Wires get super hot when tapping car battery to test auxiliary fan

[bmwsmurf]

I've always wondered about this and never figured out what the problem was.

Years ago I was testing my old E36 auxiliary fan and followed the advice from forums to directly tap it to a spare car battery that I had sitting around just to see if it at least would power up. I had an old harness that was cut out of a junked car, and connected it to the fan. I touched the wires to the bare battery terminals and the fan spun up. But within seconds, the wire got super hot and i quickly disconnected it.

Any ideas what might have caused this?

[UsernaM3]

I've always wondered about this and never figured out what the problem was.

Years ago I was testing my old E36 auxiliary fan and followed the advice from forums to directly tap it to a spare car battery that I had sitting around just to see if it at least would power up. I had an old harness that was cut out of a junked car, and connected it to the fan. I touched the wires to the bare battery terminals and the fan spun up. But within seconds, the wire got super hot and i quickly disconnected it.

Any ideas what might have caused this?

What gauge wire were you using? Amperage might have been a bit high for the size of the wire. If the fan is drawing tons of current, that will overload a small wire pretty quickly and put off heat as a byproduct.

[bmwsmurf]

What gauge wire were you using? Amperage might have been a bit high for the size of the wire. If the fan is drawing tons of current, that will overload a small wire pretty quickly and put off heat as a byproduct.

It was the same wiring that the fan would normally use since I had cut the fan wiring and connector out of a donor car.

[R Shaffner]

Several things to consider:

Are you sure the wiring and fan are exact matches for one another?
The old harness might be damaged.
The fan might not normally get full battery voltage. (Fan speed is often varied by design.)

[shogun]

Turn the fan by hand, does it turn easy? If not, some of the magnets inside the motor got lose and are jamming and that causes high ampere. In my one there are 6 magnets and 2 were lose.
http://kharon.suomiforum.com/www/bmw/pics/IMG_2594.jpg
missing magnet http://kharon.suomiforum.com/www/bmw/pics/IMG_2593.jpg
damage on the magnet http://kharon.suomiforum.com/www/bmw/pics/IMG_2592.jpg

[bmwdirtracer]

"Tapping" the wires on a battery is NOT a good connection. There's a ton of resistance caused by that poor connection. Resistance equals heat.

[Matt330LS]

Are you sure you were using the fan wires and not the smaller gauge wires that power the fan relay?