Other than a defective motor, a bad switch or blower motor resistor assembly can cause an inoperative heater blower motor.
Car problem reports
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Most reported 1991 Mercedes-Benz 420SEL problems
It is not uncommon for the idle air compensator to get stuck in one position, resulting in the engine idle speed (rpm) being too high or too low. Replacement of the failed compensator may be necessary to correct this concern.
The bushings for the shifter lever wear out to the point where they break and fall out. This causes excessive movement (loose feel) in the shifter lever and a clanging-type noise when changing gears.
Valve stem seals may wear and cause the engine to burn oil. this can contaminate the catalytic converter and cause it to fail.
The fuel pump and/or fuel pump relay can fail, causing the car to crank over but not start. Proper diagnoses will be necessary to confirm if the pump or relay is at fault.
A refrigerant leak may develop from one or more of the AC hoses. Proper leak detection procedures should be followed if refrigerant loss in encountered. Replacement of any leaking hoses will we required to restore proper AC operation.
The tie rod ends, idler arm, and/or drag link components of the steering linkage may wear prematurely. Please keep in mind the wheel alignment should be checked and adjusted when any of these items are replaced.
A vibration felt through the center floor of the vehicle can be caused by cracked and/or shredded driveshaft flex discs, resulting in excessive driveshaft movement. If not repaired, damage to the driveshaft center support bearing, transmission, or differential can occur.
High mileage vehicles may develop a transmission fluid leak from one or more of the oil seals.