Mercedez ML320 Temperature Control Unstable

My 98’ ML320’s temperature control didn’t function properly for more than a year and got worse. The knob turns OK, but air is stuck either at hot or cold. Only way to switch between hot and cold is by turning the knobs back and forth and getting lucky. I went to a shop and replaced a control unit last week.

Current Problem:
Now I face different symptoms. The good part is that turning knob from max cold to max hot or the other way around, air temperature isn’t stuck any more. The bad news is that when I turn from max cold towards 2/3 between max cold and middle, all of sudden air becomes hot big time, not even suppressible by AC on. The only way getting back to normal temperature air is turning all the way to the max heat and turning back to the same location. Another observation is if turning the knob VERY slow from the max cold, one click at a time and wait, towards the same location, air will become warm but not hot.

Any suggestions what causes the problem and what is the needed repair?

I would say you have a motor bad on your blend door.

It does sound like a blend door motor as mentioned above. It’s probably worthwhile to ask your mechanic to verify the clutch on the AC compressor is working correctly too.

Thanks for the comments. I believe the mechanic replaced blend door motor. Though there are some uneasiness about the quality of the part. He initially said that he ordered a genuine part, then used a refurbished one siting only that was available in the market.

Air temperature is not stuck any more, and AC works fine as long as air is not excessively hot. Does this point to a faulty blend door motor or actual blend door?


There’s a good chance the condenser fan isn’t coming on, because the fuse is blown

There’s a Mercedes-Benz technical service bulletin that instructs the mechanic how to install a repair kit. I won’t go into too much detail, but it takes about an hour or so and involves installing a wiring modification kit. A dealer would know about this.

For those of you wondering . . . simply replacing the 40 amp fuse will not fix the problem. If the modification hasn’t been done, the fuse will blow again. Replacing the fuse all the time gets old real fast.

Do yourself a favor . . .

Start the engine
Turn on the AC
Pop the hood

Is the condenser fan . . . the electric fan in FRONT of the condenser . . . engaged?
It may cycle on and off
If it doesn’t come on at all, you may need the modification

Also make sure the AC compressor clutch is engaging . . . if the AC compressor clutch is not engaging, you may be low on charge due to a leak

Please post back with an update