Cuando el Módulo Relé de Bomba de Combustible Mercedes W124 Deja de Funcionar
The fuel pump relay on the Mercedes W124 is a compact electromechanical component mounted in the engine bay fuse and relay box. It controls the switched 12-volt feed to the in-tank fuel pump. When the relay fails — whether through contact burn, coil breakdown, or internal corrosion — the fuel pump receives no power, and the engine either refuses to start or cuts out while driving. The relay itself is not expensive, but misdiagnosis leads many owners to replace fuel pumps, injectors or ECUs before anyone checks the relay.
How the Fuel Pump Relay Works on the W124
When the ignition is switched on, the engine control unit sends a signal that energises the relay coil. The relay contacts close, connecting battery voltage to the fuel pump circuit. On early W124 models with the KE-Jetronic or HFM-SFI injection systems, the relay is part of a broader overvoltage protection relay assembly. On later M104 and M111-engined variants, a dedicated fuel pump relay handles this function separately.
The relay is typically located in the engine compartment relay box on the right side of the bulkhead, though exact positioning varies by model year and market. The socket contacts on older W124s are prone to oxidation, meaning a relay that measures correctly in isolation may still fail to deliver current reliably through a corroded socket.
Symptoms That Point to the Relay
- Crank, no start: The engine turns over normally but never fires. Fuel pressure at the injector rail reads zero because the pump is not running.
- Intermittent stalling: The engine cuts out at speed or at idle, often restarting after a short wait — classic relay contact fatigue behaviour.
- No pump prime sound: When the ignition is first switched on, a correctly functioning pump runs briefly to prime the rail. If this is absent, the relay or pump itself is the first area to investigate.
- Heat-related failure: Stalling occurs after the engine is warm but the car restarts once cooled. Relay coils and contacts expand and contract with heat, which can cause intermittent open circuits.
None of these symptoms alone confirm the relay. Fuel pressure testing and relay substitution are the two quickest ways to narrow the fault to this component.
Códigos de avería relevantes
The W124 uses older Bosch diagnostic systems. On vehicles with the diagnostic socket, a code reader may log the following:
| Código | Descripción |
|---|---|
| 1-1-1 | No fault stored (but fuel pump circuit not self-diagnosing on early KE systems) |
| 2-3-1 | Fuel system — lean mixture condition (secondary symptom of low fuel pressure) |
| 3-1-1 | Oxygen sensor signal — loss of fuel delivery causes lean codes on closed-loop systems |
On many early W124 variants with KE-Jetronic, the fuel pump relay circuit is outside the ECU’s direct self-diagnostic scope. No fault code will be stored for a relay that has simply failed to close. The absence of codes does not mean the relay is healthy.
Confirming the Diagnosis
First, listen for the pump. Turn the ignition key to position two without cranking and listen at the rear of the vehicle near the fuel tank. A functioning relay closes and the pump audibly primes for one to two seconds. Silence here is significant.
Second, substitute the relay. W124 relay positions often share the same footprint as other relays in the box. Temporarily swapping a known-good relay of the same type into the fuel pump socket is a fast, zero-cost test. If the pump primes and the engine starts, the original relay is confirmed faulty.
Third, check the socket. Even with a new relay, corroded or bent socket terminals will cause the same symptoms. Clean the contacts with electrical contact cleaner and verify terminal tension before declaring the relay permanently failed.
OEM Reference Numbers
The following OEM numbers apply to fuel pump relay variants on the W124 platform. Always confirm against the label on the existing relay and the relay box diagram:
- 0025422619 — Overvoltage protection relay unit (early W124, KE-Jetronic)
- 0015459405 — Fuel pump relay, later HFM-SFI equipped models
- 0025421119 — Relay variant for 6-cylinder M104 engine applications
- 0015453405 — Relay variant for 4-cylinder M111 engine applications
- 0025420719 — Combination relay used in certain market-specific W124 builds
The W124 was built from 1984 to 1995 and received several injection system updates during that period. The relay type is tied to the injection system, not simply the chassis number. When sourcing a replacement, confirming the injection system variant — KE-Jetronic, LH-Jetronic or HFM-SFI — is essential for correct part selection.
Vehículos afectados
All W124 body variants are potentially affected: the saloon (W124), estate (S124), coupe (C124) and cabriolet (A124). Engine variants range from the 200E four-cylinder through to the 300E-24, 320E and the E 500 with the M119 V8. The M119-engined E 500 uses a different power supply architecture, so relay identification must be specific. Six-cylinder M104 models from approximately 1992 onwards are the most frequently encountered in workshops for this fault, largely because of age and relay contact fatigue.
Preguntas frecuentes
Can the W124 fuel pump relay cause the engine to start and then immediately stall?
Yes. A relay with marginal contact resistance may close briefly when first energised, allowing the engine to fire, then drop out as the contacts heat or vibrate. This produces a characteristic pattern where the engine catches for one or two seconds before dying. Substituting the relay is the fastest test for this specific symptom.
Is the fuel pump relay the same component as the overvoltage protection relay?
On early W124 models with KE-Jetronic injection, the overvoltage protection relay incorporates the fuel pump relay function within a single housing. On later HFM-SFI models, these are separate components. The relay box diagram on the underside of the fuse cover identifies which applies to a specific vehicle.
Can a corroded relay socket cause the same symptoms as a failed relay?
Absolutely. The socket terminals on W124 relay boxes are over 30 years old on most surviving vehicles. Green oxidation or bent terminals prevent the relay from making proper contact even when the relay itself is serviceable. Always inspect and clean the socket when replacing any relay on a vehicle of this age.
Does the replacement fuel pump relay need any programming or coding?
No. The fuel pump relay is a passive electromechanical component. It requires no programming, coding, or adaptation after fitting. Substitute the correct OEM reference, ensure the socket contacts are clean, and the system should function immediately.
Should the fuel pump itself be tested before condemning the relay?
Yes, ideally both are tested. Bypassing the relay to apply direct 12-volt power to the pump circuit confirms whether the pump is mechanically operational. If the pump runs on direct power but not through the relay, the relay is the confirmed fault. If the pump fails to run on direct power, the pump itself or its wiring should be investigated alongside the relay.
Encuentra componentes y módulos electrónicos OEM probados en: https://giantparts.eu/
