Thump/Water Drop sound from Dash w202

[Eiknujrac]

1998 C280

So when I pulled up to a stoplight the other day, I noticed my AC system was making a really strange noise. About every 3-4 seconds, there will be a thump noise, which is loud enough to be felt through the dash by my knee, and most of the time, this thump is accompanied by a water droplet sound. The AC seems to be working fine, blowing fast and cold. But this sound is very annoying when I want a quiet ride home.

Anyone ever have this noise before? or have any idea what it is?

Thanks in advance.

[Eiknujrac]

UPDATE: found that the sound only occurs with the AC on, no sound with the heat on. Anyone?

Just a guess, but is you AC drain plugged? I'm not sure about the thump, but it seems that water may be building up, then draining all at once.

[michakaveli]

Our '96 C220 also has the loud THUMP, it occurs at all times that the blower is working, AC on or off - although there is no water drop sound to follow. It driver me up the wall, but since everything works fine I have just left it alone.

[Ethan]

there have been other post about the thump, also check where I think it is also discussed.

The thump is from the auxillary water pump or maybe an item called the duo-valve that is activated by the climate control.

I have always been able to sense that thump in my car, but cannot say I flet it through the dash by my knee.

Oddly enough for the first time in 77K miles I got a similar dripping noise, as a test I ran my HVAC with the EC light on, fan speed at maximum, and temp on the hottest, also adjusted vents to downward airflow icon on the HVAC unit. I ran it that way while driving for around ten minutes. The dripping sound went away.

[benzfan]

The sound is indeed the duovalve. It cycles at a rate depending on the need for heating in the heater core. With the A/C on, the duovalve will still be required to help maintain the core temperature. In the spring and fall, when minimal heating is required in the cabin, you will hear the valve cycling more. Both of my 202s do it. The gurgling is the water being let through the heater core a small slug at a time as the valve cycles. I recall posting a lengthy reply on this subject to someone on club202.com.

[Eiknujrac]

Thanks alot for the replies guys.

But just to clear things up, this is a normal sound right? And even the newer valves make the same noise?

[Ethan]

the thunp is normal

the drip sound I am not so sure, but try what I described and hopefully the drip sound will stop

[Eiknujrac]

Thanks guys, I really appreciate it.

I'll try the drip thing tommorow and post an update.