Dear Car Talk:
I have a 1999 Mercedes ML430 with 150,000 miles, and for the past bunch of years, occasionally there would be a ticking/clicking/clacking sound coming from the dash area. I determined that it occurs when the temperature dial is set all the way to the cold side, and my mechanic thinks it's the heater gate getting stuck or trying repeatedly to close when it may already be closed. I suspect that pieces of insulating foam may be blocking the gate. (I've seen pieces of gray foam fly out of the dash vents when the fan speed was on high!) Is there an easy fix for this? The vehicle is quite old (although it drives well), so I don't want to pay to have the entire dash taken apart to get to the problem area; however, the clicking noise is quite annoying in the summer, and I'm concerned that the gate motor might fail eventually. Thanks!
Well, there's your easy fix for the clicking sound, Ken: The gate motor will eventually fail.
Those are called servo motors. They use little gears to move things -- in this case, your ventilation system's blend doors. The gears usually are made of plastic. And when a gear gets stripped, you get that clicking sound when it repeatedly tries to close the blend door and can't.
If you can keep it from clicking by turning the temperature selector almost all the way to hot instead of all the way to cold, that'd be the cheapest and easiest solution. But I'm guessing you've tried that.
In most cases, to replace those servo motors, the dashboard has to come out. That'll easily run you about $1,500.
So, is the clicking sound $1,500 worth of annoying? If it is, then I would have your mechanic replace all the servo motors and the fan blower motor while he's in there. You'd hate to spend that kind of money and then have another electric motor fail a week later. And they're all the same age.
I'd also have him replace the heater core while he's in there. That also requires removing the dashboard, and if it starts to leak three weeks after you put the dashboard together, that will redefine "annoying" for you. Good luck.