I know this topic came up many times before but I need some help to start with.

I have an 1978 300cd with airco, the airco is not working anymore. The problem is that my heater blows air, but it doesn't get warm. Where do I start finding the problem?

Thanks

You should look for threads run on the ACC Servo. That is that miserable unit with 20 vacuum tubes running into it, mounted inside the right front fender.
An aluminum body rebuild is made, most have found that is the cheapest and best, as I recall it runs about $275. The new replacement runs a lot more.
Those units tend to get cracked cases and the inside parts rot away after 25 years. I took one apart and it just poured out onto the floor. I routed the heater hoses aroung it to get by, then was able to transplant a manual heat control system from a Euro 300D I have. Not an easy solution, but have good heat control now.