On my 85 SD, it warms up just fine, but the climate control will not deliver any amount of hot air to the car until I stop the car, and turn it off for a second. Immediately upon restarting the car, the heater will blast me out. This is all well and good, but very dangerous on a freeway.
I was told to check the monovalve, located on the firewall. Exactly what does it look like. There is a rectangular metal can with a connector sort of dead center and a smaller device between it and the battery. Suggestions would be welcomed.

To heat or not to heat
 

Ron, the monovalve is located about 3 inches to left of the battery (left as if you are sitting in the drivers seat), on the top of this valve will be a small electrical connector with two wires (mine is brown in color)attached to a metal plate which is held down by four small sheet metal screw. You will need a small or offset screwdriver to undo the screws. The plate is very close to the firewall and just under the lip of the hood panel. Once you locate the monovalve, it is very easy to open. Once open their is a plunger which has a rubber diaphragm attach. It this rubber diaphragm is torn or ripped the heating system will not work. The parts are about $38.00. Now for a very easy test you should do before dissembling the monovalve, try this; remove the electrical connector from the monovalve start the car and see if you now have heat. If you do have heat its not the monovalve which is giving you trouble. Its the temp switch on the dash. The monovalve has power 12 volts on it when the ignition switch is on. The temp switch, when activated completes a ground to the monovalve which then close the valve and allows heat. It could also be the results of a vacuum leak. Go luck.