.
• Some taxis retro-fitted aftermarket dual-fan kits wired directly to ignition; procedures above do not apply to modified vehicles.
• Daimler WIS documents AR83.30-P-6010W and PE83.30-P-2205A (fan circuit troubleshooting).
• Technical Service Bulletins LI83.30-P-051702 (SAM water ingress) and LI83.00-P-058989 (pressure transducer software).
• Study PWM control MOSFET failures in Infineon BTS6143D to understand thermal derating.
The condenser / radiator electric fans in the Mercedes Vito depend on three pillars: solid 12 V power through 40–60 A fuses, functioning fan motors, and correct sensor data processed by the Front-SAM/ECU. Loss of any of these—most often a blown fuse, seized motor, corroded wiring, or a faulty refrigerant-pressure sensor—prevents the SAM from driving the fans. A structured check of power, load, and control, supported by a scan tool, pinpoints the fault quickly while avoiding unnecessary parts swapping.
