1980 300TD no heat

1980 300TD wagon, 150k, runs like a top, temp guage when running hovers around 85C and lower when running down the road when the outside temp is 20F. I verified that the vacuum line connected to the coolant valve
diaphragm operates the diaphragm and the valve, but no matter what position that coolant valve is in, no heat. Here's a clue: the vehicle spent 23 years in Florida. No nead for heat down there...
I have several theories:
1) There is a flap open somewhere letting in a large amount of outside air.
2) The coolant valve is broken, corroded, and though it moves easily with fingers or vaccum via diaphragm, it's not letting hot coolant into the core.
3) Since I can put my hand on the lower radiator hose at full running temperature and it's not even warm, my thought is a bad thermostat. What is the regular operating temp of these 300 diesels? Seems like it ought to be around 90-100C. I've put cardboard in front of the radiator but haven't had time to see if that helps on a long enough trip, it's only 7 miles to work.
4) The "amplifier", whatever that is, isn't working.
5) All the insulation is torn off the underside of the hood. I wondered about using contact cement to attach a layer of Reflectix www.reflectixinc.com from the hardware store to keep heat in the engine bay, allowing the motor to reach operating temp.
Our 2001 VW TDI provides heat like hell...what am I missing?
Sincerely,
Kevin
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Starting from #5, hood "insulation" is primarily for sound deadening, not temperature related so forget it.
'80 models have a "servo" under the hood and an "amplifier" under the dash, behind the glove box. The servo is mushroom shaped and has a vacuum harness and electrical harness connected to its top and the heater lines connected to its base. The "servo" is an expensive ($300 - $400 rebuilt) replacement, the "amplifier" is the logic board and costs a lot less ($100 rebuilt). Both need to work for the climate control to be automatic.
There's also an Auxiliary (electric) Water Pump on the right fender, near the servo. Its function is to push coolant through the heater, especially at engine idle. The aux. pump may be broken or disconnected.
If your car has a "monovalve" instead of a servo the car is an '81 model and you're a lucky guy to not have the servo system. The monovalve defaults to full heat if it's broken. Monovalve repair kits are available for about $30; its a relatively simple DIY job.
The engine should run at 80 degrees C. Diesels don't warm up as quickly as gas engines. The engine's heat is in the TOP radiator hose the cooled water from the radiator to the engine is in the lower hose - no wonder the lower is cool - good radiator!
After some driving you should feel the heater hoses at the firewall, where they enter the cabin. They should be hot - 80 degrees C. If they're not you need to replace / rebuild the monovalve or servo or amplifier etc.
The DEF position calls for maximum heat - try it.
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actually, it's also for insulation to the upper outside... since mine is gone...on rainy days...the water on the hood steams up at a red light...loooks scary... but I know...that it's just the water... :O)
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Just a technical point. The amplifier is really just that, an analog amplifier. It only amplifies the small difference voltage generated by the resistor chain which then drives the servo left or right for more or less heating/cooling. It doesn't have any logic, digital circuits, etc.
One thing is for sure, if his car does have auto climate control, he'll have lot's of experience on that part of the car before long. lol

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