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Old 02-05-2009, 01:57 AM
Strife's Avatar
General Purpose Geek
Join Date: Apr 2004
Location: KY USA
Posts: 2,238
Emergency Mr. Badwrench Blower Motor Repair

My blower motor finally stopped working. I had earlier gotten the squealing out of it by (with great difficulty) regreasing the bearings. Now, I suspected the brushes.

You can detect bad brushes by taking a volt-ohm meter, putting it on ~1K ohms setting, and then SLOWLY spin the motor by hand. If you see the motor at any point show infinite ohms (usually shown with a "1" on a digital meter), your brushes are likely bad. Because you may be holding the two meter probes to the pins, an assistant may be required to spin the motor for you.

Another way to test the motor (bearings in particular) out of the car is to find a junk linear "wall wart" wall-mounted power supply of (approximately) 12 volts from an old modem, etc., of at least 3/4 amp but not more than two, cut off the plug on the end, strip, separate the wires, and, after locating the pin that does not go through the resistors inside of the connector (pin 7 on a later model 107), connect one wire from the wall wart to that and the other wire to any other pin. If the motor is working properly and the brushes are good, you should be able to connect to the other pins and get various speeds out of the motor. If the motor doesn't spin, the brushes are bad or the bearings are so bad that the relatively weak wall wart won't have enough power to spin the motor. This is also useful for finding "squeaks".

So...what to do about brush replacements. I didn't have any, and I really needed this to work. So I did a BAD, but hopefully reversible thing: After measuring the length of the springs, I stretched them out a bit, which forced the remaining brush carbon closer to the commutator. This is NOT GOOD for a long term solution, because it will put additional wear on the commutator. I'm mentioning this because it is useful in a pinch.

I am aware of the guy selling brushes on the 'Net, but I already know how to do this and he's asking a lot of money for them, even with instructions; I think I've found an alternate source and will be getting some in. I'll install them in the spring (and squish the springs back to normal), when I also clean up the beginnings of rust in the motor/cowl area and replace the seal between the motor and the cowl.

The wall wart trick is also very useful for testing (briefly, see below!) aux water pump motors. Hard to believe, but a 9V transistor radio battery will power one of these if it is in good shape. Do not test it for long because some pump designs do not like to run dry and the seals may become damaged.

The cheaper, the better with the junk wall wart. The old, "linear" wall warts are better for this purpose. Laptops and some very modern equipment use miniature switching power supplies that don't like to be shorted and are generally internally fused - so a short can effectively destroy them.

Because the wall wart has no on/off switch and because you have now exposed bare wires, avoid shorting the wires, and unplug it immediately after testing! Most are current-limited, but some of them aren't and can be damaged and/or get something very hot if shorted.
86 560SL
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85 380SL
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