W124/W201 A/C not cooling? Check the A/C relay
Short answer: If the air conditioning in the Mercedes W124, the 190 E (W201), or the G-Class stops cooling and the compressor doesn't engage at all when switched on, the cause on the four-cylinder models is often the A/C relay A 002 545 12 05 the cause. The replacement is a function-tested genuine part (Kostal, 12 V) for 49,00 € – plugs in, no intervention in the refrigerant circuit.
A fault pattern that regularly lands on our bench: the car runs, the blower blows, but as soon as the A/C button is pressed, nothing happens at the compressor up front – no jolt as it engages, no cold air. Many people then immediately suspect an empty refrigerant circuit or a faulty compressor, i.e. two expensive repairs. Before starting there, it's worth looking at the switching side: on the four-cylinder model series W124, W201, and the G-Class, the magnetic clutch engaging depends on a single plug-in relay – and that's now three to four decades old.
Symptoms
What matters is separating the switching side (relay, contacts, control signal) from the refrigerant circuit – only the first group has anything to do with the A/C relay.
| Symptom | Likely cause |
|---|---|
| A/C button pressed, compressor doesn't engage | A/C relay not switching through |
| A/C works intermittently, fails in warm weather | Burnt or oxidized switching contacts in the relay |
| No switching sound audible at the relay holder when switching on | Relay coil not working, or control signal has no voltage |
| Compressor audibly engages, air still stays lukewarm | Cause in the refrigerant circuit – not not the relay |
| Blower doesn't run at all, or only on certain settings | Not to be confused with: more likely the control panel – see W201 heater control panel defective? Check the blower & controls |
| Heating doesn't warm up, or stays permanently hot | Not to be confused with: water side, not the A/C relay – see Check W124 duo valve & fix |
Causes
A relay is a wear part. Every time the compressor's magnetic clutch engages, a high inrush current flows through the contacts; when switching off, the inductive load produces an arcing spark. Over tens of thousands of switching cycles, material migrates from one contact to the other, the surface roughens, and contact resistance rises. Eventually the current is no longer enough to reliably pull in the clutch – at first only in heat, later not at all.
Add to this the installation situation. Depending on the model series and equipment, the relay holder sits in different places; for the position and pin assignment, refer to your model series' wiring diagram. Common to all installation locations is that temperature swings and moisture work on the plug contacts over the decades. Oxidized connector tabs produce the same fault pattern as a burnt-out relay – so checking the contacts is a must.
Testing it yourself
Work your way from cheapest to most expensive – a multimeter is enough for the first steps.
- Check fuses and plug connections. Pull the suspect relay out of its socket and look at the connector tabs. Verdigris, dark discoloration, or scorch marks on the socket are already the diagnosis. Clean the contacts, plug it back in, test again – that costs nothing.
- Do a listening test. Ignition on, blower on, press the A/C button and listen at the relay holder – ideally with two people. A clear click means the coil is being driven and pulls in. No click means the relay is faulty or there's no control voltage.
- Do a swap test. The most reliable check in the classic car world: plug in an identical relay with the same part number. If the compressor engages afterward, the matter is settled. Visually similar relays have different internal wiring – the part number has to match.
- Measure contact resistance. With the relay engaged, check the resistance across the switching contacts. A clean contact reads close to zero ohms; anything noticeably higher, or that jumps when tapped, is burnt. Refer to the wiring diagram for the terminals.
- Narrow down the control signal. If the relay stays silent despite being electrically fine, the cause lies upstream – for example at the control panel or in the safety interlock chain. Then check whether any voltage reaches the control side at all.
Solution
If the suspicion is confirmed, the replacement is straightforward: disconnect the battery, pull the old relay straight out of the socket, plug in the new one in the same orientation. The refrigerant circuit stays sealed – no need to evacuate or refill. We supply for this a genuine Mercedes-Benz A/C relay A 002 545 12 05 (Kostal, four-cylinder version, 12 V), function-tested before shipping.
Mercedes A/C relay A0025451205 – tested
Part number A 002 545 12 05 · Kostal · 4-cylinder · 12 V
Fits W124, W201 (190 E), and G-Class
49,00 €
View A/C relayYou'll find more tested switching parts for these model series in our categories Mercedes relays & control units and Heating & A/C. If sporadic electrical faults also occur in the same vehicle, it's also worth taking a look at the overvoltage protection relay – it sits in the same age bracket and causes similarly erratic failures.
Compatible vehicles
The overview reflects only what's documented for this part.
| Model series | Version | Note |
|---|---|---|
| W124 | 4-cylinder, 12 V | fits per parts data |
| W201 (190 E) | 4-cylinder, 12 V | fits per parts data |
| G-Class | 4-cylinder, 12 V | fits per parts data |
| Six-cylinder models | not covered | Limitation: this is the 4-cylinder version |
Other model series aren't documented for this relay. What always matters is the number stamped on your removed part: if it matches A 002 545 12 05, the replacement part fits. Compatible vehicle categories: W124 and W201.
FAQ
How can I tell whether the A/C relay, not the compressor, is faulty?
If the switching sound stays absent at the relay holder when the A/C is switched on and the magnetic clutch doesn't engage, that points to the switching side – relay, plug contact, or control signal. If the compressor audibly engages but the air stays lukewarm, the cause lies in the refrigerant circuit.
What part number does the A/C relay for the four-cylinder W124 and 190 E have?
The Mercedes part number is A 002 545 12 05 (A0025451205), made by Kostal, rated for 12 volts. Before buying, compare it with the number stamped on your removed relay – the fit is only confirmed if the numbers match exactly.
Does this relay also fit six-cylinder models?
No. This relay is the four-cylinder version for W124, W201, and the G-Class. For other engine variants the fit isn't documented – in that case, compare the number on your original part first.
Can I replace the relay myself?
Yes. The relay plugs in and can be replaced without any intervention in the refrigerant circuit: disconnect the battery, pull the old relay straight out of the socket, plug in the replacement in the same orientation. The A/C system doesn't need to be opened for this.
What does a tested genuine A/C relay A0025451205 cost?
The tested genuine part costs 49.00 euros. Every relay is function-tested before shipping; 8 units are currently in stock.
KFZTeile-Thot is a specialist for tested and refurbished Mercedes parts with its own test benches. Relays, control units, and instrument clusters are tested under voltage before shipping with us – over 1,000 eBay reviews (almost exclusively positive) reflect that. Not sure whether this relay fits your vehicle? Call us on 0179 8991069 (Mon–Fri 8 am–6 pm) or write via our contact form – it's fastest with your chassis number and the number of your removed relay.