Drizzle66

I've searched pretty extensively and found several posts that are ALMOST the same as my problem -- but not quite...



My '96 LS400 runs very well and is well maintained. But I've noticed that when I go on a long trip (two hours or more -- it usually takes quite a bit more than that), my heat or A/C airflow fades and eventually quits blowing. It's not a coolant level issue -- it does the same with A/C as with heat. (And it's an AIR FLOW ISSUE, not the temp of the air...) Different RPMs or engine load don't make a difference either. And I keep the cabin air filter changed regularly, so it's not that. In order to get the air-flow control back, the car has to sit for a period of time (sometimes stopping for lunch will make it 'all better', and sometimes it requires an over-night stop).

Just driving around my normal daily life, it never happens -- so of course, I can never get it to my mechanic while it's acting up because I have to be several hours from home before the problem ever shows up!

It would be my guess that the problem is located in a control that directs an airflow 'door' somewhere. When the problem occurs, you can hear the fan running as normal (responding aurally to manual adjustments in fan-speed). But the air simply quits flowing from ANY of the vents. The air always comes from the correct ports (defrost, face, feet) when it IS flowing. But when the problem surfaces, the air-flow simply tapers off to nothing.

It's really maddening since this car is my preferred driver, and it's BEAUTIFUL for long trips -- except for this! If it's during the need for A/C, it's merely an inconvenience, as I don't mind driving with my windows down for a while. But when it's cold out (like on our trip home from our daughter's place more than three hours away last night), it can become a hazard, since I have no way to provide ANY airflow to the windshield, and keeping the windows from fogging up or even frosting over can become very challenging. (Especially with two adults and two dogs in the car, like I had last night...)

Anyone else every experience this? With winter here, I really need to find a way to address this. But since I can never experience the problem except when I'm a few hundred miles from home, it's never practical to examine things while it's acting up!

Thanks in advance for any advice or shared experience...
Dennis

Scraape

run A/C diagnostics! Hold Auto & Recirculate buttons, then turn ignition to on

https://www.clublexus.com/forums/per...eed-codes.html

Drizzle66

Thank you Scraape.

I tried that, and got the "00" flashing at me in response. I'll keep the code sheet with me and retry it the next time it 'acts up'. Perhaps it might be one of those "Code only appears when problem is current" issues... (Though that seems unlikely, given what the two codes are that include that remark.)

Dennis

Drizzle66

Okay -- I finally ran into an opportunity to look for active codes in my system last night. (Remember, this situation will only show itself after several hours of driving...)

I stopped for gas a half hour or so after the symptoms reappeared, and learned the following.

While still running, before shutting down to run the diagnostic, I noticed that while there was never even a modicum of warm air-flow to be felt from any vent, turning the temp control down would deliver some cool air-flow from the face vents, once the display showed either both floor and vents should be active, or with further depression of the temp value, it showed only air from the vents. Given what the display showed (or my ear could hear) for the fan speed, it felt like perhaps less air-flow than there should have been -- but still there was some.

(I'll add to the above the fact that on a trip to Florida a couple of years or so ago, it did the same thing with A/C air-flow -- refusing to supply cool air through the vents when desired, even though both the display and my ear determined that the fan was running as it should be.)

Okay -- so I shut the car off just briefly enough to allow me to enter diagnostics and check codes. When it finished its blinking, it flashed 21 and 24 alternatively, and showed the 'face-vent' icon on the display.

Noticing that the recirc button was lit, I tried pressing that. The system seemed to enter a loop of test circuits -- sequencing through all possible air delivery methods -- cool and warm, and through various ports (floor, face-vent, defrost). All of those settings appeared to operate properly.

Restarting the car and continuing on our journey home, we had full heat again -- but only for about 10 minutes of driving. When it again tapered off to nothing.

The codes confuse me. 21 is supposed to concern the "Solar Sensor Circuit". Okay -- maybe. But what the heck is 24?? That's not on the list! And if the solar sensor is causing it to decide not to deliver warm air to the floor (or anywhere), why would it have refused to produce A/C while in Florida a couple of years ago? Certainly the solar sensor fault that might cause it to refuse to deliver warm air would encourage it to give me all the cold air it could, right??

Anyone have any thoughts or experience?

Thanks again!!

