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Espar Hydronic Booster Heater — Diagnosis & Repair

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The factory Espar Hydronic booster heater (DW5Z/D5 variants) fitted to 2000–2006 T1N Sprinters is a diesel-fired coolant heater designed to quickly warm the engine and cabin. It is a known reliability weak point, with common failures including a seized combustion fan, failed electric coolant circulation pump, faulty flame sensor, and ECU/control module faults.

Symptoms

  • Heater completely fails to start — no fan spin-up, no relay clicks, and no ignition attempt [4]
  • Heater starts but shuts itself down after running for a short period, often an overheat lockout caused by insufficient coolant flow [0, 22]
  • Loud howling or screeching noise from the heater during operation, typically indicating a dry or worn combustion fan bearing [10, 13, 17]
  • Grinding noise from the heater unit during attempted startup [6]
  • Large white cloud of unburned diesel and strong diesel smell with no successful ignition [16]
  • Heater worked previously but now produces weak or no heat despite appearing to attempt startup [15]
  • Unit appears completely dead — no response — after an extended period of non-use [3, 5]
  • Heater runs when the engine is running but refuses to run engine-off, possibly indicating a failed electric coolant circulation pump [21]

Causes

  • Seized or stuck combustion fan/turbine motor — the single most common mechanical failure, especially after extended periods of non-use or on high-salt-exposure vehicles [4, 5]
  • Failed electric coolant circulation pump (ECP) on the firewall under the brake booster — a very common wear item; the pump may spin audibly but still fail to move coolant, triggering an overheat lockout [0, 22]
  • Faulty flame sensor — causes the unit to fail to start or locks it out; resistance at ambient temperature should be approximately 1.1 kΩ [0, 5]
  • Faulty ECU/control module — a significant failure mode; at least one-third of D5WZ units encountered in the field have been found to have faulty control modules, sometimes traceable to corroded transistor solder joints driving the fan circuit [23, 26]
  • Blocked or fouled burner chamber / fuel wick — carbon and debris accumulation in the D-chamber can prevent reliable ignition [0]
  • Faulty temperature sensor(s) — short or open circuit faults on either heat exchanger sensor can prevent startup or cause shutdown [29]
  • Corroded or dissolved brush wires inside the combustion fan motor — found on teardown of non-operational units [27, 28]
  • Cracked or corroded coolant housing — road salt can cause aluminum corrosion that cracks the outer casting, leading to coolant leaks [9]

Diagnosis

  • Connect a 7-day timer with diagnostic wire to the Espar k-line connector — the timer will display specific fault codes when the unit fails to start, confirming whether the ECU is alive and identifying the failing component [3, 6]
  • Check combustion fan rotation first: remove the square plastic cover on top of the Espar unit and verify the turbine spins freely by hand before attempting any other diagnosis [5, 21]
  • Verify the electric coolant circulation pump is actually moving coolant (not just spinning): press the REST button to activate the ECP and physically confirm coolant flow by feel or by briefly disconnecting a hose — an audible pump does not guarantee adequate flow [0, 22]
  • Measure glow pin resistance: at ~70°F ambient, a good glow pin reads approximately 0.9 Ω [0]
  • Measure flame sensor resistance: at ~70°F ambient, a good flame sensor reads approximately 1.1 kΩ [0]
  • Measure heat exchanger temperature sensor resistances: at ~70°F ambient, cold sensor ≈ 9.27 kΩ and hot sensor ≈ 9.03 kΩ are considered in-range [0]
  • Apply 9V directly to the combustion fan motor to verify it spins freely; a fan that spins on direct power but throws an open-circuit blower fault (code 00802) may indicate a faulty ECU transistor rather than a bad motor [26, 29]
  • Check for fault codes using MB Star/DRB3 diagnostics or a compatible Autel scanner if no 7-day timer is present; a locked-out unit may require a scanner reset if pulling the fuse does not clear the lockout [6, 29]
  • Verify fuel supply: ensure the tank is at least one-third full (the Espar fuel pickup is positioned above the tank bottom), and if the unit has sat unused, disconnect the small fuel line and confirm fuel is present [21]

Repair

The Espar Hydronic on the T1N is repairable by a skilled DIYer, but many of the sub-repairs — fan lubrication, ECP replacement, flame sensor swap — are well within reach of an owner comfortable with mechanical work. Full removal and bench work is strongly recommended over in-van repairs, as limited access increases the risk of breaking fasteners and delicate components. Corroded fasteners on intake and exhaust tubes are a known challenge, particularly in salt-belt states, so penetrating oil and heat should be used liberally. The ECU can be repaired by re-soldering the fan-driver transistor pins, but this requires electronics skill. An Espar that has been locked out by repeated overheat faults may need a diagnostic scanner to reset, not just a fuse pull.

