Date registered: Apr 2009
Vehicle: Current: 1987 560 SEC US, former factory trained MB tech 10 years, owner of 14 others.
Location: Spokane, WA.
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Excessive amp draw on the motor (unlikely but possible) and/or continuity on the positive to ground (a short) would cause it. If you have a digital multimeter, set the meter to lowest ohms. Connect to power wire at blower connection. Then the black to ground wire. Turn it on(meter not fan) and look for any numerical reading. If you have one, you have a positive short to ground, typically a simple insulation failure. Make sure the power is off for the test.
To narrow it down, remove the fuse from the circuit of where it is from your owners manual or the fuse box cover information, and test again. If no reading, it is before the fuseblock, if not, it is under your dash.
You also may simply have a blower motor at the end of its life. Worn brushes draw more amps. The shorter the brush from wear, the lower the tension to the commutator, hence the less conductivity=more amps to get it done.
Let me add, that if the blower seems slow, or makes noise, a simple lack of lubrication will do it as well. The wiring in those years is bulletproof. I vote new motor.
Edited to add: I would also use the multimeter to bridge the connections upon the start-up of the blower first. Set it to amps, in the 20 range. Place the ground probe (black) on the back side (current in) and red to motor (current out). If you see an increase, it is the motor. If it is constant, it is a short.
Last edited by renaissance_warrior; 12-20-2009 at 06:29 PM.