My 87 300SDL is playing up again. It has no heat. It only blows air at ambient temperature. I just changed the radiator and I am pretty sure that there is no air lock as the temperature gauge is running normal at around 80 degree C.

I suspect it is the auxiliary circulation pump or the Automatic Climate Control. Questions:

1) where is the auxiliary circulation pump on a 300SDL. I cannot find it to test it out?
2) Any other things could go wrong, Automatic Climate Control Unit?
3) Vacuum line not switching the flaps?

Any help is appreciated.

 

Fixed the problem. It is the mono-valve stuck in close position. It is 12v for close and 0v for open. I remove the plug ( 0V ) and still no heat. Replace the valve and now I have heat. Also located the auxiliary circulation pump, it is next to the mono-valve buried under an electronic control box and all the hoses. It would be a pain to get to if I need to replace it.