Read first

  • The Espar burns diesel fuel inside the combustion chamber — ensure there are no fuel leaks at the metering pump line or fuel fittings before attempting a start after any service.
  • Coolant hoses must be properly clamped before disconnecting to avoid coolant loss and potential scalding from hot coolant if the engine has been recently run.
  • The combustion fan turbine is extremely fragile and press-fit on the motor shaft — applying excessive force during removal will break it; always heat the hub first and work gradually [27, 28].
  • The ECU k-line diagnostic connector should only be connected to one diagnostic device at a time (e.g., the 7-day timer OR a scan tool, not both simultaneously) [6].
  • Fan motor magnets are retained by a large spring clip to the outer housing — be careful not to break them during disassembly [10, 11].
  • When bearing-lubing with the oil-fill method, ensure all excess oil is drained and blown out before reassembly to prevent oil contamination of the combustion chamber [10, 11].

Tools

  • 7-day timer with diagnostic wire (for fault code retrieval — strongly recommended)
  • Hose pinch pliers or two pieces of wood + vice grips (coolant hose clamping)
  • Penetrating oil (e.g., PB Blaster) and heat source for corroded fasteners
  • Spray silicone lubricant (thin viscosity) for fan bearing lubrication
  • Carburetor cleaner for burner chamber cleaning
  • 500W halogen work light (for heating turbine fan hub prior to removal)
  • Thin metal blades / butter knife / putty knife (for separating press-fit turbine from shaft)
  • Small bearing puller (if replacing fan motor bearing)
  • Soldering iron and solder (if repairing ECU transistor connections)
  • Multimeter (for resistance checks on glow pin, flame sensor, and temperature sensors)
  • 9V battery (for direct fan motor spin test)
  • MB Star / DRB3 or compatible Autel scanner (for fault code reading and lockout reset)
  • Never-seize compound (for intake and exhaust tube reassembly)
  • Modeling clay (for oil-fill bearing lubrication dam method)
  • JB Weld and fine mesh screen (for coolant housing crack repair if applicable)

Steps

  1. Before removal, connect a 7-day timer diagnostic wire to confirm the ECU is alive and retrieve fault codes to focus your repair effort [3, 6].
  2. Clamp off the coolant hoses to the Espar using hose pinch pliers or two pieces of wood and vice grips, then disconnect the coolant inlet and outlet hoses [17].
  3. Disconnect the fuel line, electrical connector, and intake/exhaust tubes. Apply penetrating oil and heat to stubborn fasteners on the intake and exhaust tubes — these are a known problem especially in high-salt regions [7, 8].
  4. Remove the Espar unit completely from the van and move to a bench for all further work [7, 8].
  5. Remove the square plastic cover on top of the unit to access the combustion fan/turbine. Check that it spins freely by hand [5, 21].
  6. If the fan is stiff or seized: apply spray silicone lubricant between the fan and housing, work the turbine back and forth by hand until it rotates more freely, then attempt to run the unit [17, 18, 21].
  7. For a howling bearing: build a clay dam around the turbine area with the blower cover removed, fill with oil, allow it to migrate through the bearing, then drain and blow out the excess [10, 11].
  8. If the fan must be fully removed for motor replacement: heat the turbine with a 500W work light until very hot, then drive thin blades (e.g., putty knives) between the turbine and shaft progressively around the circumference until the turbine pops free — work carefully as the turbine is fragile and press-fit [27, 28].
  9. Once the motor is exposed, open the casing by unbending the crimp tabs, lift the top section, unclip and slide out the brush/stator assembly, and inspect the brush wires for corrosion or dissolution [27, 28].
  10. To service the burner chamber: disassemble and spray carburetor cleaner into the D-chamber and fuel wick to clear carbon fouling, then reassemble [0].
  11. To replace the flame sensor: disconnect and remove the old sensor, install the replacement (confirm correct resistance ~1.1 kΩ at room temperature before installation), and replace all seals on reassembly [0, 12].
  12. If the ECU is suspect (fan spins freely on 9V but unit will not drive the fan): attempt repair by re-soldering the transistor pins on the fan-driver circuit of the control board [26].
  13. Replace the electric coolant circulation pump on the firewall (under the brake booster) if it is not moving adequate coolant; confirm actual flow after replacement, not just audible operation [22].
  14. On reassembly, apply Never-seize to the intake and exhaust tube fittings to prevent future corrosion seizure [7, 8].
  15. After reinstallation, run the Espar with the engine running first to verify operation before testing engine-off mode [21].
  16. Perform regular maintenance exercise runs to keep the fan and pump from seizing during long periods of non-use [0, 2].

Parts

Plain part names — affiliate links and pricing are coming in a later update.

