Does anyone know how the cooling fans engages and with what parameters? I recently changed out my thermostat nd while bleeding the air, I noticed both cooling fans engage when I have the main Defrost button pressed. No a/c is running, and the engine is stone cold. (ambient temp is 0 Celsius)

Is this normal? It causes my engine temp to start dropping, from 180F to 170F and counting...Any info would help.

 

You sure the compressor is not running under defrost mode? This is common on a Lexus.

 

Yup. I can hear it engage and the rpm change when I manually press a/c, while in defrost mode.

 

Hmmm then I'm not sure. That is odd.

 

I would put eyes on it and see if the clutch hub center is spinning. As stated, defrost calls for AC and it likely overrides the softkey/physical button AC off as defrost enables AC for moisture removal.

IIRC only a single fan comes on at like 208 - 212°F (98 - 100°C) and only AC drivers both fans.

 

A/C system automatically engages any time the defrost mode is selected. The purpose for the air conditioning turning on is so that it chills the air to condense the moisture molecules within it...... Once the moisture particles are condensed, the heating system heats that cold air to evaporate the moisture and expel it from the circulating air. The reason why the cooling fans turn on up by the radiator is because it needs that air flow to condense the air before it enters the component called the dryer.

Sounds to me like your HVAC is working absolutely perfectly

 

I'll toss a wrench in the mix. If it's too cold outside the compressor won't engage under any conditions. Might account for the observation of the compressor engaging sometimes and others not.

 

This is common on every defrost system developed since the defrost system was developed. It's not unique to Lexus. It's the standard by which all defrost systems operate be it a brand new $2,500 Geo Metro or a pre-owned $125,000 Mercedes G63.

The only vehicles that I'm aware of which don't operate on these principals are EV powered cars.

 

While the defrost mode is selected (physical button one), in the climate control menu, the A/C button shows its turned off. I can manually turn it on while defrosting and it results in me hearing the compressor kick on and the engine load change slightly. With the a/c option showing "on" the fans remain in the same state.

 

*facepalm*
I went over to look at the a/c pressure switch because I suspected it was bad or had loose wiring...Thats when I noticed liquid moving around in that clear glass peephole. I instantly look up and boom you were right. A/C clutch is engaged. Defrost does indeed override the touch A/C button. I guess the load change and rpm is for the fans drawing current...

I still get both fans on low when defrost is activated whether the car is stone cold or at operating temp...is this due to the a/c clutch being engaged, but not running because of the low ambient temps as mentioned by LeX2K, making the system think the compressor is running all the time?

Thanks for all the help guys this has been very enlightening..

PS I found a sequence for hondas that decouples the a/c from the defrost button. No luck finding one lexus.

 

You will need Techstream to program that. It is in one of the customize settings or A/C settings

 

No worries....
The flip side is your stone cold engine has its thermostat shut and isn't circulating coolant to the radiator until it hits (WAG) ~168°F or so. So the fans although on, really aren't slowing the warmup process down.

Given the temps there, these cars are equipped with two coolant heaters that should be checked for function. Although normally only found on diesels, we have Glow Heaters at the back of the engine where the coolant routes to the cabin.

Several names come up, PTC - heater, 'Plug, Glow' PN: 87343-30010. 'Plug, water by-pass glow'
Here's a picture...
https://www.lexuspartsnow.com/parts/...343-30010.html

These things get two 50A fuses 'GLW PLG1 & GLW PLG2' and two big relays. I'm guessing that if you pulled GLW RLY1 and GLW RLY2 and ohm between pin #5 of each socket, you measure across the heater and get something like 0.2 to 0.7 ohms. Meaning the heater elements are okay. If you get like 15 ohms or higher, they are dead. I need to try it on mine.




Tag #2IS Electrical Glow Plug Heater

 

Thats nuts hahah I never came across anything mentioning coolant glow plug heaters.
The fuse diagram is a little confusing, as it has 2 glow plugs labeled "GLW", and 2 more labeled "Diesel GLW". And the relays say "Diesel" on them. Anyways I visually looked and saw two glow plugs side by side near the heater core hoses. I get 1.2 Ohm for both relays (I think I was probing #5 as the others didn't give any reading.)

 

The parentheses around 'diesel' section indicate they are used for fans for the diesel applications. For me it's the labeling of E39, Combustion Heater Glow Plug...

This is all part of the of the ULE II emissions stuff. The sooner the engine is up to temp, the cleaner it runs. I would like to know if mine are working tho as the 350 seems like it takes forever to warm up. Especially if going down hill from a cold start. 10min later for the needle to move and blow warm air.

14V / 1.2 ohms ~ 12Amps which is well below the 2X 50A fuses...

I'll try to measure mine this weekend. No guarantees as its snowing...

 

OP can you confirm you saw two of these? See pic. I ask as the schematic does not align well with installation into that aluminum housing at the back of the engine. As in the E39 box in the schematic shows two 12V sources going in, and No ground. Essentially making them useless. In reality, they have a 2X 12V supplies and the case goes to ground completing the circuit when the relays close which actaully makes them independent and my test method is wrong as their schematic is wrong!

If you measure them again, go from pin #5 to ground on each relay.
Hindsight says 14V / 0.4 ohms = 35 Amps each with some safety margin.