Automotive Air Conditioning Information Forum

I have a 1999 Lexus LS400 w/ viscous cooling fan and 1 factory electric pusher fan on the condenser. I have just replaced the viscous coupling so I know that it is working properly. I live in Pheonix where daily ambient are approximately 105-110 right now. My car sits in the sun all day at work and is very hot inside when I return (I use a sunshade, crack windows, and have very dark tint).

I have noticed that after startup, the AC doesn't work particularly well at idle for at least 5+minutes until the engine has warmed up and the viscous fan engages based off radiator temperatures.

Running some quick numbers, assuming high side pressure is around 300psig with R134a, condensing temperature is about 160 degrees F. That means the air coming off the condenser is at some temperature cooler than 160 degrees, which doesn't seem like enough to engage the fan.

I did observe this, I let the engine idle for 7-8 minutes with the ac on and only then did the radiator get hot and cause the viscous fan to engage (it was audible). At that point the AC began to cool better. The electric fan was on the whole time but doesn't move a whole lot of air. It was also on low speed (it can be on low or high speed depending on what the ECU wants).

Is this working as designed? Is there any way around this? Ideally the mechanical fan would engage with the AC running even if the engine is not yet fully warmed up. I was considering getting a remote start to pre-cool the car but if the fan doesn't lock up for the first 5 minutes anyways it might be a waste. Couldn't this also run high side pressures up dangerously high?

Even when I drive off right away, the stop and go nature of my commute makes the first 5 minutes or so very hot inside the car.

I'm in Phx too. Your seats, dashboard, interior are all 150-160F after sitting in the sun. That heat needs to be removed, sounds like typical Phx AC operation.

Don't waste your time idling.

The compressor has a fixed displacement. Spinning it faster generates more mass refrigerant flow, which means more cooling capacity.

The viscous fan speed goes up with engine speed, even if not "locked". Additional engine speed heats the clutch. Additional engine speed adds heat to the whole heat exchanger stack, which decreases the time before clutch lockup.

Start the car and Drive Away. Drop a window for a minute or two to let the superheated air out of the cabin. Select a lower gear if stuck in low speed traffic until the cabin cools down - keeping the engine above 2000 rpm keeps the compressor at maximum output. Driving also provides more airflow over the condenser.

Most cars do not have any reserve cooling capacity at idle. They can maintain a comfortable cabin at idle once things cool off.

All good points, thanks for the tips. I may invest in some good electric aux fans one of these days but for now I will just take off after starting up the AC.

Thats what i usually do when starting up a hot car and then doing low speed operation- i select 1st gear when rolling around in traffic. Also if stopped for a light i'll pop it in neutral and rev it to 1500/2000 rpm. Obviously once the cabin cools off there is no need for this. But it does certainly help. I've kept a thermometer in the car on hot summer days-a light colored car-and on 90F days or so i've seen 140-145F interior temps. I can only imagine what its like in the 100F+ ambient temps...and there are always the people that leave pets and children in the car in these temps