How do I wire two seperate switches to control one fan???

[wabbit]

I'm installing a cooling fan in front of my transmission cooler. I want to be able to operate the fan manually by a toggle switch on the dash and automatically by a thermo switch in the trans fluid line.
Right now I have a fused line off the battery that splits to both the toggle switch and the thermo switch. The "outs" from the switches come together to the fan lead wire. I was expecting either switch to run the fan. The toggle does power the fan but when I turn off the toggle and turn on the thermo switch, it burns the 20A fuse on the supply off the battery.
Should this work??? (all grounds are connected)
Maybe a bad toggle or possibly some kinda line to ground when the toggle is off and the thermo is on?
Note: The toggle switch is lighted if that may be an issue. I'm hoping that the toggle switch light will also come on when the thermo switch is powering the fan.
Is a relay necessary and if so why, and how does it work?
Please help...

 

[wabbit]

still need help...

I did some research on relays and now understand how they work and their useful applications but I don't see how this would solve the problem. The fan only draws 6.2 Amps and it is on a 20A dedicated circuit.
It seems maybe the power out of the thermo switch to the fan is also running back up the power out to the toggle switch and causing a power to ground condition. In the "off" position does the power out of a toggle connect to ground? I was counting on the power running back up to the toggle to light it as a "fan on" indicator but I don't know why it seems to short the circuit?? That is my theory. But I still need a solution or perhaps a better theory...
Please help...

 

[Goober]

Wabbit, I'm trying to help here so don't throw stones if it isn't correct....LOL

first, you said you have the power split between the two. I think this might be the problem. Since the original plan was for the autoswitch to turn the fan on when at a certain temp, by putting a switch in the path the circuit won't complete if the toggle is off. when the toggle switch is 'ON' the path is complete once again and the auto can do its job as before.

If you want both to work separate, I'd think running a separate power wire for the toggle switch (grounds should be able to be ran together or spliced into) that way one doesn't work off the other in order to operate.

This is the way I see it and it seems right, but I've never hooked anything up quite like that. Hopefully someone else can agree/disagree and help you out.

 

[sink]

It would be stupid to have a toggle that shorts the 'out' lead to ground when in the off state, but the one you have may have been built for a special purpose and is doing just that. Because the toggle runs the fan fine, and the current you stated is OK, you are absolutely right in that you don't need a relay. How are you 'turning on the thermo switch' ?

Get your ohmmeter out. Disconnect the two 'out' leads where they join the fan and pop the fuse holder open in the incoming line. Check for continuity between the toggle out lead and good ground with it off. Check the thermo switch out lead to ground in its on state. You should have no path to ground in either test. If the first is grounded, get a new SPST toggle. If the thermo goes to ground, you have to replace or fix it. I take it that the thermo does not blow the fuse when it is off and toggle is off.? In that case, it is unlikely that you have a wiring short, but it must be one of the switches.

 

[kilroy]

I did a similair thing with the ramp lights on my trailer that also act as backup lights when I put it in reverse. I used 2 relays, one conected to the backup lights that sent power directly through to the lights and another relay connected to a switch at the back of the trailer that turned the lights on with the interior lights.

Lets assume that the reverse light relay is the temperature switch, that would connect to the NC connection of the relay, with the main relay leg going to the lights(your fan), then the NO side conects to battery positive, and finallly the switch inside connects to the relay coil. This will only work if the temperature switch works on the power running through it, and not a connection to ground type switch, If it is a grounding switch, then reverse the battery side to negative and have the positive to the fan to constant on, then it will be a grounding type switching system.

 

[sink]

Goober- It sounds like he already has them in Parallel and not in seires like you were thinking.

Kilroy is onto something that the thermo switch may be designed go to ground when it is on. Of-course, then it might only have one lead. Continuity test will verify. Some other stuff works like Kilroy suggested, with the ground lead switched. That will work. Just be sure you also swap the leads on the fan terminals so that it is spinning the correct direction.

 

[wabbit]

Always thankful for input...

First, the toggle is not in line to the thermoswitch. The fused out from the battery splits to the two switches and the switch outs both lead to the fan.
The thermoswitch is not installed. I am jumping it's in/out wires to simulate actuation. It is a snap disc type so no power will be needed to actuate.
I'm voting for a bad toggle switch being the culprit.
I just went to test it and the sucker came apart while trying to remove the connectors. This is a good thing... because it shows the switch was not in the best of condition. I may need to use a relay to isolate the new toggle if it is still a problem.

 

[kilroy]

Your best bet is to use a relay, only so that you can isolate the fans power from the rest of the vehicle. More of a safety issue than to anything else.

 

[wabbit]

Found the problem--- BAD TOGGLE SWITCH

I tested a different lighted toggle switch and all is good. All as in both thermo and toggle switches run the fan AND the indicator light in the toggle switch comes on when the thermo switch runs the fan. COOL!!!
Thanks for all the help guys!!!
As a note, the original toggle switch(the one that broke apart), was something I got for a quarter at a yard sale who knows when. A new one is 4-5 bucks. As it turns out, after having spent a great while dealing with this problem, it didn't turn out to be such a good deal after all.
Sometimes I get the killer deals, and sometimes the killer deals get me...

 

[sink]

Since the thermo is not installed, you are down to the toggle causing ground.

ALSO- You should wire this thing to only get power when the ignitiion is on. Otherwise you are going to inevitably forget about it and run you battery dead continuously. This means you should probably get a relay. Then power the relay coil from a jumper off something that is only powered with ignition on. Go from that lead through the toggle switch in series and then through the relay coil. Run a separate wire from positive through the thermo switch to the relay coil. This will keep current flow through all the switches nice and low. Then just power the fan from your 20 amp fused wire off the battery through the normally open relay contacts.

[edit]- I worte mine while you were writing yours. Please see the part about the realy. Low current = low temp = long life.

 

[kilroy]

Yah what Sink said

 

[wabbit]

Yea, a relay would be the way to go...

This would allow me to actuate the fan from an existing "ignition on" power source and not load that circuit with the running of the fan.
Looks like Radio Shack lists some automotive relays. I may check Auto Zone too.
Thanks...