Hello everyone,

I have a 1992 LS400 with 86,500 original miles, was a 1 owner car prior to my purchase, was garage kept and all service was done by the local Lexus dealer it was purchased from. I have noticed an interesting issue, when I turn my A/C button on, the A/C will control will turn off leaving the clock on, however if I leave the A/C button off it will stay on and operate like normal. With this I have also noticed that the front A/C fans do not seem to be turning or are turning at extremely sluggish rates. I did some research already and have seen people saying to check/replace the ECT Sensor (not the one near the shifter), its supposed to be Engine Coolant Temperature Switch/Sensor. Is this the correct fix for this issue, or am I looking at the wrong part? Is my A/C control unit shutting itself down to save the system from further damage?

92 LS400 (UCF10) OEM Part #: 8942220010

Delphi Part #: TS10198


Just to clarify I am talking about the 2 front fans that are supposed to be cooling the A/C condenser not the single large fan acting as a pull fan cooling the radiator.

 

ECT is the engine coolant temperature sensor, and is to control the engine not the AC. The fan control temp sensor is in the bottom of the radiator, and fans not just for AC.
You need to run the climate control diagnostic and see what it reports. I think a search of you tube will show how to do it. First thing I'd check is does the ac compressor inner part of the clutch turn by hand? The engine already turns the pulley part, so no need to check that. If it won't turn then that is the problem. AC fixing can be a mess.
If a compressor puts debris in the system it has to be flushed, all seals should be replaced, and there are some on the evaporator and means dash removal and major taking the rest of the stuff behind it to get to it.

 

I ran the A/C diagnostics twice, once while the engine was cold and again when it was at running temperature (slightly below the 2nd indicator bar). Car was in the sunlight both times, and both times the report was 00 indicating no issues. I cycled through the test modes 3 complete circuits each time, no issues reported by the A/C unit.

I checked while the engine was both cold and at operating temperature, both times regardless of the A/C being on or off the front 2 fans were not moving at all, the radiator fan was moving as required.

Do you think this is a bad fan control temp sensor or bad fan motors?

 

So is the compressor turning? Your too focused on something that may not be a problem.

 

The electric fans do not by default with the AC on. You'll need to troubleshoot fan operation right at the fan if you want to rule out a bad fan or not. The electric fans will run when the computer sees coolant temp rise and doesn't feel like the belt driven fan is providing enough air flow. The electric fans will cycle under such condition. The computer will also look at AC demands and environmental conditions to determine whether to run the fans or not. One way you can check the fans is to give it a few day's when it's hot as hell down here in FL and leave the car idling still with the hood shut after a trip. The car will probably have trouble maintaining a coolant temp it likes without cycling the electric fans on and off. You can let it idle for 5 mins or so on a hot day and you should see some fan operation. If not, you'll need to troubleshoot them and work backwards if you don't find a problem with them.

This is the electric fan logic on a 95-97. The earlier models are most likely pretty similar.

 

So I managed to pull the Fan Control Sensor Connector out from near the bottom drivers side of the car, when I did that the fans immediately kicked on, so no dead motors, it seems like its a faulty Fan Control Sensor.

As for "waiting till its hot as hell down there in FL".... its been 88 - 91 the past few days down here, hard to believe that waiting 1 1/2 more months for the "dog days of summer" to hit where it gets to 110 with humidity before I can really test out the coolant.


As I previously stated I have tested the car at cold and at RUNNING TEMPERATURE, perhaps I am not properly stating that temperature or what that means. Using my mechanics wording (he has been working on cars professionally for 31 years). A vehicles RUNNING TEMPERATURE is its NOMINAL temperature bracket usually defined to the average driver by the temperature needle being at, just below, or just above the middle temperature line in the instrument cluster.

Taking that definition and applying it to my Lexus Owners Manual about nominal temperatures that should equate to around 180F - 210F

Also please note that the A/C system test found no errors with the system and the system was just recharged with R12 by my mechanic a little over 1 month ago, and a dye was placed in at my request just incase there were any leaks in the system as that was when I purchased the vehicle.

Also yes the compressor turns and the clutch kicks in, and my air gets VERY COLD even with the fans off, but after 5 minutes of idle in traffic the A/C system completely powers off and remains unresponsive/unpowered until I turn off the car completely and restart. Now if I turn the A/C button off but leave the system going it stays on no problem, after browsing this forum and Lexus Owners Club I noticed a regular trend with the LS400, that the Fan Control Sensor (Radiator) seems to wear out at around 75K miles on average and it's a cheap and easy fix if you have the socket set. So I did the unplug test and even without starting the car the fans kicked on in high mode, and you can't mistake the air and noise those fans make on high, holy crap!

 

Regarding my weather comment I was just making a joke as, I'm not sure what part of FL you're in but, it's been pretty awesome here the past few days actually. The hot day with the car idling in park is more of an observation than a diagnostic process. I'm certainly not saying you need to wait until July to figure out what is wrong with your car...

