W124 4-Cylinder A/C Off After Kickdown? Relay A0035455405
Short answer: If the air conditioning in a Mercedes W124 with a four-cylinder engine switches off on kickdown and does not come back – or the compressor keeps running regardless at full load –, the cause is often the relay for the kickdown cut-off, part number A 003 545 54 05. The replacement is a tested genuine Mercedes-Benz relay for 79,99 €, swapped by plugging it in, with no intervention in the refrigerant circuit.
The fault only appears under certain conditions: In city traffic the air conditioning cools normally, but after overtaking with the accelerator floored the air stays warm – often until the next engine start. Many owners of a W124 then suspect too little refrigerant or a worn compressor. Before you spend money on that, it is worth looking at the switching side: On the four-cylinder, a separate relay handles the kickdown cut-off, and after decades its contacts are the weak point.
Important when looking for the right part: The six-cylinder has its own number. This guide covers the four-cylinder version A 003 545 54 05.
Symptoms
What matters is when the cooling fails. Only a failure that depends on the load state points to the kickdown path.
| Symptom | Likely cause |
|---|---|
| A/C switches off on kickdown and doesn't come back | Kickdown relay drops out but does not switch back |
| Compressor keeps running at full throttle, noticeable power loss | Cut-off not working, relay contacts stuck |
| Cooling cuts out sporadically, especially in hot weather | Oxidized or burnt contacts in the relay or socket |
| Compressor never engages, not even at standstill | Not to be confused with: more likely basic activation than the kickdown path – see W124/W201 A/C not cooling? Check the A/C relay |
| Same fault pattern, but six-cylinder engine | Not to be confused with: different part number A 003 545 56 05 – see W124 A/C cuts out at full throttle? Check the kickdown relay |
Causes
The relay switches an inductive load: the magnetic clutch of the A/C compressor. When switching on, a high inrush current flows across the contacts, and switching off produces a small arc. Every kickdown is such a switching operation under load. Over decades, contact material migrates and the contact resistance rises. In the end there are two typical states: The contacts burn off and the relay no longer switches back cleanly, or they stick together and the cut-off fails completely.
On top of that comes the age of the plug connection. Temperature changes and moisture cause the terminal blades in the socket to oxidize – this produces the same fault pattern as a worn relay. For the installation location and terminal assignment, please refer to the wiring diagram of your vehicle, as Mercedes made changes here over the production period.
Testing it yourself
Start with the steps that cost nothing. For the rest, a simple multimeter is enough.
- Pull the relay, inspect the socket. Verdigris, dark discoloration or scorch marks on the contacts are already a clear finding. Clean the contacts, plug the relay back in and go for a test drive.
- Compare the part number. The stamping on the removed relay must read A 003 545 54 05. Relays that look similar in the same carrier are wired differently – without comparing the numbers, you may end up testing the wrong part.
- Do a listening test. Ideally with two people: ignition on, switch the A/C on and off and listen at the relay. An audible click shows that the coil is working. If it stays silent, the activation is missing or the coil is defective.
- Measure the contact. In the switched state, an intact contact should show almost zero ohms. Clearly higher values, or values that jump when tapped lightly, indicate worn contacts. Refer to the wiring diagram for the terminals.
- Swap test. The most reliable test: plug in a working relay with an identical number and recreate the fault situation. If the cooling is retained after kickdown, the cause has been found.
Solution
Once the relay is confirmed as the cause, replacing it takes only a few minutes: ignition off, pull the old relay straight out of the socket, plug in the replacement in the same orientation. The refrigerant circuit stays closed – no evacuating, no refilling. For this we supply a genuine Mercedes-Benz relay A 003 545 54 05 for the W124 with four-cylinder engine, personally function-tested, at a price of 79,99 €.
Mercedes W124 Relay Air Conditioning Kickdown 4-Cylinder – tested
Part number A 003 545 54 05 · genuine part · four-cylinder
Fits W124 (4-cylinder) · personally tested, works
79,99 €
To the kickdown relayCompare the number before ordering
Four-cylinder: A 003 545 54 05. Six-cylinder: A 003 545 56 05 – two separately listed part numbers.
Other tested switching parts
Relays and control units for this model series: Mercedes relays & control units, Relays and W124.
Compatible vehicles
| Model series | Engine | Part number |
|---|---|---|
| Mercedes-Benz W124 | 4-cylinder (with air conditioning) | A 003 545 54 05 |
This information comes from the part description. Individual model variants are deliberately not listed here – if in doubt, send us your VIN (chassis number) and model year and we will check the fit free of charge.
FAQ
How do I know whether my W124 needs the relay A 003 545 54 05?
According to the part description, the relay is intended for the W124 with a four-cylinder engine. What matters is the stamping on the installed relay: If it reads A 003 545 54 05, the replacement part fits. If in doubt, we will check the fit free of charge based on your VIN (chassis number).
Can I install the six-cylinder relay A 003 545 56 05 instead of A 003 545 54 05?
We advise against it. Both relays look similar but are listed as separate part numbers for four- and six-cylinder engines. Install the number that is printed on the removed part.
Does the air conditioning have to be opened or refilled for the replacement?
No. The relay is simply plugged in. The refrigerant circuit stays closed; an A/C service is not needed for the relay swap.
Is it normal for the air conditioning to switch off briefly on kickdown?
Yes, that is the intended function: With the accelerator fully depressed, the compressor is briefly switched off so that the engine power reaches the drivetrain. It only becomes noticeable when the cooling does not return on its own afterwards or the cut-off no longer happens at all.
How is the relay tested before shipping?
Every relay is personally function-tested before shipping. It is a used Mercedes-Benz genuine part, not aftermarket.
KFZTeile-Thot is a specialist business for tested and refurbished Mercedes parts with its own test benches and has over 1,000 positive eBay reviews. Not sure whether the relay fits your W124? Call us at +49 179 8991069 (Mon–Fri 8 am–6 pm) or write to us via the contact form.