The AC blower fan on my 2007 Mustang, convertible, base model takes 15-20 seconds after the key is turned on to come on. It is operating properly except for the time delay. The problem started about two weeks ago with no additional symptoms. I tried replacing the fan relay and it made no difference. The fan speeds are all working properly.
I hope someone has a suggestion to remedy the problem.

OK....... Here is an update for my troubleshooting saga:
1) I replaced the fan control relay.
2) Pulled the blower fan and tested. Worked perfectly.
3) Tested blower fan resistor function. Have all 4 speeds, no issue.
4) Checked the power at the blower fan power plug. The delay is 10 seconds, same as with the blower fan hooked up.
More detailed info of the issue: The blower fan does not come on until 10 seconds after the ignition is switched on. I tested this 10 times and it was 10 seconds every time. The fan comes on immediately when the ac/htr control is switched from the "Off" position to any of the htr/ac functions. The delay only occurs when the fan comes on with the ignition switch is actuated.
This started happening about two weeks ago, with no other issues occurring.

I hope someone has seen this issue before and has a suggestion as to the cause.
Thanks!
The

 

Since it is not too difficult to remove the blower motor, I would remove it and supply power and ground directly to it and see how it behaves.
If it does the same thing , I would consider getting another motor.
If the problem is no longer there, we know there is a problem with the power or ground to the motor.

 

I was hoping for an easier way to troubleshoot the problem..
Oh well, out comes the blower.
Thanks!