  • Electric coolant circulation pump (ECP) — firewall-mounted unit under brake booster (exact replacement for 2003–2006 T1N OM612/OM647)
  • Flame sensor (Espar D5/DW5Z)
  • Combustion fan / turbine blower motor assembly (Espar D5/DW5Z)
  • Temperature sensor(s) — heat exchanger cold and hot sensors
  • Glow pin / glow plug (Espar D5/DW5Z)
  • Metering (dosing) pump (M19) — if fault codes 00701/00702 are present
  • ECU / control module (E18/ZHE) — if transistor repair is not feasible
  • O-ring set for inner coolant body section seal
  • Fan motor bearing (measure spacing before removal if replacing)
  • JB Weld + fine nickel mesh screen — for coolant housing crack repair
  • Never-seize compound
  • 7-day timer with diagnostic wire — strongly recommended upgrade for ongoing diagnostics

Related forum threads

From the manuals

  • Mercedes fault-code reference

    "VQltage supply to ZHE [CHM] control unit (E18) is faulty 00301 03 01 Undervoltage (voltage >10.5V) or wiring is damaged. c. Check Fuse F9. d . Inspect vehicle charging system. Alternator or voltage regulator is faulty. a . Inspect vehicle charging system. Alternator or voltage 00302 03 02 Overvoltage {voltage <15.5V) regulator is faulty. b. ZHE [CHM) control unit is faulty. 00400 04 00 Premature flame detected a . Flame sensor is faulty. b . ZHE [CHM] control unit is faulty. 00501 05 01 Flame sensor has short circuit a. Flame sensor is faulty. b. ZHE [CHM] control unit is faulty. 00502 05 02 F"

  • Mercedes fault-code reference

    "Cable from (terminal 85) ground circuit Kn/4 conditioning auxiliary fan (M45) to ground point W5 has a short circuit to voltage(+) or open circuit. d. Relay for auxiliary fan (K77) is faulty. e. Fuse F119 is faulty. f. Auxiliary fan motor (M45) is faulty or fan has excessive debris preventing it from operating. D The Complete Sprinter Fault Code Guide -HVAC Systems --Automatic Climate Control Daimler Dodge DRB3 Component or variable monitored Fault Sub Fault Sub Code Fault Code Fault (MB Component Number) B100C 015 900C OF Self test failure HZR [ATC] control unit (A81): Heating mode Self-test"

  • Workshop manual (2000–2003)

    "As air flows over the fins of the evaporator, the humidity in the air condenses on the fins, and the heat from the air is absorbed by the refrigerant. Heat absorption causes the refrigerant to boil and vaporize. The refrigerant becomes a low-pressure gas before it leaves the evaporator. The evaporator cannot be repaired and, if faulty or damaged, it must be replaced. Fig. 4 A/C Compressor - Refrigerant Lines 1 - A/C COMPRESSOR 2 - O-RING SEAL 3 - SUCTION LINE 4 - BOLT 5 - RETAINING CLIP 6 - BOLT 7 - DISCHARGE LINE 8 - O-RING SEAL 24 - 54PLUMBINGVA A/C DISCHARGE LINE (Continued) REMOVAL WARNING"

  • Workshop manual (2004–2006)

    "Inspect the blend-air door(s) for proper operation and sealing. Repair if required. The low side pressure is normal or slightly low, and the high side pressure is too low. 1. Low refrigerant system charge. 1. See Refrigerant System Leaks in this group. Test the refrigerant system for leaks. Repair, evacuate and charge the refrigerant system, if required. 2. Refrigerant flow through the A/C evaporator is restricted. 2. See A/C Evaporator in this group. Replace the restricted A/C evaporator, if required. 3. Faulty A/C compressor.3. See A/C Compressor in this group. Replace the A/C compressor, if"

  • Workshop manual (2000–2003)

    "(6) Install the instrument panel (Refer to 23 - BODY/INSTRUMENTPANEL/INSTRUMENT PANEL ASSEMBLY - INSTALLATION). HVAC HOUSING REMOVAL The heater housing assembly must be removed from the vehicle and the two halves of the housing separated for service access of the heater core, evaporator and each of the various mode doors. WARNING: ON VEHICLES EQUIPPED WITH AIRBAGS, DISABLE THE AIRBAG SYSTEM BEFORE ATTEMPTING ANY STEERING WHEEL, STEERING COLUMN, OR INSTRUMENT PANEL COMPONENT DIAGNOSIS OR SERVICE. DISCONNECT AND ISOLATE THE BATTERY NEGATIVE (GROUND) CABLE, THEN WAIT TWO MINUTES FOR THE AIRBAG SY"

  • Workshop manual (2004–2006)

    "(2) Install the screws that secure the HVAC housing to the dash panel. Tighten the screws to 4.5 N·m (40 in. lbs.). (3) Connect the HVAC housing wire harness connectors and install the wiring harness to the bracket. Fig. 8 Heater Housing 1 - HEATER HOUSING 2 - AIR TEMP SENSOR WIRE CONNECTOR 3 - WIRE HARNESS 4 - WIRE HARNESS CONNECTOR 5 - BOLT (4) 6 - EVAP TEMP SENSOR WIRE CONNECTOR 7 - EVAPORATOR DRAIN TUBE Fig. 9 HVAC Housing 1 - SCREW (12) 2 - UPPER HOUSING 3 - A/C EVAPORATOR 4 - EVAPORATOR O-RING SEAL (2) 5 - EVAPORATOR GASKET 6 - VENTILATION HOUSING GASKET 7 - LOWER HOUSING 8 - WIRING HARN"

Sources

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