A/C signal request at BC9
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A/C signal request at BC9
I want to have the cooling fan turn on when the A/C is turned on.
From looking at the schematic, it appears that 12V is supplied to location BC9 when the A/C is turned on, as the "A/C REQUEST SIGNAL".
Now, when 12V is supplied there, does it turn on the fan? What does the ECM do with that signal?
There also appears to be an "A/C cooling fan pressure switch" that connects the Fan Request Signal at BD12 to ground. But that seems to bypass the ECM internal switch and it would have the fan on continuously (even when the car is above the fan shut off speed).
When I'm driving at speed, I still want the fan to turn off.
From looking at the schematic, it appears that 12V is supplied to location BC9 when the A/C is turned on, as the "A/C REQUEST SIGNAL".
Now, when 12V is supplied there, does it turn on the fan? What does the ECM do with that signal?
There also appears to be an "A/C cooling fan pressure switch" that connects the Fan Request Signal at BD12 to ground. But that seems to bypass the ECM internal switch and it would have the fan on continuously (even when the car is above the fan shut off speed).
When I'm driving at speed, I still want the fan to turn off.
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From: Chasing Electrons
Car: check
Engine: check
Transmission: check
Re: A/C signal request at BC9
I briefly took a look at the code to refresh my memory. This may not be exact but should be close.
The compressor clutch input to pin C9 is used for the IAC idle learn with A/C active. In the fan logic it is used for a delayed fan off once the compressor clutch is off and the head temp switch (only used on F body cars) and A/C pressure switch are inactive.
It is also used for an async fuel pulse when the compressor is enabled.
As for the D12 input, secondary fan status. This is used by the ECM to enable the primary fan. Either the head temperature switch or the A/C pressure switch can enable this input.
Note that GM used two different wiring schemes for the fans & A/C. The Y body has the A/C compressor and secondary fan both under ECM control.
While the F body cars the A/C compressor clutch is switch controlled (not ECM controlled). And the secondary fan is controlled by the A/C pressure switch and head temperature switch.
Note that if the engine coolant temperature is above the fan on set point, there is no MPH threshold to turn it off. The MPH thresholds for fan off is for when the A/C is requesting the fans to run and the engine coolant is below the fan on threshold.
RBob.
The compressor clutch input to pin C9 is used for the IAC idle learn with A/C active. In the fan logic it is used for a delayed fan off once the compressor clutch is off and the head temp switch (only used on F body cars) and A/C pressure switch are inactive.
It is also used for an async fuel pulse when the compressor is enabled.
As for the D12 input, secondary fan status. This is used by the ECM to enable the primary fan. Either the head temperature switch or the A/C pressure switch can enable this input.
Note that GM used two different wiring schemes for the fans & A/C. The Y body has the A/C compressor and secondary fan both under ECM control.
While the F body cars the A/C compressor clutch is switch controlled (not ECM controlled). And the secondary fan is controlled by the A/C pressure switch and head temperature switch.
Note that if the engine coolant temperature is above the fan on set point, there is no MPH threshold to turn it off. The MPH thresholds for fan off is for when the A/C is requesting the fans to run and the engine coolant is below the fan on threshold.
RBob.