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Starter Relay

Starter relays can go faulty in two ways.

The contacts can weld themselves shut, so that the starter runs all the time the battery is connected, in which case you'll have a very flattened battery that will need replacing along with the starter relay.

The contacts can become high-resistance, so not enough current will flow to begin to turn the starter motor. They are cheap to replace, and some types can be easily opened and the contacts cleaned.

The starter relay may "chatter" or sound like a faint buzzer when you attempt to start the bike. This is a classic symptom of a battery with a shorted cell or a high resistance between a couple of cells. What happens is that there's enough voltage to switch the relay on, but as soon as the starter draws its stall current the battery voltage drops and there's not enough to hold the relay on, so it goes off again. The voltage recovers, rinse and repeat. You will definitely need a new battery, and it is probable that the relay contacts have become pitted and dirty, and will soon exhibit one of the two conditions above - either is just about equally likely, so a new relay would be a good idea as well.


Some newer Piaggio models (MP3 etc) have a new style of starter relay that has a 'hidden' fuse (and its spare). This relay is often located in a place where it can gather crud and damp. This will inevitably start electrolytic corrosion of one or both of the fuse connections, which will start to have a slight resistance. This fuse is in series with the regulator output to the battery, so high currents pass through it. At some point the resistance rises to a point where the connections get very hot, and all the plastic around the fuse and the connections melts into a heap. A new relay is required, and the connections should be given a liberal dosing of dielectric (silicone) grease and then covered up somehow - anything to stop a repeat! Good dealers will do this at PDI. Wise readers of this wiki will do it ASAP as a precaution!
Last Updated Mon Nov 30, 2009 11:18 am
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