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Heater Stuck In 100% Mode


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I have the opposite problem as this one:

http://us.lexusownersclub.com/forums/index...755&hl=heat

When I turn on the fan to ANY temperature, I get maximum heating, even with the A/C setting on. This happened suddenly yesterday.

I know the A/C works (or used to work) because I had major service on it about 6 months ago. Also, when I turn on the A/C and get a blast of blow-dryer hot air, I can feel the compressor kick in.

When I start the car cold, and turn on the A/C - I get even colder air, so I know the A/C is working. But once the engine heats up, I think the heater max 100% issue overtakes my A/C and then all I get is fire-hot air.

Is there a thermostat or something that regulates the cabin heater? Because I think it's stuck in maximum temperature ON.

Any and all fan settings send out max. heat.

I read in the linked post above there is a cable or something under the hood? What's that?

Thanks

-Dave

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There is a little valve that adjusts the flow of hot coolant from the cooling system to the heat-exchanger in the car (the one that is running you out with hellish heat)

I would have to guess that this valve is in the "stuck open" position allowing maximum hot coolant to enter into the car. One of the recent threads about "Heat not working" had a post with a photo of what that valve looks like under the hood. If that valve sticks in the closed position, it does the opposite of what you are experiencing.

Good luck!

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Does the valve look like this? (have hopefully attached a photo I found of a new valve)

I don't have a camera with me right now - but I can see a (really dirty) version of this kind of valve, under the hood, on the driver's side, near the firewall, and there is a cable attached to it. Is this the heater valve?

post-17334-1232749935_thumb.jpg

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Place the controller into diag mode and see if there are any codes.

With the key off,

Press and hold the AUTO Button while also pressing and holding the recirc/fresh button.

Turn the key to the ON position ( not the start position )

The indicators light should flash four times, (at 1 sec intervals ). After the lights have flashed, any codes should be displayed.

IE the lights will flash at one sec intervals for the first number, there will then be a short pause, and it will then flash the second number.

It could be the heater valve, one of the temp sensors, or blend door problems. But checking for codes is easy and should take only a couple min. Note you might have to read the codes a couple times, I know it took me a couple times before I got use to counting the flashs.

PS to exit the diag mode, press the OFF button.

If you have any codes, let us know,

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I think this diagram may help...

Heater piping stuff

Does the valve look like this? (have hopefully attached a photo I found of a new valve)

Could possibly be it. Sounds like you are on the right track. Follow the smallish rubber hoses that go through the firewall and one will have a fitting like you are talking about with a cable that controls the valve.

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Thanks

I tried to wiggle that valve/cable assembly. Then I started the car, and ran the system in Auto mode, and went Max cold to max hot, and back and forth a few times, and my friend could see the valve/cable moving under the hood. I think I just now unstuck the thing!

I drove for about 20 minutes, and selected various temperatures, and it seems to work fine now. I think my poking around down there unstuck it or something?!

I will run the diagnostics as explained 2 posts above, and see if there are any codes. But I think somehow the valve was stuck in the HOT position.

Intermittent stuff like this seems to be typical of my particular car.

Thanks

-Dave

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Thanks

I tried to wiggle that valve/cable assembly. Then I started the car, and ran the system in Auto mode, and went Max cold to max hot, and back and forth a few times, and my friend could see the valve/cable moving under the hood. I think I just now unstuck the thing!

I drove for about 20 minutes, and selected various temperatures, and it seems to work fine now. I think my poking around down there unstuck it or something?!

I will run the diagnostics as explained 2 posts above, and see if there are any codes. But I think somehow the valve was stuck in the HOT position.

Intermittent stuff like this seems to be typical of my particular car.

Thanks

-Dave

Now that you've found the prob, I would do a flush to clean out the heater core and the valve and also lube the cable to make sure it doesn't happen again.

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I drove for about 20 minutes, and selected various temperatures, and it seems to work fine now. I think my poking around down there unstuck it or something?!

I bet so!

Great news! :)

I think that with 100k you will see some of that. I wonder if you can use a spray lubricant like white lithium in that cable assembly to make sure it isn't hanging up in there? Just thinking out loud here...

I am sure one of the more knowledgable guys on this type of thing will weigh in with some suggestions on what maintenance you can do now that you have located the root cause.

Edit: dang...these knowledgable guys are quick. They got in before I could finish typing!

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Code 21 is for a open or short in the solar sensor circuit.

Code 41 is for the air mix dampner position sensor does not change value even if the ecm operates the air mix servo motor.

Do you have a manual? It appears that you have a couple different issues. Of course if there has been any dash work, IE new stereo or such, make sure the connectors are fully seated.

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Wow - the diagnostic mode really works, just like you said it would

The codes were: 21 then 41, flashing.

-Dave

So, those codes indicate:

21 (3) .............................. Solar Sensor Circuit

41 ....................... Air Mix Door Servomotor Circuit

The solar sensor controls cabin temps according to what the sensor is reading. In my LS it's on the dash at the bottom of the windshield pass side. Not sure where it is on yours. Btw you didn't mention the year of your RX. You might want to update your profile to include it. Helps sort out the advice posters will give you. If this sensor is failing you will lose some control of your cabin temps.

The AMDSC controls the blending flaps as G_J mentioned in his post. This servo controls the direction and amount of air flow. Could be that there's some strain on the circuit from trying to move stiff flaps or resistors are failing. If you begin to experience probs again I would have these checked out.

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Thanks for the diags info

I mentioned this car is 1993 ES300 in my first/second post. My user name refers to my RX300 2002 because that was my first (used CPO) Lexus.

This ES300 I inherited from my Dad when he passed away last year...

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Thanks for the diags info

I mentioned this car is 1993 ES300 in my first/second post. My user name refers to my RX300 2002 because that was my first (used CPO) Lexus.

This ES300 I inherited from my Dad when he passed away last year...

Sorry to hear about your dad. Yeah, I read the first post. Sorry about that, I just forgot which car we were talking about. :chairshot: You know confusion starts to set in on Fridays, been a long week. I'm ready for some sleep. Anyway Let us know how it goes.

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Hey George, how did you sneak in there? :D

Hi, I've looked at my heater controller before, and thought the diag info might help here. Alldata has some good info on this, but it appears that these controllers are more sophisticated than I would have quessed. BTW according to alldata, the temp sensor should measure app 1.6 to 1.8 Kohms at 77 deg F

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