W126 won't start? Fuel pump relay 0025455105

The car sat overnight, the battery is fully charged, the starter motor turns over strongly – and yet the engine still isn't getting any fuel. Anyone with a 260 SE, 300 SE or 300 SEL who switches on the ignition and no longer hears the brief whirr of the fuel pump usually looks at the pump, filter or injection first. But the control sits one station earlier: in the relay box in the engine compartment, at the fuel pump relay of the KE-Jetronic.

This relay is not a simple on-off switch. It enables the pump depending on engine speed and cuts it off as soon as there is no speed signal – a safety function so that no fuel is delivered while the engine is stopped. During starting it also controls the cold start valve. If the relay fails, the engine is left without fuel even though the pump itself is perfectly fine.

Symptoms

Symptom	Likely cause
Engine cranks fine but doesn't start; the fuel pump cannot be heard when the ignition is switched on	Fuel pump relay no longer enables the pump
Vehicle stalls while running without warning, restarts after a pause	Intermittent contact in the relay, the pump control drops out at times
Relay clicks audibly, but the pump still doesn't run	Control side works, the load contacts no longer switch through
Poor cold start, engine starts normally at operating temperature	The relay also controls the cold start valve during starting – rule out the valve first: Cold start valve: testing and replacing
After several hours of standing, long cranking, then immediate start	Not the relay – missing residual pressure in the system: Check the fuel accumulator (W116, W126)
Poor hot start, fine when cold	More likely the system pressure regulator: KE-Jetronic hot-start problems
Idle hunts or the engine stalls while the pump audibly keeps running	More likely the supply to the injection electronics: Checking the overvoltage protection relay

Causes

The relay switches a load that draws considerably more current when starting up than in continuous operation. Every switching operation stresses the contacts, and after several decades and tens of thousands of starts they are burned or welded. The typical intermediate state: the relay audibly pulls in, but the contact resistance on the load side is so high that the pump no longer starts up reliably.

Then there is the installation location. The relay box sits in the engine compartment and goes through decades of heating and cooling cycles, plus moisture and road salt. Solder joints on the board age under these cycles, and plug pins and socket contacts oxidize. In a vehicle that still has the original relay from first delivery, the component is therefore a plausible suspect – it is confirmed by testing, not by age.

Testing it yourself

1. Inspect the relay box and plug contacts. Remove the cover, pull the fuel pump relay and check pins and socket for verdigris, corrosion and burn marks. Clean the contacts, plug the relay back in firmly, and try starting. It costs nothing and resolves some cases right here.
2. Listen at the pump. In a quiet environment, switch on the ignition and listen for the brief start-up sound of the fuel pump – ideally with a second person at the rear of the car. If it stays silent, the control is a stronger lead than the pump itself.
3. Check fuses and supply. Use a multimeter at the relay socket to check whether constant positive and switched positive arrive at all and whether the ground is clean. If voltage is already missing here, the fault lies upstream of the relay: fuse, wiring or ground point.
4. Keep the speed signal in mind. Because the relay deliberately switches the pump off when the speed signal is missing, a failed signal looks exactly like a defective relay from the outside. Rule out this branch before ordering a spare part.
5. Bridge the load side and compare the number. Following the wiring diagram for your build level, briefly bridge the load side of the relay socket, with a fuse in the circuit. If the pump then starts up, the relay is confirmed. Work only with a cold engine, no open flame, and no longer than necessary – the system holds fuel under pressure. Afterwards, note the number printed on the removed relay and compare it 1:1 with the spare part number.

Solution

Once the relay is confirmed, it is replaced as a whole – repairing the load contacts is not worthwhile. We supply the genuine part with the Mercedes number A0025455105 (002 545 51 05) as a professionally refurbished part tested for function, with a 5-year warranty, no core deposit and no obligation to return your old relay. The build level of this suffix number is 08/1986 to 08/1988; compare the number on your removed relay before ordering. You can find more parts under Mercedes relays & control units, Relays and W126.

If your suffix number doesn't match, we carry further KPR versions for the W126:

Compatible vehicles

The overview reflects the fitment of relay A0025455105 documented in the product data. Mercedes changed the KPR of the 6-cylinder W126 several times – among others 002 545 27 05, 002 545 51 05, 002 545 92 05 and 93 05, 003 545 16 05 and 17 05 as well as 003 545 23 05 and 24 05, depending on model year and cut-off speed. Comparing the suffix number is therefore mandatory.

Model	Engine	Build level	Category in the shop
260 SE (126.020)	M103, 6 cylinders	08/1986 – 08/1988	W126
300 SE (126.024)	M103, 6 cylinders	08/1986 – 08/1988	W126
300 SEL (126.025)	M103, 6 cylinders	08/1986 – 08/1988	W126

Does not fit: V8 models (420, 500, 560 SE/SEL/SEC), 4-cylinder models with M102, diesel models, and other build levels of the 6-cylinder W126 with a different suffix number.

FAQ

How do I tell a defective fuel pump relay from a defective fuel pump?

By the order of checks: first verify whether the pump is being driven at all. If voltage arrives at the relay socket and the pump runs as soon as the load side is bridged according to the wiring diagram, the pump is fine and the relay is the fault. If it doesn't run even then, the cause is the pump, the wiring or the ground.

Does fuel pump relay 002 545 51 05 fit my W126?

According to the product data, this suffix number is assigned to the W126 with the 6-cylinder engine M103: 260 SE (126.020), 300 SE (126.024) and 300 SEL (126.025) at build level 08/1986 to 08/1988. Because different versions were installed within the model series, we verify the assignment in advance using the VIN (chassis number).

Why are there so many KPR suffix numbers for the 6-cylinder W126?

Mercedes changed the relay several times during the production run, among other reasons because of different cut-off speeds and build levels. That is why the number on the removed relay, not the model, is the deciding criterion when replacing it.

Can I keep driving with a defective fuel pump relay?

We advise against it. A relay with intermittent contacts can switch the pump off while driving – the vehicle then stops without warning, in the worst case in moving traffic. Confirmed defects should be replaced before the next longer trip.

Do I have to send in my old relay?

No. No core deposit is charged, and you don't have to send anything back. The refurbished part comes with a 5-year warranty, plus a 60-day return policy and free shipping within Germany.

KFZTeile-Thot is a specialist business for tested and refurbished Mercedes parts with its own test benches – every part is tested before shipping. Over 1,000 positive eBay reviews confirm this. Not sure whether the suffix number fits, or whether it really is the relay and not the pump at fault? Call us at +49 179 8991069 (Mon–Fri 8 am–6 pm) or write to us via the contact form – with the VIN (chassis number) and the number on the removed relay.