2023 Drake Smith - Please do not use or reproduce this elsewhere.  Feel free to link to it though.

Radiator Fan Testing

K75, K100, K1, K1100


K bikes have robust cooling system such that if you're moving at any speed then the airflow through the radiator is sufficient to keep things cool without the need for the electric fan to kick in. The only time the fan will usually come on is when it is warm out and the bike is stopped or in stop-n-go traffic.

Due to this, in cooler climates, K bike fans virtually never come on. For bikes run in cooler climates or for bikes that have sat for an extended period (years, not months) it's a good idea to test the fan.

Spin By Hand:

Since K bike fans usually seize up when they the fail, simply being able to spin the fan blades by hand usually indicates that the fan is still operational.

For plain K75s, plain K100s and K75C models you can do this without removing any bodywork. For more fully faired K models you'll need to remove a side panel to gain access to the fan.

(It's probably a good idea not to stick your fingers down by the fan blades when the bike is running.)

Direct Power Jump:

To be 100% certain that your fan works you can jump 12V directly to the fan's electrical connector. The fan connector is located under the left side of the gas tank and usually buried under all of the other wires there. It is a square black connector with violet/yellow and brown wires.

When you ground the brown wire and apply 12V to the violet/yellow wire the fan should spin.

Relay Box Jump:

An alternative way to jump the fan is to jump power to it in the relay box. The method for this is different for 2 valve K bikes and 4 valve K bikes. The ignition does not need to be on in order to do this.

Relay Box Jump - K75s and 2V K100 Models:

Remove the white temp relay from its connector and jump 12V to the right rear terminal of the connector where the relay's A2 terminal connects.

Temp Relay Connector Fan Power Terminal

Relay Box Jump - K100RS4V, K1 & K1100 Models:

Remove the fan relay from it's connector and jump 12V to the center terminal as shown below.

Fan Relay Connector Fan Power Terminal

Inoperative Fan:

If your fan has failed then it is not the end of the world. You can actually ride thousands of miles without a functioning fan. (I have myself when I had a dead fan while out touring.)

If you're in hot weather (like I was) then just hit the kill (like at a long stop light) and then restart the bike when you need to get moving again. If you're in cool weather you don't need to do that. As stated in the introduction above, when a K bike is moving or in moderately cool weather it really doesn't need a fan.

As long as the temperature warning lamp in the instrument cluster does not come on there is nothing to worry about.

Fan Replacement:

Since the fan is attached to the backside of the radiator, replacing the fan is a somewhat significant project because you'll have to drain the cooling system and remove the radiator. On the fully faired Ks removing the bodywork to get to the radiator is half the project.

For more information on fan repair/replacement click here.


  2023 Drake Smith - Please do not use or reproduce this elsewhere.  Feel free to link to it though.