Max, if your lo aux fan does not engage with the A/C running, but you've powered the aux fan directly from the battery and it runs, you've verified that the fan motor is OK. Jumpering the pigtails verifies that the fuses, the relay (K10), a series resistor (R15) and wiring are also OK. Does your A/C system work correctly? It blows cold? If not, low refrigerant level would explain why the lo aux fan isn't engaging. Charge the A/C system and try again. But if A/C works, then it suggests that the high pressure switch on your A/C drier (the red base component with the 6-inch pigtails) is bad (P/N 004-820-68-10). You should also test to see if the hi aux fan engages as it is supposed to, altho if your main engine fan is working and your lo aux fan is working, it should be very rare that hi aux fan is ever needed.

It is a bad idea to leave the pigtails jumpered. As Arthur pointed out, this is a TEST only. The wiring on the lo fan circuit is undersized for continuous operation.

__________________
1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't