open the plug (screw) at the bottom of the radiator, open the radiator cap and let the system slowly gravity bleed while pouring more fluid into the radiator. do this with vehicle off.
Your low on coolent! system set up to protect engine first so routs coolent away from heater.
Empty the hose and and more coolent
undo the hoses from the fire wall and poor coolent into both of the hoses until the coolent fills up and starts comng out the hoses at both ends
Go to Walmart or an auto parts store. Buy an a/c recharge kit for R134 freon coolent. Follow the directions on the can..
You should never attempt to bleed your a/c system unless your are quailfied, the reason why is there is a low pressure side and high pressure side to your a/c system, and besides the fact that knownly releasing the coolent gas in to the air is a crime punnishable with fines, because it bad for the enviorment and to your health. if you don't do it correctly and release the high pressure side your very likely to injure youself. Beside the odd that the coolent gas is used up is slim, normally if your a/c isn't working anymore it mean you have a leak in your system and the gas has escaped.
Coolent leaks from pasenger side of engine.
Add coolent add some R134a but don't over charge, you will blow the ac compressor seals
chesk if there is coolent and if not put some in and if that doesnt work bring it to a toyota dealer
check the radiator coolent level...If it is low make sure the engine is running when you add the coolent. Heres a tip, Take the radiator cap off while the car is cold and not running. Then start the enigine and add coolent...this will prevent air bubbles that may cause overheating. The coolent should circulate fill up the radiator then wait, the level will fall and then add some more...then just repete until full.
Unlike most other BMWs the Z3 does not have an electric water pump for the heater. This means it is sensitve to air locks in the system. Eventuly these should clear themselves. Mine took a week to come back to full operation after replacing the coolent and bleading the system as per the instructions. I think the overdrive observation is just a red herring and the heater will work once the engine exceeds about 1500 rpm for a while. Make sure the coolent and oil levels are correct and try holding the car in a lower gear for a minute or so at 4000-5000 rpm while driving. This should force the air out of the system.
i like to give a short application of specific heast energyland and sea breeze,air conditioners as a coolent, radiators ,
on the passenger side right by the fire wall you will see a nozzle that looks like a valve on a tire, that is your low side where you put your coolent. Follow the hose from the compressor and it will take you right there