94 E320 Heater

My heater fan quit working, is there anything simple to check or should I assume it's the fan itself? When the heater is turned on I can hear the vents opening anf closing but no fan.
Thanks
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Chuck, I have an Owners Workshop Manual for the W124 through 1995 and it shows the fan controlled by resistors (3) which are located on the firewall behind the brake booster. There's a protective grill over them that must be unscrewed. The fan is four-stage. The resistors are preset for the three lower speeds. For the highest speed the resistors are switched off. The fan should run at one or more speeds if the problem is a resistor. The problem could be 1) the fuse, 2) a bad fan switch, or 3) a defective fan motor. Good Luck troubleshooting, Ken
Chuck wrote:

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There are no resistors for speed on 124.032 chassis. The speed is controlled on these via the Control panel sending a variable voltage signal [ 3v. to 7v.] to a transistorized voltage regulator/heat sink with a switched ground next to the blower motor. The test is for voltage at the 3 prong plug behind the brake booster. The yellow is trigger volts and the others are load power. I would check the main feed fuse /circuit first.
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