Relay - Priced Each

Your Mercedes-Benz is heavily reliant on a sophisticated network of Signal Acquisition Modules (SAMs) and fuse boxes to distribute power. At the heart of this high-draw power distribution is the 5-prong, 40-amp multi-purpose relay. Identifiable by its green casing (and sometimes grey/black depending on OE supplier updates), this little relay acts as the gatekeeper for some of the most critical systems on your vehicle.

Because it controls high-amperage components like the starter motor and the Airmatic suspension compressor, its internal electrical contacts are subjected to intense heat and frequent arcing. Over time, these contacts become pitted and can either fail to close (causing a dead circuit) or, much worse, physically fuse together (causing a component to run continuously until it burns out).

Common Signs of a Failing Relay:

• Air Suspension Collapse: The Airmatic compressor fails to turn on, leaving your vehicle "slammed" to the ground with a suspension warning on the dash.

• Airmatic Compressor Burnout: The relay contacts weld closed, causing the suspension air compressor to run continuously—even with the key out of the ignition—until the compressor motor burns itself out.

• The "No-Click, No-Crank" Condition: You turn the key to start the car, all your dash lights come on, but the starter does absolutely nothing.

• Accessory Power Loss: Sudden, intermittent loss of climate control, power windows, power seats, or other interior comfort features tied to the front accessory power circuit.

ECS Tuning Recommends:

If you are replacing a burnt-out Airmatic air suspension compressor, you must replace this relay at the exact same time. It is incredibly common for a failing relay to be the root cause of a dead compressor. Hooking up a brand-new, expensive compressor to an old, sticky relay is a guaranteed way to void your warranty and burn out your new part in a matter of days!

Tech Tip: Because this single relay is used for so many critical functions (like the starter), we highly recommend keeping a spare in your glovebox. A $20 part can literally be the difference between driving home and waiting hours for a tow truck!

Comprehensive Chassis & System Application Guide

Because this is a "Multi-Purpose" 40A relay, its job changes depending on where it is plugged into your fuse box. Below is a breakdown of what this specific relay controls across various Mercedes-Benz platforms:

W164 ML-Class, X164 GL-Class, & W251 R-Class (2006–2012)

• Airmatic Air Suspension Compressor Relay: Provides power to the air suspension pump.

• Starter Relay: Sends the high-amperage signal to the starter solenoid to crank the engine.

W220 / W221 S-Class & W215 / W216 CL-Class (2000–2013)

• Airmatic Suspension Relay: Controls the heavy electrical load of the air ride compressor.

• Starter Relay: Essential for engine cranking, especially on massive V8 and V12 (S600/CL600/AMG) powerplants.

W204 C-Class & X204 GLK-Class (2008–2015)

• Starter Relay: Triggers the starter motor.

• Front Accessory Power Relay (Circuit 15 / Circuit 87): Manages auxiliary power distribution in the front SAM, waking up electrical accessories, climate control, and safety systems when the ignition is turned on.

W211 / W212 E-Class & W219 / W218 CLS-Class (2003–2017)

• Circuit 87 / Accessory Power Distribution: Crucial for delivering stable voltage to the engine electronics and interior cabin convenience features.

• Starter Relay: Controls the starting sequence for both standard gasoline and BlueTEC diesel engines.

• Airmatic Compressor Relay: For models equipped with optional air suspension.

R230 / R231 SL-Class & R172 SLC-Class Roadsters (2003–2019)

• Accessory Power Relay: Feeds power to the complex folding hardtop modules and interior electronics.

• Starter Relay: Handles the ignition sequence.