I would like to wire up either a 12v LED or my unused check engine light to come on when my electric radiator fan is off. Can anyone point me in the right direction. Car is a 91 firebird with a 350 from an older monte carlo. Its carb'd. I also have a summit fan temperate switch.

Thanks in advance.

 

Usual way is have light showing fan is ON. ( just a extra wire from fan relay to light)
In your case you will need a another relay to run the light so relay is turned off when fan is turned on

 

No - he 'could' use stock relay with no additional relays - but the light WILL be on always (even with car off).

Fan relay should have 5 terminals, one is not used. Connect wire to that terminal, and to positive side of the light - then ground other side of light. BUT - like I said, the light will ALWAYS be on.

To make a light that comes on when fan is off, but does not burn when car is off, you would need another relay. Wire the second relay as:

85 - 12v+ from fan positive wire (at fan or at fan relay)
86 - ground
30 - ground
87a - ground side of your light/LED
87 - not used

Then give the light/LED 12v+ from any hot-in-run source

This will do the following:

Turn on key, light is on. Car gets hot, fan comes on - this sends 12v+ to 85, 86 already grounded, so relay activated and a ground is sent from 30 to 87 - nothing connected to 87, so this ground goes nowhere. Car cools, fan turns off, thus 85 gets no power, so relay is "un-activated", thus ground is sent from 30 to 87a - your light is on 87a, and is still getting 12v+ since key is on, so light comes on.

Make sense? Use standard 12v 30 amp auto SPDT relay (about $9) any parts store.

{I think I got that right )

 

Makes perfect sense... Thanks will try that. Much appreciated