Blackbird radiator fan switches on the blink

Q. I have a 1998 Honda Blackbird that 'chews up' radiator fan switches. Two years ago it started heading for the red zone so I investigated and found the switch was faulty (it looks like a stumpy spark plug that screws into the back of the radiator infront of your left knee).

David Silver Spares provided a new one which turned out to be faulty (unless fitting it damaged it?) but they kindly replaced that free of charge and all was well. With this spring it has happened again but I don't understand why it might give up the ghost so quickly and I'm wary of plugging a new one in as at £60 a pop it could get expensive if there's something I'm missing that causes this reduced life span.
John Turnbull, email
A. That sensor is a single wire sensor which uses resistance changes in its circuit to monitor the coolant temperature. The most common cause of a problem like this is a poorearth somewhere. With this switch it would have to be the radiator itself at fault as it gets its earth from the radiator thread.

Failing that, it could be a ‘surge’ from a shorting connection elsewhere tracking through the switch instead and blowing it. Any component needs an earth, and it looks for another one nearby. Ever seen the brake lights ‘flashing’ on a white van as it slows down to turn off the road? That’s a poor earth on the indicator circuit.

Rather than lash out £60 you can pick up a Nissan switch that fits and cost a tenner which will work on Blades and VFR750s. The part number is: 21595-01A00. A guy on the FireBlade forum worked it out.

The only mod you have to do is wire one of the terminals to earth by making up an earth wire, one spade, one ring and taking it to an existing earthing wire attached to the radiator mount bracket on the left side. Take out the bolt and add your new earth with the other one and crank it down. Make sure you’ve got the right size o-ring or coolant will seep out.

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