Scraape

solar sensor circuit might be because it was dark outside at the time or you did the test in the garage or somewhere without much light?

i'm not sure what 24 is, but might be a similar code related to the solar sensor.

try running the diagnostics in sunlight

Drizzle66

When I ran the test, it was night. I was parked under the lit pump area at a fuel station. There was quite a sufficient amount of LIGHT, but virtually no radiant HEAT for that sensor to detect.

I think it's important to understand the significance of code 24. But it's completely absent from the list you provided above. Any other means of learning what that code means?

"Running the diagnostics in sunlight" is frankly a fool's errand because as mentioned earlier, this event will only occur after several HOURS of driving. In yesterday's case, it was after about six hours. I don't drive that far that often. But when I do, it can be damned inconvenient to have the heat (or A/C) crap out on me.

Certainly, I will seize any opportunity to do this again when it is daylight. But predicting when that might be is difficult at best. And honestly, taking this car on long trips during the possibility of NEEDING that HVAC system to work properly is something I am disinclined to do because of this problem. Which further procrastinates the likelihood of learning anything further...

Yamae

The code 24 is for the passenger side of the solar sensor. I don't think it's related to your problem.
Is the fan speed still controllable any little after the long trip?
I worry that the fan speed controller is defective. The confirmation can be done applying the heat to the semiconductors in the controller using a hair dryer or something similar which can heat up them about 100 degrees C.

Drizzle66

Thank you Yamae,

The fan speed seems to be completely unaffected. I can HEAR the fan running normally. I am pretty sure the issue lies with something that controls an air-flow door. The fan continues to run -- it sounds normal. But the air-flow from the floor vent gradually tapers off to nothing.

If there is a circumstance where the climate control system intends to deliver warm air to the floor and cool air to the face, then the cool air still flows through the A/C vents while the floor vents cease flowing.

Thanks for the suggestion though!

Scraape

last piece of advice i forgot to mention:

not sure if your 96 comes with the factory phone in it or not, but i was having some issues with my radio cutting out randomly as well as my A/C / Heat randomly stopping airflow.

I removed the factory phone ECU that was in the trunk, plugged the old connectors together, and haven't had any issues since.

the phone ECU was designed to mute the radio and lower the fan speed of the A/C when on a phone call , from what i've read.

maybe you don't have a factory phone, but I figured it's worth a try since that solved my issue. It's easy to remove the phone ECU too so couldn't hurt

Drizzle66

Thanks for that Scraape. No, mine doesn't have the factory phone. But again -- I appreciate your sharing the thought!

YODAONE

A possible solution;

https://www.clublexus.com/forums/ls-1st-and-2nd-gen-1990-2000/846574-interior-temperature-sensor-1995-2000-ls400.html

Drizzle66

Thank you YodaOne. I'll mix that into the pot of possibilities...

I spent a great deal of time on the phone with the foreman of the Lexus service dept today and MAY have landed on the problem. He suspects the Amplifier (on the back of the Climate Control controller head). It makes sense since the symptom reflects the possibility of a capacitor losing its charge, or something similar to that. Which is essentially a function of the Amplifier.

I have two identical LS400s, so I'm going to try swapping them (Amplifiers) and see if my next long trip shows different behavior... It will take some time to be sure, but we'll see what I learn...

Thanks again to everyone for their ideas!!!

Losiracer2

I'm not sure if this could be a symptom, so it's more of my learning benefit too...but would the leaky capacitors in the ECU affect the climate control?

I've still yet to replace the 5 in my 97 that's required....I purchased them from Digikey in case it begins showing peculiar symptoms that aren't logically traceable through troubleshooting with traditional methods.

Drizzle66

Losiracer -- I wasn't even aware of that issue. I have no idea. Hopefully someone with more experience will chime in...

YODAONE

While replacing the climate control head LCD, on 1993 LS400, I replaced all electrolytic capacaitors.

The earlier controls used random value capacitors.

Climate control function appeared to perk up a bit.

I ordered replacement capacitors for the 99 from Digi-Key awhile ago and recall just one value used.

There was minor "crusting" on several.of the traces. (1999 LS400 ) on a spare climate control head I acquired, so reasoned to go through mine at some point.

Have yet to install and will post values on 99 if interest on anyone's part.