My auxilary fan on a 87 420sel does not run at all. What I did with ignition on; jumped #1,#2 connector on top of thermostat, fan does not come on. Jumped wires from switch on dryer, fan does not come on but I hear a "click" in the fuse box (ac controll unit?). Jumped the lower connects on the dryer unit, nothing. I did jump the fan direct so I know it does work. Fuses are good. How can I test/jump the C and D high and low speed relays to see if the fan turns on? And one last thing could this be a burnt resistor located below the fuse box area? I jumped from the positive batt to one side of the resistor, nothing, I jumped the other side of the resistor and the fan turned on high.
JD

With the key on & the relay in question removed, use a jumper wire on the connector where pins #
30 & 87 marked on the relay plug into the connector.

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With key on I jumped relays C pins 87 and 30 fan runs on high and D pins 87 and 30 do nothing.

Could be the burnt resistor, if I remember the system correctly on those it needs to run through both relays to get to the fan for output. A wiring diagram of the system would tell you for sure.

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Alex
MB Star certified, now independent.
Owner/Operator FDMotorsport Mpls. MN
Mercedes and GM certified transmission rebuilder
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1998 E320 4MATIC-- stolen by my sister
1989 300TE-- sold
1999 C230 Kompressor