85 300D no more heat for me, help its cold
 

So on my way this morning i realized that there was no heat driving down the highway. Once I was almost frozen to death and drove off the exit it started to blow warm air. On my way back from work as soon as I got on the highway it started blowing cold again. I am guessing a flap might be the problem, but do not know where to start looking.

That is the classic symptom of a torn diaphragm in the monovalve. A monovalve repair kit and ten minutes of your time will solve the problem.

My 85 300D would do that
 

Usually a fix that lasted for the remainder of the winter was to pull the electrical connector on the monovalve, clean up the electrical connections real good and connect it back together. Real simple and very effective. A little of that electrical contact grease is helpful on the connection.

A poor electrical connection at the monovalve causes the heat to work when it is not supposed to. The OP has the opposite problem.

where is the monovalve

Firewall, inboard of the battery.

http://www.google.com/imgres?imgurl=http://www.dieselgiant.com/monovalve48.JPG&imgrefurl=http://www.dieselgiant.com/coolingsystem.htm&usg=__u34H50oRLjZHdFjRYLwFcxGPx6o=&h=480&w=640&sz=66&hl=en&start=0&zoom=1&tbnid=qM vxvNPPjr4FBM:&tbnh=170&tbnw=227&prev=/images%3Fq%3D300d%2Bmonovalve%26um%3D1%26hl%3Den%26client%3Dsafari%26sa%3DN%26rls%3Den%26biw%3D1647% 26bih%3D831%26tbs%3Disch:1&um=1&itbs=1&iact=rc&dur=439&ei=bw7STPyMFYG0lQfyoPC1Dg&oei=bw7STPyMFYG0lQf yoPC1Dg&esq=1&page=1&ndsp=28&ved=1t:429,r:5,s:0&tx=129&ty=93

If you need a good monovalve I have some in my toolbox for cheap.