Hi, Eric.
First, please check your blower motor itself by applying 12V to the BLU/RED wire and GND to the BLK/BLU one. It’s the 2pin 250 connector located on the blower motor case itself.
The blower relay will be always ON as soon as you turn the IG key into P2 position regardless of your CCU status.
For the ease of understanding, please imagine that this relay is at the upper stream of the blower motor and supplying the +IG power to one wire of the motor.
The other wire for the lower stream (GND side) is controlled by the CCU by chopping the base current of the Power Tr resulting in voltage change of the applied voltage to the motor and hence, changes in fan speed.
Blower Hi relay is at the lower stream (GND side) of the motor and its purpose is to bypass the Power Tr when CCU decides that it requires the max fan speed.
This is the reason why we experience one of the most common CCU failure mode of ‘fan speed control only operates when set at max speed’ because while the Power Tr control circuit inside the CCU is dead, the one for bypassing it and switching On the blower Hi relay survives.
There are several conditions that trigger the blower Hi relay but the easiest one is to set the target temperature (left dial) at Max or Min setup (32/18degC or 90/60degF) while keeping the fan speed control (right dial) in auto (all the way anti-clockwise) or simply turn the right dial at max setting, all the way clockwise.
Since blower Hi relay is at the lower side, the condition for it to be able to bypass the power Tr and make the fan operate at max speed is that the blower relay is functioning fine and supplying the +IG power to the motor.
In other words, even if the blower Hi relay is operating fine and activated by the CCU, the fan won't work at any fan speed setting if the blower relay is faulty,
It’s very unlikely for both relays to fail at the same time especially the blower Hi relay and if I remember correctly, both relays are of the same type.
Assuming that your CCU board and blower motor itself are fine (if not, please send the CCU to BrianK), just swap the two relays and see whether the blower motor works fine all the time when the speed control (right dial) was set at somewhere in the middle but not at the Max position.
This will confirm that the relay originally used as the blower motor one was faulty as it’s at the upper stream supplying the IG power and if it fails, regardless of the status of CCU and the blower Hi relay, your fan won’t work simply because no IG power to it.
If the issue continues and as it’s intermittent, you need to replace both relays with new one and look at other areas such as the wiring, terminal contacts, blower motor itself, etc.
Kaz
