Cooling fan questions

[Megavolt-380]

TGP 1989
When operating without the air conditioner on, everything is fine. But when we turn on the air conditioner, even on a cold engine, both the main and additional fans turn on immediately. The question is - is this a normal operating mode? After all, the pressure sensor should be responsible for turning on the fans, but the control unit ignores any state of the sensor. 

[55trucker]

Indeed it is........

both the primary & secondary fans will engage.

The low pressure sensor engages the entire system when the system is above approx 40lbs, the compressor does NOT cycle, once engaged it runs 100% of the time the a/c is turned on (depending on the volume of system charge).

[jiggity76]

I just rebuilt the A/C system in the International and yes, both fans run when the A/C is on.  The fans and the compressor is doing exactly how 55trucker is describing.

[55trucker]

I should also add that the coolant fans engagement is speed dependent, when the a/c is on the pcm will disengage the fans as the vehicle speed increases to the point when they no longer need to be.

[jiggity76]

9 hours ago, 55trucker said:

I should also add that the coolant fans engagement is speed dependent, when the a/c is on the pcm will disengage the fans as the vehicle speed increases to the point when they no longer need to be.

When I first started charging the A/C system, the driver side fan kicked on.  It might have kicked on when I actually pressed the climate control panel.  As I started filling the system...I think with the start of the 2nd bottle of freon, the compressor kicked on as well as the passenger side fan.  Yes, when I'm on the HWY or Interstate, I'm assuming both fans have shut down long ago by that point.  I can then hear them, (and feel a little bit), when I'm back in city driving mode.

[Megavolt-380]

But it turns out that if the car is stuck in a traffic jam at idle speed and the brakes are pressed, and plus two fan, the voltage drops

alternator - new  105a

Edited 3 hours ago by Megavolt-380

[Megavolt-380]

Was this the mode from the factory?
then what is the purpose of the pressure switch near the dryer?

[55trucker]

2 hours ago, Megavolt-380 said:

But it turns out that if the car is stuck in a traffic jam at idle speed and the brakes are pressed, and plus two fan, the voltage drops

alternator - new  105a

Yep, that's what happens at idle, the idle speed needs to be at least 800rpm to combat the load on the charging system. In those situations put the trans in neutral, raise the engine rpm to get the charge rate back up. Something one can do is to pull out the secondary fan relay to prevent the secondary fan engaging. The secondary fan only engages when the a/c is on, it very seldom engages for engine temp cooling purposes as it doesn't engage until the engine temp is near 220F degrees. The side effect of this is the condenser will not dissipate as much hot air as both fans running.

[55trucker]

2 hours ago, Megavolt-380 said:

Was this the mode from the factory?
then what is the purpose of the pressure switch near the dryer?

The pressure switch is a necessary device to prevent the compressor engaging when there is not enough refrigerant in the system.