W210/W170 relay unit faulty? Check & replace A2105400472

The pattern is typical for vehicles that have two to three decades behind them: first the rear window defroster stops working, then the blower fan stays off the next time it rains, and a few days later everything works again — as if nothing had happened. Anyone who then replaces fuse after fuse and orders relay after relay spends money without getting any closer to the cause. Because when several independent components are affected, the fault rarely lies with the individual component but at the point through which their current flows together: the relay unit in the fuse box.

Symptoms

First, match your symptoms to the pattern. The key question is: does one component fail, or do several fail at the same time?

Symptom	Likely cause
Several components not working at the same time, fuses look intact	Contact resistance or a broken conductor track in the relay unit
Failure comes and goes, often in wet weather, in the cold or after a car wash	Corroded plug contacts, moisture in the fuse box
Component works again after light tapping or wiggling of the module	Hairline crack in a solder joint, loosened connector
Fuse holder discolored brown or deformed, smell of scorching	Heat damage from contact resistance — replace the unit, do not keep driving
Only a single component not working, the rest is fine	More likely a single plug-in relay than the unit — see Check W210/W203 relay A0025421119
Warning lights flicker, unstable idle, intermittent dropouts while the engine is running	More likely the overvoltage protection (OVP) relay or a ground problem — see Checking the overvoltage protection relay

Causes

A relay unit combines fuses, relays and their wiring in a single module. That is exactly what makes it practical — and sensitive: the current of several circuits runs through a few plug contacts and conductor tracks. Three things work against this component over the years.

Moisture and corrosion. Fuse boxes are protected against splashing water, not against permanent moisture. Leaking seals or clogged water drains are enough to let humidity reach the contacts. A thin oxide film forms that still conducts current — but with measurable resistance.

Heat. Every contact resistance produces power loss, power loss produces heat, and heat ages the contact further. This spiral accelerates itself: a contact that is only slightly tarnished today can be discolored brown and deformed a year later.

Material fatigue. Temperature changes and vibration stress the solder joints between the relay terminals and the circuit board over decades. The result is hairline cracks that still close when warm and open when cold — the classic fault that is “different in every kind of weather.”

Testing it yourself

Work your way up from the cheapest cause. Disconnect the battery before working on the electrics.

1. Battery and ground first. A weak battery and loose ground connections cause failure patterns that look very much like a faulty module. Measure the resting voltage, check ground points for corrosion and tighten them — it costs nothing and often saves you the rest.
2. Pull the fuses and test them with a meter, don't just look at them. A cracked fuse element is hardly visible to the eye. Pull each fuse of the affected circuit individually and check it for continuity.
3. Measure the voltage drop at the fuse holder. The most telling test: with the component switched on, measure against ground at both fuse contacts. A difference of more than a few tenths of a volt indicates contact resistance in the holder — that is, in the module, not in the fuse.
4. Remove the module and inspect it. Check the plug contacts for green-white corrosion, the fuse holders for brown discoloration, the plastic for deformation, and the board side for scorch marks and ring-shaped cracks around the solder pads. A single discolored holder is enough as a finding.
5. Swap in a known good part. If the fault remains unclear, the swap test is the proof: if the symptom disappears with a tested unit, it was the unit — if it comes back, the cause is in the wiring harness or in the component itself.

Solution

If you have found a discolored holder, a measurable voltage drop or a cracked solder joint, resoldering is rarely a lasting fix: the contact material has aged, and the next failure usually follows within a year. The clean solution is to replace it with a tested genuine part with the same part number.

We have the genuine relay unit / fuse box A2105400472 from Mercedes-Benz in stock — tested and fully functional, for 89,00 €. Compare the number beforehand with the stamping on your removed part; it is also given as 210 540 04 72 written out.

If the number does not match, you will find more units and plug-in relays in the category Relays as well as under Mercedes relays & control units.

Compatible vehicles

The following model series are documented for the relay unit A2105400472. We deliberately do not list engines and model years across the board, because Mercedes installed different versions within a model series.

Model series	Part number	Note
W210	A2105400472	Compare the number on the removed part
W170	A2105400472	Model series as per part data; confirm fit via VIN (chassis number)

If your module shows a different final number, that is not an exclusion criterion, but it is a reason to ask: variants often differ only in pin assignment, not in the housing.

FAQ

How can I tell whether the relay unit or just a single relay is faulty?

By the number of affected components. If exactly one component fails, its plug-in relay is the first suspect. If several independent components fail at the same time or intermittently, the trail leads to the common distribution point — the relay unit. The voltage drop at the fuse holder settles it.

Does the relay unit A2105400472 fit my car?

The model series W170 and W210 are documented. Ultimately, what matters is the number on your removed part: if it matches A2105400472 or 210 540 04 72, the unit fits. If you are unsure, send us your VIN (chassis number) and a photo of the installed module.

What does the tested relay unit A2105400472 cost?

It costs 89,00 € as a tested genuine part from Mercedes-Benz. We have several in stock, so you do not have to rely on a single piece.

Can I have a faulty relay unit repaired instead of replacing it?

Individual cracked solder joints can be resoldered, and a slightly tarnished contact can be cleaned. With fuse holders that are discolored brown or deformed, however, the plastic is thermally damaged — here replacement is the only reliable solution, because otherwise the contact resistance will return.

What information do you need to confirm the fit?

VIN (chassis number), model series and the part number printed on your removed module — ideally as a photo. That lets us identify the variant unambiguously before you order.

KFZTeile-Thot is a specialist business for tested and refurbished Mercedes parts with its own test benches. Every relay and every control unit goes through a functional test with us before it ships — over 1,000 positive eBay reviews back that up. Unsure about the diagnosis or the part number? Call us at +49 179 8991069 (Mon–Fri 8 am–6 pm) or write to us via our contact form. We will look at your fault symptoms before you order a part.