radiator fan wiring

I know about electronics but i am puzzled..My radiator fan has two wires.  The blue is going straight to the 12v batt. fused, the black is going to a switch I have on the armrest and then to ground so I can turn it on and off at will. Why would power, 12v, be coming back out of the fan on the black wire that is supposed to go to ground. See what I mean now? In other words the if the fan is out of the car..it has two wires blue and black. I hook the blue wire up the the battery charger pos. Then I take my volt meter and test the ground wire which is hook up to nothing...and it has 12v on it. This is supposed to be going to ground...I don't think that the ground wire should have power coming out of it from the fan. This makes no sense...it shorts when i put it to the neg. I have tried another exact fan from another car and it seems to do this too.  Are both shorting inside the fan sending 12v back up through the ground? or is this normal.  I think another fan I tested that was a different brand did not do this.

Voltage is traveling through the motor,  without ground connected, this is normal. 

a motor is basically coiled wire, when a circuit is not completed across the windings, it act the same as straight wire. When it is part of a circuit the inductance in the winding will prevent a short.This information is provided only as a reference.

All circuits should be verified with a digital multi-meter prior to making any connections.

Oh so this is why it sparks then when connected to ground....so the 12v stops coming out of it when the ground is finally hooked up.

Sort of When the circuit is open, not complete, the motor does not act like a load. There is negligible voltage drop because the windings have low resistance. When the motor is part of a completed circuit, you will be able to measure the potential difference between the power side and the ground side. You get a spark when you connect the ground because for a motor or any inductive load, for that matter, in the initial milliseconds the circuit is completed the motor windings act like a short until inductance kicks in. The load of a motor is a result of inductance. This is pretty simplified but I think gets the point across.This information is provided only as a reference.

All circuits should be verified with a digital multi-meter prior to making any connections.

Keep in mind that it doesn't matter WHERE the switch is. The power/electricity must "loop" to flow... EG we think of it "conventionally" as current flowing out the +ve terminal thru wires thru the load & back thru wires in to the -ve terminal (thru the battery out the +ve terminal thru wires...). Break that loop ANYWHERE and the electricity flow stops, hence no power (flow) and no fan..... FYI: (stop here - do not read below....) What you describe is an earth or ground switched system, aka cold-switched (as opposed to HOT = +12V above ground etc). Its advantage: no "hot" wire to switch and back so that if the wire is damaged, there is no hot exposure (ie could short to gnd). Also that you can use less wire the GND connection can be to the chassis etc. Also for "mixed" loads or supplies - you could use the same switch to turn on a 12V & 24V or 6V fan (assuming the different voltage supplies share the same GND). (Not that that's a practical example, but controlling things with different power sources is - see f.ex "Open Collector" switching as used in computers & logic/digital circuits - it's the same as many vehicle ignition coils, horns etc; sometimes lights or relays....)

How much current is the switch rated for?  It is normal for a medium/large cooling fan to pull 40 to 50 amps of current at startup.  You would really be better off using a relay.

Good point! I had a 16A rated switch for an 80W fan (~7A) until the switch melted. I've always used a relay since then and never had any problem. (Inductive loads - always a hot topic.)

Good news...I have two relays one for each fan (two fans).  The switch is still on the arm rest, 30A and the switch triggers grounds on the relays.  The fans are 80w each.  I have the pos+ to 14 guage hot at all times, fused, each of those wires less than 3 ft.  The other fan whiched I checked like 3 times does not flow pos 12v through the ground wire...but it works fine...that is why thought there was something wrong with the one I was asking about.....they are two different fans but off the same car.  One is a radiator fan the other is a condenser fan but I rewired them like ubove and they cool the new radiator i got.

Thanks guys

If both fans turning on at the same time are dimming your headlights too much, it is very easy to delay one of the fans for a second or 2.