I have a 1988 300SEL and the heat sometimes works and sometimes doesn't work ( by not working I mean it doesn't blow warm air)and now it seems like if it does start to work then a little while later it starts to blow cold air.

You need to check your sampler blower motor. The quickest way to do this is to put a piece of paper cut 1cm square over the little grille near the rear view mirror with the ignition turned on. It should stay in place, if not, you will have to find the sampler blower motor, see that there is voltage to it. If there is voltage, replace the motor, if not, trace the circuit.

Good luck,

Thanks Larry, But the sampler motor is apparently OK because the paper stays there. Do you think it is the climate control unit itself is there a way to test that? Thanks again

The temperature sensor is right behind the grill. I found my sensor had a coating of lint on it which prevented it from sensing the air temperature very well. I used a, plastic safe, electronics spray cleaner on the sensor. My ACC worked much better afterward. The temperature was more stable, still not perfect but much better.

If you will do some searching on climate control trouble codes or some such, you will find instructions on how to draw trouble codes on your system using an LED. Make sure that there are no trouble codes set before proceeding. There are trouble codes for each of the temperature sensors. If a code is set it should lead you to the sensor or other electronic component that might be the culprit.

Hope this helps,