Car is a 1995 E420 and I've been having to hunt down a popping/ticking sound from inside my dash for the past few months and have finally found the culprit. The heater control valve on the passenger side. When it's active, it ticks/pulses every 4-5 seconds constantly as long as the heat is on in the car. This noise is transferred to the cabin and is very audible. Anyone have any insight into what the problem could be. I always thought the ticking of the valve was normal operation. My guesses are that the valve is simply louder than it should be (replace) or that its operating too often (either the valve itself or the CCU). Any feedback would be great! Thanks!

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1995 E420
1998 SL500 Sport
2009 E350 Sport
1995 S500 Coupe
1992 500 SEL

I just wanted to post an update to see if anyone can shed a little light on the problem. The 'clicking' occurs as soon as the car is up to cabin temperature. It is always at regular 4-5 second intervals. If I have the thermostat at a moderate cabin temperature it will continue to click non-stop once at temperature. If I crank the heat up the clicking will stop for a while until the car gets back up to the higher requested temp. I'm assuming this is the valve's normal response for the increase in demand for more heat. The the more heat the heater core demands, the more the valve lets through. This is pointing me to believe that the valve is just louder than it should be (why I have no idea). Keep me posted!

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1995 E420
1998 SL500 Sport
2009 E350 Sport
1995 S500 Coupe
1992 500 SEL

The duovalve on my 89 260e clicks every 4 - 5 seconds. I don't know if you have the same setup in your car.