Ok boys, I have noticed that when i turn on my heating/cooling system I hear a kncking from under the hood. Any idea what this may be? Once i turn it off it goes away.

 

anyone have any idea what this may be?

 

There might be a malfunction lock out…(this is a known issue that causes a knocking noise when the heater is activated. This is what you hope the problem is!)

Before anything try this:

(Ignition and heater must have been switched off for at least 2 mins)

- Remove heater fuse (B) for 10 seconds and reconnect (located rear left of engine compartment)
- Switch on heater via timer switch
- Remove heater fuse B (Located in the left rear of the engine compartment)
- Wait 10 seconds then switch off heater and reconnect fuse
(The fault will then be eliminated)

OR


you can take it to MB to have them use their Star Diagnostics Scanner to eliminate the fault.

 

tried this..didnt work. any other ideas?

 

I tracked down what I would describe as a "rythmic knocking" tied to the ventilation system this last weekend to the duo-valve on the passenger side of the engine compartment. It sounded like something trying to cycle unsuccesfully every second or so, very noticable "thunk ... thunk ... thunk" that sounded like it was coming from the center of the dash originally. I live in AZ where this would be closed most of the time, but about 2 weeks ago the morning temps were low enough the system tries to provide just a little heat and it started the "just loud enough to be annoying" knock. A search led to a few threads, all say the valve is "not repairable" and would need to be replaced. Part number is 001 830 34 84 and runs ~$230. Since mine is functioning but noisy, I might pull it out and see if maybe one of the valves is stuck shut or not cycling properly and can be cleaned up. Anyone have advice or hints on this particular unit? -Steve