MG MGB and MGB GT Tech Talk
Hi everyone,
I have an electric fan on my MG. I would like to wire up a light would be on when my fan is turned off, and turns off when my fan turns on. I assume this could be done with some sort of relay, but I've been searching the Internet for about an hour and haven't been able to find anything.
Does anyone have any ideas how this could be done?
Thanks,
Darian
Go to the Library link at the top of this page. Under "Electrical and Instruments" heading is an article about adding fan relays.
Also, Limey (Eric Marshall) is a regular contributor here and can help you out with a relay kit he has developed...
Get a small square cube relay and socket with a normally open and a normally closed circuit in it. Pretty standard relay. Wire it so it will kick the fan on when the thermostatic switch calls for it and wire the normally closed circuit to the light. This way when the fan kicks on the light will go out and visa versa. Make sure the light is wired to the ignition switch so it goes off when the ignition is off. I hope I didn't screw this up and not explain that correctly. PJ
Just to expand on what Paul has said, you'll need a relay with 2 sets of contacts in it. When the relay is de-energized, one set of contacts will be made providing power to your light, and the other set of contacts will be open not allowing power to the fan. When you energize the relay, the contacts for the light will open causing the light to go out and the contacts for the fan will close allowing the fan to operate. Pretty simple little circuit, you just need to get the correct relay.
Hi all,
Thanks, that makes it clear. I suspect, however, that if I ask my Auto Zone guy for a relay with a "normally open and a normally closed circuit that has two sets of contacts" he won't have any idea what I'm talking about. Does Radio Shack sell something that would work with 12v? This is a shot in the dark, but if someone has a part number for an automotive relay that would work, that would be great.
Thanks again,
Darian
A relay with 2 separate sets of contacts will work as noted, but you can also use a relay that has Single Pole Double Throw (SPDT) contact arrangement.
The common pole for the contacts is for power, then use the NC contact for your light and the NO contact for the fan.
For those who like pictures.
The 87a terminal (for the light) is hot only when the fan is off
You can get these types of relays at Radio shack, I believe
Radio Shack - at least their on-line site, doesn't show SPDT automotive relays. I got my SPDT relays from Susquehanna. The carry Hella relays http://www.rallylights.com/detail.aspx?ID=441
George showed the diagram already and you can even modify it when putting the hot lead to the fan on the spade 87 of the relay that is titeld 'not used'
With a 20 Amps fuse in the +12 lead to the spade 30, it will power the fan perfectly and cut off the light on the dash.
Do make use of the right gauge wires fo that installation!
The 85 spade can be enrgized by a thermo switch or/and an overrun switch.