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Ask a Mechanic Series – What is a duo valve system?

Post Date: 27th September 2017


Mercedes vehicles have what is known as a duo valve system.

What is a duo valve system and what does it refer to?

It refers to the flow of air into the cabin.

Why is it dualed?

Because you need separate heat controls for the driver and passenger sides, the duo valves system means you can have different temperatures for each side of the car. It’s located near the brake fluid vessel, at the back of the engine bay.

Checking that the system is working properly

To see if the duo valve is working properly, it’s essential that you make sure the coolant isn’t too hot – you don’t want third degree finger burns! Unplug the power connector and wait for it to cool down. Once it’s reasonably cool, take a Torx star bit and and unscrew the bolts that hold the metal case in place. You can now take off the two covers – just slide them off.

Once you’ve got into the system, you’ll see two orange cylinders. These are the electrical coils, and make the valve pistons move in and out. You’ll also see two blue parts – these are the capacitors, which are cheap and convenient to replace if needed.

Next, you’ll need to split the valve. However, before you do this, it’s absolutely essential that you put a piece of card in place to stop components falling into the engine bay. The reason being, when you separate the duo valve’s two parts, there are two small valves that will fall out. They will look like little mushrooms.

Once you’ve taken it apart, you need to check the duo valve for blockages. If you’re happy with what you see / feel, it’s worth giving the whole thing a wipe down to remove any dirt or rust.

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