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If this should be in the HP section please feel free to move it.

I have an electric fan and had a problem with my temperature sensor such that the fan did not turn on - the sensor has since been replaced. However, I thought it would be a good idea to have an LED on my dash that would indicate when the fan has power. I spliced the LED into the power wire leading to the fan (after the relay) but the LED is constantly on regardless of the fan running.

Any recommendations on where I need to spice in the LED to indicate when the fan is running (or at least should be running electrically speaking) would be appreciated.

I would think it should be between the temp sensor and the relay, so it’s not exposed to high amperage. Can’t figure why it on all the time.

You should not have any power to the motor when the relay is off, so its odd the LED is lighting up. Add a second smaller relay, that is energized when the main fan relay is energized, that should isolate the LED from any leakage voltage that may be keeping it on.

You should be able to tap right off the positive wire at the relay for the fan, just be sure you are on the output side and not the input, post #87 should be the switched on/off post, #30 is the battery feed, the other two are the switching wire from temp sender and ground.

If the fan has a permanent magnet motor, (most do) if the fan windmills at road speed the motor can probably generate enough voltage to turn the light on. LEDs only require a tiny bit of voltage to operate. You might try wiring the LED to the wire from the sensor that triggers the relay for the motor. Or, you could just use a conventional peanut bulb indicator, not a LED.
Jerry

Thank you all for the input.

I will try relocating it from the fan positive wire at the relay to the relay temperature sensor wire. Hopefully that does not influence the relay from properly sensing the temperature sensor.

Okay, confession time.

I did some more checking and the reason the LED was on was because the relay was wired wrong. The relay was always activated with the power to the fan being controlled directly by the thermo switch. It worked that way but the thermo switch was providing the ground side of the fan power rather than controlling the relay. I suspect that is why my original thermo switch failed (not rated for that type of power).

Now that I have rewired things correctly the relay is controlled by the thermo switch and the fan turns on when the thermo switch triggers the relay to power the fan. The LED is still connected to the power feed to the fan (after the relay) but does not light unless the fan has power (or is moving). When the fan power is removed the LED dims as the fan is slowing and goes out when the fan stops. I suspect that if the fan does windmill due to airflow from the truck moving it may light the LED with the brightness bing an indication of how fast the fan is moving.

I'm glad I decided to add the LED because the confusion it caused resulting in my finding my previous error before I damaged another thermo switch.

I'm sorry that my wiring error resulted in you all needing to try and help me out but once again I appreciate your help confirming that my original intention should have worked if not for my error.

Back in the dark ages, the days of DC generators and 3-unit regulators, there used to be a problem with "no charge" lights glowing faintly, even when the charging system was supposedly working properly. These bulbs were the standard incandescent peanut bulbs, a few decades before LEDs came along. The problem was usually dirty contact points on the cutout relay of the regulator, and the generator would try to find another path to the battery- - - - -through the idiot light! The shade tree fix was to put a higher wattage bulb in the dash to let a little current pass without illuminating the filament as brightly. The correct fix was to clean the cutout points, or replace the regulator.
Jerry