Date registered: Nov 2005
Vehicle: 1996 210.020
Location: Adrift on a continent...
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I don't believe that my '96 E300 has a fuse for the auxiliary fan.
If you have the M4 auxiliary fan (located in front of the condenser), it is not connected to a typical relay and resistor. The fan speed is controlled with pulse-width modulation. In addition, the controller pulses the fan to get it started, even at full demand. If the wrong electrical signal is detected during startup, the attempt is aborted and tried again in a few seconds.
Because of this, connecting a straight 12V to the fan is not a viable way to detect a bad fan motor. It can be good enough to run with 12V but not good enough to make the controller happy.
To see if you are getting power, disconnect the fan and connect the leads to a test lamp. Be sure to not short the leads in the process, due to the lack of a fuse. Then set the fan test mode to get full demand by holding both AUTO buttons for >10 seconds. If all is good with the controller, you should see the lamp flash quickly a few times, go dark for 5 or so seconds, then flash again.
Note that there is an additional fuse location, on the K40 relay module. But I doubt that you have a fan fuse there either.