AC and Heating Question

I have a 98 Mercedes C280, the heater does not blow hot air and the AC will not blow cold air, but the blower motor is working

First are you saying it blows air but the air is not cold or hot, or are you saying it does not blow air into the cabin, but you can hear the fan running?

My first guess is a vacuum problem. I on your car those systems are controlled by a vacuum system.

If the blower motor is running, it’s probably blowing air somewhere, even if it’s not heated or cooled as it should be. As Joseph says, the climate system vents are vacuum controlled, and a big vac leak will cause the vent doors to not open correctly, all except the defrost vents, which default open so that you will always have a clear windshield, even with catastrophic vacuum failure (safety measure, by design). Beyond that, there could be several causes, including that the blower “squirrel cage” is not turning even though the blower motor is OK, or even that it’s been installed backwards. Give us a bit more info on what work, if any, has been done to the climate system recently. And while you’re at it, answer Joseph’s question to clarify its current state.

The blower motor is blowing air into the cabin, it’s just not hot or cold. It was working fine and one day during the winter it just stopped blowing hot air, then when the temperature outside heated up and I turned on the AC, the air wasn’t cold.

Are you saying that the temp of the air being moved by the blower motor is the same temp as the outside air (ambient)?

Yes. The when I set the thermostat to 90 degrees or 53 degrees, it just blows out ambient air, which is strange because I thought that at least the heater would work, since that’s just engine heated air.

You will get hot air only if the incoming air is directed through the heater core. You will get cold air only if the incoming air is directed through the AC evaporator. The heater core may be hot, but the air may not be going through it. Same with the AC.

Airflow is a function of the flaps, or doors, within the HVAC system. These flaps are controlled by vacuum in some cars and electric motors in others, either of which is operated by a computer if your vehicle has Automatic Climate Control, which I assume it does.

You can find out if the heater core is hot by feeling the heater hoses under the hood. If the hoses leading to and from the heater core are hot after the engine warms up, the the core is hot.

Same for the AC. Turn on the AC, verify that the compressor is running, and feel the hoses going to the evaporator. Are they cold?

If the heater is hot and the AC is cold, the problem is with the climate control system.

I replace the climate control unit and I’m still not getting any cold or hot air. The compressor isn’t even coming on.