For some reason in a swamp cooler I have a fairly thick line of "black dust" that appears to be emanating from the belt that goes from the motor to the fan. It's like a black line.

black line.

2 Answers 2


The black spread everywhere very much looks to me like the belt itself is disintegrating due to age and wear. Additionally, the edge of the belt itself is turning white, indicating that the cords that strengthen it are showing through. It's definitely time to replace the belt.

As bits of rubber let go, they simply get flung in a tangential line to the belt's rotation and stick to whatever they hit. Since the belt is in a line, it creates a line on the inside of the housing.

If you want, you should be able to clean the black gunk off with something like brake parts cleaner (available at your local auto parts store) and/or a razor blade to scrape it off. Likely, brake parts cleaner and a rag will remove the majority of it. Cleaning it now will help in the future as you'll see it start to build up again as the belt ages, giving you an easy indicator that it's probably time to replace the belt again.

  • Yeah I think most of the black gunk was from that belt's predecessor, though that one's showing some signs of wear. You can see a couple of places on the roof where I had wiped off the gunk to see if it came back with the collar tightened up and it seems like it didn't after a year :)
    – rogerdpack
    Commented Jan 6, 2023 at 18:47

In this case when I checked on the big pulley and its blower, they were slightly loose. Like the fan could slide sideways a bit, wasn't fully secured. I believe a collar had gotten loose on the "other side" of the blower wheel shaft, that keeps the shaft steady. So it was sliding around. So the belt was trying to keep the two pulleys aligned (one was loose), and then shedding itself as it went. Tighten or replace collar and line up the pulleys (motor and fan).

Another reason for this can be if the motor pulley and the blower pulley aren't lined up right. You can adjust the motor pulley by loosening it and pulling it in or out, relative to the motor, to line things up straight.

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