My 1999 RX300 heater only works in cold weather. Today I had an appointment because it was again not working (since spring). It was 42 degrees traveling to the mechanic and it worked fine. Did the same thing last winter.
thank you!
Zenshin

 

The problem is that it does not work at let’s say 60° where it would be real nice to have a heater. Since there is no direct control, to turn the heater on, is there a temperature sensor or something like that which could be faulty?

 

start with the basics ... check the coolant level in the radiator ... engine cold. Top up if necessary. If it is small amount, you can use distilled water.

Next is share with us the temperature gauge reading. Assuming it pegged to C when you start and in about 15 minutes of driving it should be near the 50% mark. Next tell us if it stays at that level or fluctuates as you drive.

Once the coolant issue is verified then we need to look at the air-mix servo. Search you lead you more discussion on the mix servo.

Please do the diagnosis in the order I listed above.

Salim

 

Thank you Salim. Radiator is full to the top and the temp gauge rock steady a few ticks above half way.
To jump a little ahead: where is the thermometer that reads the inside cabin temperature. From the way this heater works and actually also the air conditioner I believe the reading to be faulty. Since there is no heater control button and it is all dependent on temperature to call for heat I do believe that it isn’t sensing the right temperature.If the car thinks it is hotter inside than it is it will not react in mild weather but works fine when it is truly Cold. I’ve had two winters like this and the heater itself works fine once it gets cold. I was fooled again this year in to thinking that maybe it really had gone out And for the second time on my way to the mechanic it started to work after not coming on when it would have been nice this fall.
cordially,
Zenshin

 

From my memory, the temperature sensor is behind the little vent below the steering wheel to the right side. When the plastic panel is removed you will see the sensor attached to the panel.

 

I have not delved into the cabin temperature sensor and associated controls. I bet there is an ECU involved that governs the mix-servo. Repair manual can help.

Salim

 

The mode and blend door actuators are a known issue on these cars that can lead to incorrect temp air being sent to the wrong places. The sensor can be ruled out very easily by using a dealer level scan tool or a handheld blowdryer/heat gun to see if the expected response occurs when cold air is requested, that being the other vents blow cold and tbt actuator swings the blend to full cold.

 

Does the sensor matter when you are in manual mode, like when you manually change the mode or the blower speed?

 

I don’t think the sensor matters as far a influencing mode or blower speed. When you change them there is no effect on the output temperature.
Now that it is quite cool in the morning the heater works fine, which is very good! I plan to put the sensor to the test soon. We are just getting settled into a new place and there is nothing but work right now. I can’t really afford a trip to the dealer so I’ll try to do some logical investigation before buying a new sensor.
Clublexus is the greatest!
Zenshin

 

Correct the blower speed is just the fan speed and mode just directs the air output. In AUTO the switch positions are ignored and the computer takes over and adjusts the blower speed and the mode.

I vaguely recall a test mode that you can get into to test out the system and if I remember it right it can tell that the blend door is stuck or not. Please verify my statement before accepting it.

Salim

 

Wouldn't it make sense that when in auto the computer would control the blower speed, mode, AND mix? That way the mix and speed are adjusted depending on how far the set temperature is from ambient. And if you aren't in auto, turning the temperature dial should have a direct effect on the mix regardless of the ambient temperature, like it does in a car that doesn't have automatic climate control.

 

Interesting point of view. I am making my best guess and the vehicle may be doing something different. In my view the mix-servo is directly controlled by temp setting and cabin temperature sensor. Heater compartment door opens and closes based on the difference in the temp-set and cabin temp sensor. To me the "AUTO" means self regulated fan speed and selection of air flow ports.

I just thought of another thing that can be one of the possible issues. The fan in Heat mode is also controlled by coolant temperature [for sure in Auto mode]. The fan does not kick into higher speed till the temperature of the coolant is raised to a set value.

Salim

ps: There are two temperature sensors in the coolant for RX. One is a switch and the other is analog sensor for ECU.

Salim