Also, for the two wires sizes that you may have, the following is the current capacity:

14 ga maximum load is 32 amps
12 ga maximum load is 41 amps

both are wires in free air, not in a wire bundle

Is it possible that the air flow at speed is causing an increased load on the fan motor, thus drawing more current?

OR, is it possible that, under acceleration, there is a binding of the fan causing an increased amp draw? I'm not sure what kind of motor is used in an automotive fan....some can take stall without damage (like a shaded-pole motor) and some cant.

The ammeter idea is good EXCEPT that you have no fuse in the line....try to run your ammeter test with the fuse included. Use at least 12 ga wire.

mike in tucson