Hi all: one of my forum friends was asking about the starter relay and so I typed up a little tutorial as a PM for these mysterious devices. I'm going to stick it here as well in case anyone else would like to know what a relay is and how they work.
To you electrical wizards out there - if I muff this, please jump in and correct me.
Here's a not very good photo of the starter relay in place on Lucille. It's the thingy hidden by lots of wires just above the silvery box inside of which is the starter safety relay (SSR) that prevents the engine from cranking if you happen to hit the starter button after it is running. As I recall it, you have to access one of the mounting screws for the starter relay through the oval hole where the battery positive cable passes. You'll need a big JIS screwdriver for that.
Just in case you're not aware of what the starter relay is for and how it works - here is a little tutorial. BTW - some people refer to starter relays as
solenoids - but that is not correct.
Starter motors on bikes and cars requires a very large electrical current - like 50-300 amps or even more - depending on the engine. Such large currents require very big wires and if that current had to go directly from the battery to the handlebar starter button and back to the starter - those big wires would be all over the bike. Also, the switch itself would have to be huge to handle those current levels.
Sooo...those wacky engineers use the handlebar button to control a relay (aka
starter switch). The relay contains an little iron plug which is magnetic and Is connected to the battery by the large diameter positive wire. This plug is usually called an armature and it is surrounded by a coil of electrical wire through which it can move a short distance.
When you push the starter button, power goes through the coil (which only requires a small current as controlled by the handlebar button switch) and this creates a magnetic field inside the coil - and that field causes the armature to rapidly move through the coil (a distance of maybe 1/8" or about 2-3 mm). When the armature reaches the end of its travel it hits an electrical contact which is connected to the starter motor by another big wire - and that completes the circuit and power flows to the starter which cranks the engine.
That's why most relays - no matter what they control - have four connector terminals:
- 2 smaller ones connected to the coil for the control circuit - this is referred to as the primary circuit or
primary side;
- 2 larger terminals for the battery-armature and armature starter motor circuit and this called the
secondary side.
Some relays have five terminals - one of which is a ground, but I don't think these are used on motorcycles.
Anyhow, all of this relay stuff is to prevent having huge wires running up to the handlebar switch and back down to the starter motor. Horns are another device which require large electrical current and so they often use relays too.
When people talk about "jumping" or
bypassing the starter relay, they are simply connecting the two big terminals on the
outside of the relay which allows current to flow without requiring the armature to move. If you ever do that - you'll need to use a fairly hefty size piece of wire - like the size of one of the battery cables - because that wire will be conducting the entire starter current to the starter motor to crank the engine.
Starter relays sometimes malfunction and usually it is because a wire in the primary coil is broken or the armature is stuck or for some other reason (corrosion) it cannot make that final contact to connect the battery to the starter motor. When you hit the button and your battery is weak, you often hear just a "click" and that is the armature moving to complete the high current secondary circuit - but there isn't enough power available to operate the high current draw starter motor - so no cranking occurs.
Here endeth the lesson.
Today is clean-up day in the
Disaster Central Workshop and I'll be assembling my snazzy new combo service cart-tool box that I bought this week on sale at Canadian Tire (sort of a Canadian combo-version of Ace Hardware and Pep Boys). It's a neat little unit and it should help bring law and order to the shop. I'll post a photo when it's done.
Cheers and have a great day!
Pete
Edited to incorporate the correction provided by RetiredGentleman (with my thanks).
