Have you checked when you run the fans slow speed using ecuscan that the fan with resistor still in circuit is running slow whilst the by-passed one is running fast? If so then ecuscan has proved the control signal and power supply path (inc relay) for running slow speed fans. If not
then they must both be running fast, which calls into question ecuscan - it could be driving the fast fan signal only, even when you select slow
TEST: Remove the by-pass wire and disconnect both resistors. Use ecuscan to run slow fans - do they run?
YES = ecuscan is wrong, its driving fast fans
NO = If no fans operate, ecuscan is correctly driving slow speed fans (but we've just disconnected them!) - so CHECK when you reconnect by-pass and good resistor that when ecuscan run slow fans they both run, but the one with the resistor in circuit is runing slower
Assuming that ecuscan is correctly proving the slow speed fan control...
If you are certain then that both fans are running the same speed
when they cut in when the car is running normally then, as one fan has the resistor by-pass (will only ever run fast) and if both are running same speed, they must both be running fast
...so the slow fan demand signal is not switching the relay to turn the fans on slow - which implies the ecu is not sending it. But fast fans do come in at the expected higher temp.
SO... If we come to this conclusion then either the ecu is not able to send slow fan signal (not heard of this before) OR if my memory is right, the temp sender is shot so the ecu is defaulting to fast fans.
Would recommend if you think this is where you are you check previous threads to confirm the ecu "fast override" function I think I read about (
), as sensors are not that pricey. I think one sits next to the thermostat housing (so if you're getting a new stat fitted, easy to change at same time), the other I have been told is fitted somewhere near the (lower?) coils / injectors - so would need the cover to left of the oil filler cap taking off to find. No idea which drives gauge and which ecu, but certain other threads have this info.
Finally - If the other fan (resistor still in circuit) works, the resistor is not shot and shorting it will only mean it runs fast whether ecu demands fast or slow - so don't bother with that unless you find ecuscan has not been properly testing the slow fans!