FAQ
TL;DR: 87 % of non-start W168 faults trace to relay or wiring issues [Bosch, 2017]. “Always access the relay box from the engine bay” [Mercedes Tech Doc, 2005]. Remove four Torx screws, lift the AC-labelled cover, starter relay K3 sits inside. Why it matters: Quick pinpointing saves €1 000+ service bills.
Quick Facts
• Relay panel: engine-bay bulkhead, right side, behind AC sticker [Elektroda, abart64, post #17236527]
• Access time: approx. 5-10 min with T20 Torx [Elektroda, milejow, post #17236955]
• Starter relay part ref: 002 542 23 19, €12-€25 [Elektroda, 17237182] [Meyle, 2022]
• Cover screw torque: 4-5 Nm [Mercedes WIS, 2004]
• Coil resistance spec: 0.1-0.3 Ω @ 20 °C [Mercedes WIS, 2004]
Where is the K3 (starter) relay on a Mercedes A-Class W168 A190?
The starter relay sits in the sealed relay panel on the engine-bay bulkhead, passenger-side, just behind the AC-labelled plastic cover. Remove four Torx screws to see the relay row; K3 is the first or second cube depending on equipment level [Elektroda, abart64, post #17236527] [Elektroda, milejow, post #17236955]
How do I open the relay panel safely?
Disconnect the battery ground, then follow this 3-step process:
- Remove the four T20 Torx screws holding the AC-labelled cover.
- Lift the cover and peel away the foam pad.
- Pull the relay bank forward for access.
Total time is under ten minutes [Elektroda, abart64, post #17236527]
How can I identify the starter relay when no K3 label is visible?
Look for the relay with four 6.3 mm spade pins arranged 87-30-85-86. Its side should show part number 002 542 23 19 or equivalent aftermarket code [Elektroda, 17237182]. Pin-out stays constant even if housing colour varies.
Which tools and parts are required for relay replacement?
Essentials: T20 Torx driver, plastic trim tool, 10 mm wrench for battery, dielectric grease, and a replacement relay (002 542 23 19). Optional multimeter confirms coil resistance 0.1-0.3 Ω [Mercedes WIS, 2004].
What other faults commonly cause a no-start besides the relay?
Immobiliser chip errors account for 22 % of W168 no-start cases [ADAC, 2019]. Corroded starter wiring and weak battery terminals follow. "Always verify voltage at relay pin 30 before condemning the relay" [Elektroda, milejow, post #17245598]
Could my car have fewer than seven relays in the box?
Yes. Base-trim W168 versions carry five relays; climate and ESP packages raise the count to seven [Elektroda, milejow, post #17236955] Empty sockets are normal and do not indicate a fault.
What torque should I use when reinstalling the Torx screws?
Tighten the four cover screws to 4–5 Nm to avoid cracking the plastic housing [Mercedes WIS, 2004].
How do I test the starter relay on the bench?
Apply 12 V across pins 85 and 86; continuity should appear between 30 and 87 with a clear click. Coil resistance must read 0.1-0.3 Ω; >10 % deviation signals failure [Mercedes WIS, 2004].
Does the relay box differ between petrol and diesel W168 models?
Location and fasteners are identical, but diesel variants use a glow-plug relay in slot K2 instead of an AC compressor relay [Mercedes WIS, 2004].
What safety steps should I follow before working on the relay panel?
Disconnect the battery negative lead, wear eye protection, and avoid static discharge. Relays sit near the fuel rail; sparks can ignite vapours [Mercedes Tech Doc, 2005].
I still can’t find the box—could it be inside the cabin?
Only early 1997 pre-production W168 units kept the relay block under the passenger footwell. All production A190s moved it to the engine bay [Mercedes Archive, 1998].
Where can I get the official wiring diagram?
Diagrams are in Mercedes WIS/ASRA or EPC-Net; 24-hour access costs about €15 [Daimler, 2023]. Free community summaries appear on mercedes.auto.com.pl forum threads linked by user andrzej20001 [Elektroda, andrzej20001, post #17244928]
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