SilverLS400

when i put the temp button to 85 it blows out cool air. The a/c switch is off and it get the whole car cold instead of hot. Does anyone have the same problem?

wwest

If its an earlier model, say 92, there is a cable that runs through the firewall and actuates the hotwater flow valve mounted at the rear of the engine compartment on the very top of the firewall. This control cable has been known to slip off.

Later models have a servomotor dedicated to this valve and you can visbibly see it working as you cycle from hot to cold cabin needs.

damon

My 94 LS 400 has a slow leaking Heater Control Valve. It has a thru-firewall actuating line as you describe above.

Does anyone have a part # , & a good replacement source?? Any help is greatly appreciated. Thanks,

wwest

is likely the only possible source.

petrojeff

My heater throws out cold air until I set it to the maximum temperature of 85 degrees. It then begins to put out very hot air. After reading wwest's post I looked and saw the cable that comes throught the firewall. I cycled the heater and the cable only moved when I got to the max setting of 85 degrees, and the cable made a full stroke on the valve. My question is.... Is the motor that controls the cable replaceable? Or do I have to get a new heater control unit? Any definitive answer is appreciated.

wwest

Your cabin temperature sensor (thermister)(above the drivers right knee) is likely shorted, the lower its resistance the hotter the ECU "thinks" the car is. MAX hot or MAX cold over-rides all
sensors.

It might also be and of three other sensors gone whacko, the OAT in front of the radiator for instance.

good luck

hyperopt

You need to test all sensors associated with temperature control systems. Heater control unit could also be bad or the actuator (least likely). Follow the troubleshooting instructions in the service manual before making a definite decision.

wwest

He said the cable actuator to the hot water valve moves full travel from off to on when he goes to max heat. That cable is driven by the mixing vane servomotor. The means that the mixing vane servo motor is in the (max) cold position and the only reason for it being there is if the ecu "thinks" the car is too hot.

The most likely sensor to do that is the passenger cabin temperature sensor shorted, even though an open sensor, wire, connector, would be a more likely occurrance. My second choice would be the OAT sensor being shorted, making the ecu "think" the outside world is extremely HOT. Third on my list would be the solar sensor, possibly making the ECU think the sun has gone supernova.

hyperopt

Logical explanation, and again, check all relevant sensors before blaming the problem on ECU or actuator. There are instances in which bad actuators only repond to full duty cycle, but this rarely happens.

petrojeff

Okay, room temp sensor pulled, reads 6.1K-Ohms @ 66Degrees F and about .65K-Ohms @ hair dryer temperature. Book says room temp should read about 1.65-1.75 K-ohms @ 77 degrees F and .55-.65 K-ohms@ 122 Degrees Farenheit. I just called Lexus and ordered a new resistor @$42. It will be in on 4/8. It seems that it has failed in a semi open instead of semi closed state and the car should be getting feedback that the car is cold and needs warming. Hyperopt and WW please theorize....

wwest

Makes NO sense....

Maybe the sensor was shorted until you "joggled" it?

Now that you have disconnected the sensor how does the system act?

On my 92 LS I have added a switch and a 200 ohm resister in series with my IAT sensor. I can activate the switch, which adds 200 ohms to the circuit, before I activate the defrost/defog/demist function so I always get heated airflow and higher blower speeds for more adequate defog functionality.

Higher IAT resistance = colder passenger cabin = more heat.

Puzzle.

petrojeff

I'm going outside to rehook up thermistor now. stay tuned...............

petrojeff

I reattached the thermistor and cycled the heater/AC. It behaved same as ususal. When I hit auto clilmate control, every time the A/C goes on. Blows only cool air. Ambient air temp is about 60 F. When I go to max, it blows hot. When I am in between, it only blows cool (not refrigerated) air. I completely removed the thermistor and the system acted the exact same. I measured the resistance again and shook the thermistor while I did this, it caused the thermistor to vary from5K to 180K ohms. It is obviously bad. It appears the system fails in this mode when it is open. I am looking at the electrical diagram to see if something else makes sense.

wwest

do you have a 1k resister you can substitute for the IAT sensor as a test?

petrojeff

No, but my buddy probably does. I will give it a try in the AM.Thanx WW.