I'm not sure what you are referring to when you mention the temperature 'bracket.' If you're saying that the temp gauge is designed to where the cars operating temp puts the needle near the middle of the gauge then that is correct. If your mechanic was talking about the needle sitting in the middle of the gauge nice and pretty when the temperature is between a range as you mentioned then that is not true with your Lexus. That is common on some domestic vehicles like Ford's. Your temp needle moves proportionally with your coolant temp. You'll see that it rides in the same spot for the most part but you will also see very small fluctuations in temp from coming to a stop and such. Your car has 3 different engine coolant temperature sensors. One for the ECM, one for the gauge in the cluster, and one for the electric fans. The one you are talking about on the bottom drivers side of the radiator is the sensor that tells the electric fans when to turn on. When you unplug the sensor the ECM either sees a resistance of zero which looks like molten lava in your radiator, or it see's the open in the circuit and turns the fan on high by default for safety.

You're not really troubleshooting fan operation by unplugging the sensor but at least you're seeing the fans turn on so you can most likely rule them out. I'm really not familiar with the fan coolant temp sensor 'wearing out' like you mention, but maybe. You may possibly have an issue inside your actual AC amplifier with the issues that you are describing but I'm not mentioning that rather than saying that is what's wrong with your car.

Your problem sounds a little weird. And weird problems usually mean internal shorts in electrical components and things like that. I would also inspect your ECM. The failing capacitors within the ECM's of the car can cause all sorts of weird issues including some AC issues. When experiencing any sort of weird electrical problems an LS400 owner must consider the ECM. If you don't keep the ECM in mind you can find yourself in a wild goose chase while chasing your tail at the same time. Been there.

 

Thanks for that, also its kind of hard to detect humor or sarcasm through text, you had my eye kind of twitching there :P

In regards to what my mechanic was telling me, he was stating if the needle is fluctuating between those temps it's perfectly normal so long as its not bouncing like its on a trampoline and going nuts.

I do know from the MyCarfax report that the ECU was serviced by Wilde Lexus of Sarasota on 06/15/2012 @ 84,034 mi (Current mileage is 85,566 mi) the following was done on that date and mileage:

• Recommended maintenance performed
• Electrical system checked
• Engine/powertrain computer/module checked

Here are the diagnostic links & PDF that I followed to get to my current line of thought.....

http://www.lexls.com/tutorials/air/accodes.html

http://us.lexusownersclub.com/forums...nt.php?id=9075

 

I just replaced the ECT Sensor near the bottom of the radiator on the drivers side in my 92 LS400, the fans stay on high even at cold start, does this mean I need to disconnect my ground to my battery and replace the #1 & #2 fan relays underneath the drivers side headlight?

Additional information:

PRIOR to the change the fans would not turn on at all

When I disconnected the ECT switch from the sensor they turned on and stayed on high

No other issues seem to be plaguing the car, ECU/ECM was checked less than 1500 miles ago by the Lexus Dealership

A/C magnetic clutch engages just fine

A/C diagnostics report no issues

 

ECT is on the front engine coolant cross over coolant manifold, next to the cold start switch and the sensor for the temperature gauge. There is not an ECT on the radiator.
Maybe a fan temp switch or sensor.
Could be a relay problem. If not them then likely the control unit.

 

The part I replaced is called the Radiator Fan Switch on RockAuto, I purchased mine through AutoZone (Duralast) for $33.99, and I installed it. *Note I am using the specific RED Import coolant.

I need to find a voltmeter/multimeter, I know my father has one lying around, I just don't know if I have one or if my father in law took it (I know my wife has one but her dad takes stuff all the time).

I know for a fact its not battery or alternator issues, the Alternator was just changed over 1 month ago with a REMAN Denso one and the power steering is NOT leaking, I have checked this thoroughly.

As for the battery, I have had 2 battery tests done, one by my mechanic and the other by AutoZone, both reported the same charge of 783 CCA, positive charge, good battery, it's around 3 years old now.

Going from fans not working to fans staying on full blast is kind of interesting.... I do wonder if driving the car in its current state is going to cause damage... I can't see why driving the car with the fans operating at full speed would be harmful???

Also what is the control unit you speak of?

 

The deally with the buttons on it.

 

Ok.... now that my ECU issues are fixed thanks to LSCowboyLS (I recommend him for rebuilding your ECU's), I can now get back to replacing that damned Integreation Relay and my A/C Control Assembly issue.

Here is what is going on with my A/C Control Assembly...

1. It will work INTERMITTENTLY.

2. When it works, my cold air is cold, to the exact temp set, same when I select heating, when it's working it freaking works like a luxury sedan should.

3. The clock and LED backlighting works 100% of the time

4. The intermittent portion I am referring to is that the unit will become unresponsive to any button commands outside of the clock (clock always works).

5. I have sent the unit out to Tanin Auto Electronix months ago, they did some work on it, and sent it back with a "clean bill of health". Had my backlighting LED's changed from OEM Green to Bright White, kept the indicator LED's with OEM Green.

6. I have replaced the Radio-A/C-Mirror 10A fuse on the drivers side kick panel with an appropriate Japanese 10A fuse

7. When the A/C Control Assembly is working and I turn on the A/C, the compressor kicks in no problem, magnetic clutch engages just fine.

8. When the A/C Control Assembly was working during its intermittent cycle I was able to do a system diagnostic, returned code 00 indicating no system fault.


Let me make this as clear as I can....

I am having an intermittent power issue with the controls on my A/C Control Assembly (this is the big black box that has the buttons on it in your cabin that you use to control your A/C system with). No matter what I have LED backlighting including the display, the clock works 100% of the time as does the lighting. The power issue I am having comes from the Control Assembly turning on and off the buttons so when they are pressed it is as if the OFF button has been pressed.

Thoughts?