Finding wiring problem - help?

[mike31154]

Lights function but not the fans, is that the issue? Fans controlled by a sensor? What kind of fixture is this, T5, LED, hybrid?

So far with the info you've given, my gut feel is that if there's circuitry controlling the switching and/or speed of your fans, that's likely the culprit. You've confirmed the fans run with 9 volts applied, so that's a good start. Unless you disturb the connectors on a regular basis, I doubt it would be a loose connection, although corrosion could be an issue, depending on salt exposure etc. Wiring & connections don't generally go faulty unless subjected to vibration and/or extreme environmental conditions.

Assuming the fans are 12 VDC, you can check for DC power at the connector with the multimeter & work your way back to the controlling circuit board. The scale on the meter should be well marked to differentiate DC & AC voltages. Start on the 20 volt setting, if the voltage measured is higher, you won't hurt the meter, it will likely show a flashing 1. If that's the case, move to the next higher scale.

To test the wiring for continuity (broken wire, loose connection) you need to unplug the fixture (no power) and use the ohm scale, again starting on the lowest setting, most likely 200. Back in the day of analog meters, one would touch the meter leads together and zero the meter movement with an adjustment screw on the meter face. This would ensure an accurate resistance reading of the component being measured. It's still a good idea to do this with an electronic meter to ensure you read close to 0 before you start. It lets you know if the battery(ies) in the meter is/are ok. On the ohm setting, the meter will generally show a 1 (infinity) until you touch the leads together. Once done, you check the integrity of a wire run by disconnecting at both ends and touching a meter lead to each end. If you get a much higher reading than when touching leads together, or infinity, there's a problem in that segment.

Best of luck in your troubleshooting endeavour. Again, I suspect it may be a component on the controlling circuit board, probably an IC, transistor, solid state relay etc. Some are heat sensitive & can go bad at any time.

One more thing you might check for before getting into all the measuring, is to look for a fuse somewhere in the fan circuit.