99 Suburban A/C Aux fan

Does any one have AUX fan testing procedure?

I have aux ac fan that doesn't turn on.
The fan tests ok
The wires test ok
the relay test ok
new a/c-heater control in dash
fuse ok
+13v power at relay contact. (fan runs if relay contacts jumpered)
+13v Power at relay coil
Not getting relay ground (blue wire) switched by vehicle control module(computer)

I have read that this fan only turns on at high coolant temp or high a/c pressure.Is this correct?

Does anyone have testing details on this? Switching Temps,pressures etc.
Thanks

Sounds like you are referring to the auxlliary cooling fan out front of radiator, and not the rear blower motor inside-

So how do you know it does not turn on? - have the proper conditions ever been met? Is your engine a 7.4L?

The cooling fan is controlled by the VCM based on the following inputs:

The A/C system
The Engine Coolant Temperature (ECT) sensor
The Vehicle Speed Sensor (VSS).

The VCM will turn the auxiliary cooling fan on when any of the following conditions exist:

Certain VCM DTCs are set.
ECT above 225°F
A/C head pressure above 300 psi.

Once the auxiliary cooling fan has been turned on by the ECT sensor, the VCM will turn the fan OFF when that temperature has dropped about 7°F. If the auxiliary cooling fan has been turned on by high A/C head pressure, the VCM will turn the fan OFF when the pressure has dropped to 250 psi. The minimum on time for the auxiliary cooling fan is 30 seconds

Sounds like you are referring to the auxlliary cooling fan out front of radiator, and not the rear blower motor inside-

So how do you know it does not turn on? - have the proper conditions ever been met? Is your engine a 7.4L?

The cooling fan is controlled by the VCM based on the following inputs:

The A/C system
The Engine Coolant Temperature (ECT) sensor
The Vehicle Speed Sensor (VSS).

The VCM will turn the auxiliary cooling fan on when any of the following conditions exist:

Certain VCM DTCs are set.
ECT above 225°F
A/C head pressure above 300 psi.

Once the auxiliary cooling fan has been turned on by the ECT sensor, the VCM will turn the fan OFF when that temperature has dropped about 7°F. If the auxiliary cooling fan has been turned on by high A/C head pressure, the VCM will turn the fan OFF when the pressure has dropped to 250 psi. The minimum on time for the auxiliary cooling fan is 30 seconds

The engine is 7.4
I didn't know that the proper conditions have been met to activate fan, that was the purpose of my question.
I just replaced the A/C heater control in dash. I know it was defective but I don't know if it was effecting the operation of the aux fan in front of the radiator. I know the circuit goes thru it.
I replaced it because there was a blown resistor on it that was causing the heater hose bypass vacuum valve not to close. Replacing the resistor solved the vacuum valve issue, but I was concerned that there may have been additional problems with the electronics of the ac/ heater control so I replaced it.

Before I replaced the A/C heater control, I ran the engine at idle in 105+ degree air temp for about an hour. I can't say the exact ECT degrees, but it was pretty high. The fan never turned on. I am assuming the VCM speed control setting would allow the fan to turn on with the vehicle stopped at idle in park. Is this correct?

I just tried shorting out the pressure switch circuit at the compressor. The fan turned on. It's not hot enough out to get coolant temp up enough to test that way. Which sensor on engine is the ECT sensor? I'll try jumping that to turn on fan. That should at least eliminate the VCM and dash control as the cause.

Thanks so much for the advice so far.
Dave

You've already proven that it works- by jumpering the a/c high pressure switch connector- you don't short it- shorting implies you grounded it- there is no need to try to jumper the ECT- it is a variable resistance sensor- not a switch- not good to go full scale with it- and no need to - since you have already proved the fan is being controlled properly by the computer...a good scan tool will watch the actual engine temperature and which is what the ECM is monitoring as well.