You know…you can remove either relay and bridge the connection between terminals 30 and 87 in order to trigger either speed… I would remove the relays one at a time and perform the relays job for it…then see what the fan does. If the fan is able to perform both speeds…then the issue is whatever triggers the low speed fan. This would be a sensor telling the ECU to then trigger the ground on the respective relay at the required temp… Methinks one of the temp sensors is out to lunch…probably the low temp/low fan speed sensor. Or if it is handled by a single sensor…that sensor is no longer able to detect the lower temp mark.
Blackbird