I am working on an automotive project where we are using an LED as an indicator for the operation of an engine cooling fan, which is turned on and off automatically by the vehicle's ECM.

The module that activates the fan receives ground as an activation signal, so we connected the ground source of the LED to the wire that activates the module and the hot wire of the LED to a power source on the vehicles fuse box. The problem is that the LED remains lit in a low dim at all times and it only turns brighter when the ECM activates the cooling fan.

We tried installing a relay but the actions reverse and only the LED turns on, but not the cooling fan; is there a simple solution for this?

Thanks in advance.

  • \$\begingroup\$ Suppress noise current on floating LED lead with pull up R. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 17 '18 at 20:28
  • \$\begingroup\$ Your LED is probably pulling the line up. Does your LED include a series, current-limiting resistor? \$\endgroup\$ – jonk Aug 17 '18 at 21:21
  • \$\begingroup\$ It does not, it is a direct on/off plug and play diode. \$\endgroup\$ – Ed M Aug 17 '18 at 22:01
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    \$\begingroup\$ Was it designed for direct 12 VDC use? (Most LEDs, as parts, run off of about 2 V to perhaps as high as 4 V; but they do not run off of 12 V directly.) \$\endgroup\$ – jonk Aug 17 '18 at 22:44
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    \$\begingroup\$ What voltage is that "activation signal" when the fan is not on? \$\endgroup\$ – Spehro Pefhany Aug 18 '18 at 3:31

Your best bet is to use a linear current regulator with an enable input. A PWM input often can be used as an Enable.

You would would turn the LED off an on with the Enable/PWM input.

Something like this: Infineon TLE4241GM

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