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I am having problems with condensation building up and freezing up on the inside of the car.

I am using the defrost with outside air-auto set up.

Its very cold in Ontario so we have to use the heater all the time, the car is being used for mainly short hop drives in the city.

Any recommendations would be welcome.


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Basically the problem is that your lexus is too damn well sealed.

In order to decrease the operational of load the A/C compressor during the summer months very little air "flow through" capability is provided.

As a temporary measure it helps if you lower one of the rear windows slightly to help move the HUMID airflow from the vehicle. While you are in frost/defog/demist mode I would be CERTAIN to lower a rear window slightly.

It also helps to lower the windows in the garage at night so most of the day's moisture laden atmosphere can escape.

It will help also if you disconnect the electrical connection to the A/C compressor during the winter months. By factory design the A/C compressor is disabled if/when the OAT declines below about 33F. So when you leave your nice warm garage in the early morning the A/C will operate briefly, until the OAT sensor "feels" that outside air temperature, and then it will be disabled.

In the meantime it has become saturated with moisture from the incoming airflow and now that moisture will be released into the passenger cabin.

As a long term fix I added four 12 volt low volume air exhauster fans within the trunk area to MOVE the passenger cabin atmosphere OUT anytime the defrost/defog/demist mode or the rear window heater is activated.

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Thanks wwest, highend solutions! Currently we have a reno going on so there is no garage available and we have had lots of snow...With your long term fix how did you find the proper cable to tap in to the defrost\RW defroster? are the fans an OEM option? did you have to drill flow holes into the rear trunk panel (back of rear seats) to let the air flow? Thanks for your help.

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The is an exhauster port at the very bottom of the driver's side rear quarter panel, if you close up the car and turn the blower to high with fresh air source you can feel the flow with your hand in that area easily.

At highway cruise speed there is positive air pressure under the car in this area so very little passenger cabin flow through occurs.

If you look at the bottom of th erear window you will notice a small gap between the glass and the rear "shelf" cover, this is the flow through path from the cabin into a channel in the car body immdiately below. I added the fans, two at each end of this channel, within the trunk's quarter panel area at each end of the flow channel.

To power the fans I used a diode "or" gate, one from the rear window defroster heating power source and another from a second "pole" of the switch used to modify the IAT sensor.

Lexus relies EXCLUSIVELY on the ability of the A/C to dehumidify the incoming airstream and thereby remove and prevent interior windshield and window condensation from forming, EVEN WHEN/AFTER THE A/C HAS BEEN DISABLED BY THEIR OWN DESIGN!

So when you switch the system to defrost/defog/demist mode you will often get airflow to the windshield that is as much as 20F cooler than otherwise footwell heating airflow. Airflow that is FULLY SATURATED WITH MOISTURE if it happens that you have driven into a COLD area from an area wherein the temperature was say, above 40F (Seattle to any one of the nearby mountain passes on a winter day). In that case you just might find yourself fully blinded by actuation of the defrost/defog/demist function.

To assure that I always get MAX HEAT and MAX BLOWER SPEED in defrost/defog/demist mode I first actuate a switch mounted under the dash by my right knee that adds a resistance into the IAT sensor circuit such that the automatic climate control system sudden "thinks" that the passenger cabin air temperature has just gotten VERY COLD.

One side of the DPDT switch adds the resistance and the other side energizes the fans.

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