W114 chassis 280 and 280C standard versions use three five-pin relays for: auxiliary fan, window lifts and air conditioner / starter (the latter disables air conditioner compressor during starting). If your power windows operate and compressor runs with two relays in place, you may have a missing auxiliary fan relay. Most all five-pin relays used during this era are all the same type (sans brands and plastic versus metal housings). Install a five-pin relay into vacant socket. With relay in place, unplug then ground the single-pin connector at thermostat housing (it is the coolant temperature sensor). Turn key to run position, then see if auxiliary fan is running. If not, inspect two-pin connector at fan wiring as well as use continuity tester to validate fan motor. For the Solex 4A1 choke system, a heater coil in choke housing is powered by 11-pin combination relay controlling switch-over valves and choke heater. Choke heater is utilized to assure choke mechanism opens when engine warms up, it is not needed to activate choke mechanism. When engine is cold (below 62 degrees F), the choke heater coil is energized with 12 volts directly. Once engine warms up to 62 degrees, relay switches and 8-ohm resistor is placed in series with choke heater coil. Test choke heater by turning key to run position (engine does not need to be started), then measure voltage a choke heater terminal (black wire with blue and red stripes). 12 volts should be present, when engine temperature is cold (below 62 degrees). When voltage is present, but choke fails to operate properly, this may indicate a stuck linkage (accompanied by lower idle speed when cold). This is a mechanical fault, needing choke mechanism to be serviced. If no voltage is present, unplug 11-pin relay and clean all terminals, both male pins and female sockets. FYI: 11-pin relay is very specialized component specific to a certain series of Mercedes and can be expensive to replace as well as difficult to find. It rarely fails unless it has been abused or disassembled.
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