Electrical stuff

**JamesB** · 17-01-22, 12:19 PM

I am going to run 2 cooling fans, I want these to be switched in 3 ways. I have 2 different range temperature switches mounted in the radiator and I want these to switch one fan each, not a problem with a couple of relays. These 2 will also be permanently live so that they can keep running with the ignition off if they need to. However I also want to have a switch on the dash to turn both fans on at the same time so that if I arrive in traffic in hot weather I can pre-empt the inevitable temperature rise however this will only be live when the ignition is on, not a problem. But, I have a single pole switch for the dash and at the moment I can't get my head round a way to wire this without creating a bridge allowing any one of the temperature fans to switch both fans on on at once. I can resolve this if I use a double pole switch but I have partly wired the back of the dash. Anyone got a solution?

**LDW** · 17-01-22, 01:00 PM

Use the switch to drive a relay with enough normally open poles. This would also help with the current that you are likely to need to start the fans.

**jrv8** · 17-01-22, 01:01 PM

James ,

You need to use another relay to override the fan relays . This relay will be operated by the single pole switch using an ignition supply to power it and the output of it wired to switch an earth to energise both existing fan relays .

Jim

**Rob P** · 17-01-22, 01:15 PM

Hi James

You could put a relay (normally closed) in between the dash switch and the connection to the fans. This relay can be operated by either of the thermostat connections to the two fans so when one of the thermostats switches the power to the fan it also supplies power to the relay which opens the relay thus cutting the power to both fans from the dash. In this way the temperature fans can only operate one fan.
Please see attached a rough diagram.

Hope this helps.

Rob

**JamesB** · 17-01-22, 01:43 PM

Fantastic guys, Not expecting such a fast response! Well there are 2-3 ways to do it, I will draw it out again and work out which will suit me best. I much appreciate it. Will likely resurrect this post to ask more questions! I am competent at this stuff from diagrams but sometimes struggle to get my head round the relay switching etc.

**skinnymathew** · 17-01-22, 02:26 PM

Originally Posted by Rob P

Hi James

You could put a relay (normally closed) in between the dash switch and the connection to the fans. This relay can be operated by either of the thermostat connections to the two fans so when one of the thermostats switches the power to the fan it also supplies power to the relay which opens the relay thus cutting the power to both fans from the dash. In this way the temperature fans can only operate one fan.
Please see attached a rough diagram.

Hope this helps.

Rob

Wouldn't either thermostatic switch closing power both fans?

**JamesB** · 17-01-22, 02:45 PM

I was just wondering that, Rob, is there something not quite right with that sketch or have I misunderstood? It looks like either thermostatic switch will trigger both fans.