GPZ 900R Cooling system


The cooling system on the 900R is fairly complex and cleverly designed, but with a little patience it can be understood to help with those all too common overheating problems - I'm sure that we've all had the temperature gauge in the red at some stage, and wondered why the fan hasn't come on !
The following text hopefully will help you grasp the logic and operation of the system to enable you to tackle any possible faults with gusto and confidence ! Good luck !

The cooling fan circuit consists of three temperature sensors, two relays, and of course the fan. The system automatically provides three modes of operation and to differentiate between them we will consider these as ignition off, ignition on (normal) and ignition on (standby). Whereas each mode utilises different sensors and power supplies, we shall see that the two relays are powered and in use constantly. Let's now look at the modes of operation:

Ignition off Mode

With the ignition off, the first temperature sensor, the 97 C switch, is armed and if the coolant is at 97 C or above, the fan will be commanded to run until the temperature falls to below 97 C. (Note - this sensor is not active with the ignition on during normal operation.)
It is therefore normal for the fan to run when the ignition is turned off with the engine hot.

Ignition on (normal) Mode

Throughout the ignition on (normal) mode, the 97 C switch is disabled and the second sensor (110C) on the thermostat assembly is armed. This will signal the fan to run if the coolant temperature increases to 110 C. Once the temperature decreases to below this threshold, the fan will cease.

Ignition on (standby) Mode

For the last mode, the third sensor actually measures oil temperature and switches at 120 C. This appears to be a fail-safe feature which switches temperature sensing from the 110 C switch to the 97 C switch if:
1. The 110 C switch is defective (we have to assume that if the oil reaches 120 C, the coolant temperature is above the 110 C threshold, therefore the 110 C sensor has failed).
2. The oil temperature rises due to a problem in the oil cooling system.

Under either of these conditions, the 97 C sensor will be activated to prevent cooking your engine. This will command the fan to run until the coolant temperature decreases to below 97 C.


The first of the two relays, the Fan Switch Relay is situated adjacent to the headlight and determines which sensor is used under the prevailing operating conditions. It routes an earth signal (if temperature threshold exceeded) from either the 97 C or the 110 C sensor to the Fan Relay (situated adjacent to the fuse box) to bring on the fan. Failure of either of these relays will render your cooling fan system inoperative.


Both of the engine coolant temperature sensors function in exactly the same manner, in that the resistance is inversely proportional to temperature - in other words the higher the temperature the lower the resistance. This is in contrast to the oil temperature sensor, which works in the opposite way - the resistance is conversely proportional to temperature, the resistance increases with temperature. The coolant sensors provide an earth when hot, the oil sensor provides an earth when cold. Remember this - it is invaluable when troubleshooting.

Troubleshooting the system

Before you go tearing the bike apart, first determine if there is a fault - does the fan run when it is supposed to ? If you have read this far you should a fair idea of what is supposed to happen and when. Let us start with a very simple check which does not require fairing removal etc, but it will prove about 90% of the cooling fan system system.

Turn the ignition on, (no need to start the engine) disconnect the black wire from the 110 C sensor on the right-hand side of the thermostat and ground that connector to earth. The fan should run. Refit the wire back on the sensor. Be careful not to snap the terminal off the sensor, they're delicate!)

If a fault persists, and you suspect component failure, then the place to start is the fuse box, check, the main fuse, the horn fuse, and the fan fuse, each of these has a role to play in the cooling fan circuit.

Remember that overheating can be caused by problems other than component failure, such as, chafed wiring, air locks, sticking thermostat or a particularly bizarre electrical problem.

Two 3ft lengths of wire with crocodile clips at either end is invaluable when troubleshooting, as they enable you to provide power from the battery, to virtually any component on the bike.


Remember that the status of the ignition switch is critical during most of these checks.

To prove the fan. and To Prove the Fan Relay (Situated adjacent to the fuse box)

With the ignition off, disconnect the fan switch relay. (adjacent to the headlight). from the 6 pin connector, earth the Red & White wire. The fan should operate. If it doesn't the fault lies either in the fan, or the fan relay. Disconnect the two pin connector from the fan, apply 12 Volts directly to the fan. If the fan fails to spin the fan is faulty. If the fan spins, then the fault lies in the Fan relay, or, there is a solitary blue and white wire supplying the fan relay from the rear of the fuse box, this connection often snaps as it is particularly prone to corrosion, remove the fuse box and check this wire.

To prove The Fan Switch relay (Adjacent to the headlight)

With the ignition switched off, disconnect the yellow wire from the 97C switch (rear LH side of the radiator and touch it to earth. The fan should operate. Reconnect the wire.
With the ignition switched on, disconnect the black wire from the 110C switch (side of thermostat housing) and touch it to earth. The fan should operate. Reconnect the wire.
If the fan fails to operate when checking the 97C sensor (yellow wire) it must be assumed that the Fan switch relay is at fault. This can either be replaced, or checked further using the diagnostic checks given in the manual.

If the fan fails to operate when checking the 110C sensor, the fan switch relay, or the oil temperature sensor may be at fault.

To prove the Oil temperature sensor

With the ignition either on or off, disconnect the multiplug connector from the fan switch relay and using an ammeter, check for continuity between the Green & yellow wire, and earth. If there is no continuity, it must be assumed that the oil temperature sensor or the wiring to it is faulty.
To Prove the 97C & 110C sensors

It is a very clumsy process to prove that these sensors are working properly, the procedure for this is featured in all of the manuals.
But let common sense prevail. If the system is not working with the ignition on, and you have proven all of the other components by following the above checks then the 110C switch must be at fault. If the fan does not come on when the engine is very hot, and the ignition is off, then the 97C sensor must be faulty.

Nb:- Remember that if the oil temperature sensor is faulty, it will switch the reliance of the cooling system (ignition on) mode from the 110C sensor across to the 97C sensor. In this case the 97C sensor will bring the fan on if the ignition is on, wheras normally it only operates when the ignition is off.