ST2 electric fan - Problems
 

Anyone know of a common fault with the fan staying off?
It was OK up to a couple of weeks ago, but now doesn't kick in at all. There's no dedicated fuse that I know of, and I'm loath to short the terminals across the rear cylinder temperature sensor as that sends to a CPU, rather than being a simple current switch.

Any thoughts gratefully received,
Mike Chadwick

You are right not to short across the sensor, that won't do any good at all. The fan can be tested by connecting the ECU to a Mathesis tester or the Technoreseach VDSTS but in the absense of these it is difficult to establish why it's not coming on.
You could start by disconnecting the fan and applying 12V across the pins of its connector. If the fan doesn't run then it's fecked. If it does then you'll have to look elsewhere. I'm not sure about the ST2 but on the 4s there is a relay under the seat which switches on the fan on getting a signal from the ECU. This would be a good place to start, as well as checking all the connections at both the fan and the relay.
If that turns up nothing then there are the 2 temperature sensors, they are both the same. One of them supplies the temperature readout in the instrument panel, the other is to the ECU. The ECU uses this to get information on the engine temperature and adjusts the fuel mix accordingly. It also uses it to decide when to switch on the fan. If the bike is otherwise running OK it is unlikely that the temperature sensor is faulty but you could swap them around and see if there is any difference but my money is on the relay.
Incidentally the fan on my ST4s doesn't come on until the display has reached 104C and that's only happened 2 or 3 times.

Thanks Derek,
I think you're right about the likely culprit being the relay, but first thing to do is supply a seperate 12V to the fan as you say.

The bike displays in farenheit. The fan should (and used to) come on at 200.
This was a rare occurance until you have a heatwave and only then when pratting around at slow speeds in town traffic. However, I've had a reading of up to 220 degrees on 3 occasions last week alone.

If the fan works OK, and the relay is fine, then frankly I'm out of ideas.

Thanks again for the reply.

Mike