W124/W126 A/C not working? 6-cyl relay A0025451305
Short answer: If the A/C compressor doesn't engage at all when switched on in the six-cylinder W124, W126 or 190 E 2.6, the A/C relay A 002 545 13 05 is often the cause – identifiable by the marking "A/C · 6 cyl. · 12V". The replacement is a genuine part from Kostal, tested for function, for 54,90 €, swapped by simply plugging it in, with no need to open the refrigerant circuit.
The fault pattern is the same every year from May: the engine runs, the blower blows, the A/C button engages – and nothing happens at the compressor up front. The first thought is usually an empty refrigerant circuit or a seized compressor, i.e. the most expensive options. On the six-cylinder W124, W126, and W201, it's worth looking at the switching side first: engaging the magnetic clutch depends on a single plug-in relay, which by now has been in service for three to four decades. Important here – this is where most wrong purchases go astray: Mercedes fitted different relays depending on the engine.
Symptoms
First separate the switching side (relay, plug contacts, control) from the refrigerant circuit. Only the former has anything to do with the A/C relay.
| Symptom | Likely cause |
|---|---|
| A/C button pressed, compressor doesn't engage | A/C relay doesn't switch the magnetic clutch through |
| A/C works sporadically, fails first in hot weather | Burnt or oxidized switching contacts in the relay |
| No switching sound audible at the relay box behind the battery | Relay coil not working, or control side without voltage |
| Compressor keeps running even though the V-belt is slipping | Speed comparison in the relay no longer works – protective function has failed |
| Compressor audibly engages, air still stays lukewarm | Not to be confused with: cause in the refrigerant circuit, not in the relay |
| A/C only cuts out briefly under full load | Not to be confused with: normal kickdown cutoff – see W124 A/C cuts out at full throttle? Check the kickdown relay |
Causes
The six-cylinder version isn't a simple switching relay but a small control unit: it switches the compressor's magnetic clutch and monitors V-belt slip by comparing the compressor and engine speeds – if the belt slips, it disconnects the compressor. It also switches off if the coolant temperature is too high, as well as under full load and kickdown, and delays switch-on after engine start.
Wear occurs at two points. A high inrush current flows through the power contacts when engaging, and switching off produces an arc at the inductive load; over tens of thousands of switching cycles, material migrates from contact to contact and the contact resistance rises. At first the system only fails in hot weather, later it stops engaging altogether. On top of that, there's the installation location in the relay and electronics box behind the starter battery, next to the fuel pump relay: verdigris on the spade terminals produces exactly the same fault pattern there as a burnt-out relay – which is why the contact check comes first.
Testing it yourself
Work your way from the cheapest to the most expensive check – eyes and ears are enough for the first steps.
- Check the plug connections. Disconnect the battery, pull the relay straight out of its socket, and look at the spade terminals. Verdigris, dark discolouration, or scorch marks on the socket are already the diagnosis. Clean the contacts, plug it back in, and test again – this costs nothing.
- Check the number and marking. The housing must read "A/C · 6 cyl. · 12V", and the number is A 002 545 13 05. If the four-cylinder or V8 number is fitted there instead, that alone explains erratic behaviour.
- Do a listening test. Ignition on, blower on, press the A/C button and listen at the relay box, ideally with a second person. A clear click means the coil is pulling in. No click means the relay is faulty or there's no control voltage.
- Do a swap test. The most reliable test in the classic car world: plug in an identical relay with the same number. If the compressor then engages, the matter is settled – relays that look similar can have different internal wiring.
- Narrow down the control signal. If the relay stays silent even though it's electrically fine, the cause lies upstream: the control panel, pressure switch, or safety chain. Take the terminal assignment from your model series' wiring diagram.
Solution
If the suspicion is confirmed, the replacement is done in a few minutes: disconnect the battery, pull the old relay straight out, plug in the new one in the same orientation. The refrigerant circuit stays closed – no evacuating or refilling needed. For this, we supply a Genuine Mercedes-Benz A/C relay A 002 545 13 05 in the Kostal version for six cylinders, 12 volts, function-tested before shipping. Condition: used genuine part with light storage and signs of use, contact pins with slight tarnish, complete and free of cracks.
Mercedes A/C relay A0025451305 – tested
Part number A 002 545 13 05 · Kostal · 6-cylinder · 12 V
Fits W124/S124, W126 and W201 (190 E 2.6)
54,90 €
View A/C relay
The identifying feature is the marking on the housing: "A/C · 6 cyl. · 12V".
For a four-cylinder, A 002 545 12 05 is the correct part instead – there's a separate article on that: W124/W201 A/C not cooling? Check the A/C relay. More tested switching components: Mercedes relays & control units and Heating & A/C.
Compatible vehicles
The overview reflects only what's documented for this part.
| Model series | Models | Note |
|---|---|---|
| W124 / S124 (E-Class) | 260 E, 300 E, 300 CE, 300 TE, 300 D, 300 TD, 300 Turbo D, 300 T Turbo D – each also as 4matic | Six-cylinder with A/C, approx. 1986–1992 |
| W201 (190) | E 2.6, 2.6 | approx. 1987–1993 |
| W126 (S-Class) | 260 SE, 300 SE, 300 SEL | approx. 1989–1991 |
| Four-cylinder and V8 models | not covered | Limitation: A 002 545 12 05 or A 002 545 18 05 applies there instead |
Other model series aren't documented. What counts is the number stamped on your removed part: if it matches A 002 545 13 05, the replacement part fits. Related categories: W124, W126, W201 and Relays.
FAQ
How can I tell whether the A/C relay, not the compressor, is faulty?
If pressing the A/C button produces no switching sound at the relay box and the magnetic clutch doesn't pull in, the cause is on the switching side: relay, plug contact, or control. If the compressor audibly engages and the air stays lukewarm, the refrigerant circuit is the issue.
What part number does the A/C relay for the six-cylinder W124, W126, and W201 have?
A 002 545 13 05, or A0025451305 / 0025451305 in compact form. The manufacturer is Kostal, rated for 12 volts, marked "A/C · 6 cyl. · 12V". Kostal 05 9952 11 and 3701130 are documented as cross-reference numbers.
Does this relay also fit four-cylinder or V8 models?
No. For four-cylinder engines, A 002 545 12 05 applies, marked "4 cyl.", and for V8 models, A 002 545 18 05 – both with a different pin layout. The later numbers 003 545 51 05, 53 05, and 56 05 aren't interchangeable either.
Why does the relay switch off the A/C at full throttle?
That's by design: the six-cylinder version disconnects the compressor under full load or kickdown, and when the coolant temperature is too high. A brief cooling pause under full load is normal, not a defect.
Can I replace the relay myself?
Yes. It's a plug-in unit located in the relay box behind the starter battery, next to the fuel pump relay. Disconnect the battery, pull the old relay straight out, plug in the replacement part in the same orientation – the refrigerant circuit stays closed.
KFZTeile-Thot is a specialist workshop for tested and refurbished Mercedes parts with its own test benches. Relays, control units, and instrument clusters go through our test bench under power before shipping – over 1.000 positive eBay reviews reflect that. Not sure whether this relay fits your car? Call us at +49 179 8991069 (Mon–Fri 8 am–6 pm) or write via our contact form – it's quickest with the VIN (chassis number) and the number on your removed relay.