I've resisted posting this problem until I felt like it had gone where no man has gone before, and I've hit that point, I think.

I bought my 1984 300D when the weather was warm last Summer. It has a low mileage non-turbo engine replaced by a dealership for the PO. Some bits and pieces were obviously never hooked up right after the swap. I have no temp guage or tach working. The engine shutoff was changed from vacuum to a manual pull, which I don't mind in the least.

When the weather changed, I blew fuses when I turned on heat. Swapped the fan motor with one from a parts car in the woods I located with a buddy, after doing a brush replacement. Swapped the damaged climate control unit, and re-soldered the original where it had burned from the motor problem (ouch). I discovered that the aux. water pump was bad, and swapped it too. I have replaced the monovalve with one from the parts box I got from the basement of a house scheduled for demolition (I've been on a salvage mission). I have verified that the pump, fan action and monovalve are all working. The whole system holds vacuum. If I park it for a couple days then pull a vacuum line, I hear suction. I've burped and purged and topped off coolant multiple times, and have warm heater hoses on both sides of the heater core.

Here's the deal now. If I start the car and let it idle, I get heat and can defrost my windshield. However as soon as I take off, within a few seconds, the temperature from the vents drops and I go from lukewarm to cold air. I basically have no heat at all while I drive. The really interesting thing is that if I cut the ignition, then turn it back on, the air goes almost immediately to hot, then cools back off. This leads me to believe I'm looking at an electrical issue, or the climate control #2, rather than anything clogged or not flowing.

My question is simply, has somebody got a theory? This one baffles me. I tore apart the old monovalve today, and it looks good, so I'm taking that out of the equation. Are there any thermostats or temp guages that might be shutting off heat flow inside the cabin, or electrically is the only variable the control unit itself? I've read a million posts and can't quite get a handle on this. Good news is spring is almost here.

Ben