We often get enquiries like "I can only hear a click when starter button is pressed". It's important to ascertain where the "click" comes from and observe what else happens when the button is pressed.
A strong click, often accompanied by a buzzing sound, tends to emanate from the starter solenoid. This is one of the most common complaints voiced over the Internet. The fault is almost always a low battery voltage from an old or discharged battery, loose battery connections, etc.
The solenoid coil will try to operate from a much lower voltage than its nominal 12 Volts, but it'll do so in a "half-arsed" sort of way. As the main contacts close the high starter motor current demand (>60 amps) will make the already low battery voltage drop even lower, causing the solenoid coil to de-energise and drop the contacts again. This cuts the current to the starter motor and the battery, now relieved of the load, raises its voltage slightly and the same cycle quickly repeats itself causing the characteristic "buzz".
A weaker click is more likely to come from the starter relay, also called the "headlamp relay" in some manuals. Find out for sure by removing left side panel and placing your fingers over the relay casing. Operate starter button, the click will be more audible and the movement of the relay mechanism can be clearly felt through the casing.
If that's the source of the click, what else happens when you press the button?
Does the headlight turn off? if yes, this is fine, after all the relay function is to disconnect the headlight load and divert its power momentarily to the starter solenoid coil.
Observe the idiot lights closely: Do they go dim at all? If they don't dim or flicker, the chances are that the diverted current is not going anywhere and trouble lies in a faulty solenoid coil or, far more likely, the condition of the relay contacts.
These relays are not sealed as befits a component that'll be fitted to a motorcycle and exposed to dirt and the weather, but it is cheap and Triumph's Bean counters get a stiffy out of that. The outer casing simply clips over the internals leaving small gaps and over time dirt, damp and an oily film is deposited on the contacts. This creates a high resistance to the current flow, often of the order of several Ohms. This reduces current to the solenoid coil to a level where it won't operate at all. The solenoid coil requires nearly 3 amps to operate satisfactorily.
You might find that repeated attempts to start often gets results, the dirt film being broken through or displaced by the repeated hammering together of the contacts, but the problem always comes back. It's not difficult to clean these contacts, unplug relay, remove outer casing with a small screwdriver or penknife and slide a strip of thin, absorbent card between the contacts moistened in contact cleaner or simply alcohol, meths or some other non-oily solvent. As the card is pulled out you'll see the dirt stuck on it.
First clean the normally-closed contacts, numbers 87A and 30, all the headlight current flows through these. Keep sliding a clean piece of card until it no longer comes out dirty. As a side benefit you might well be rewarded with a brighter headlight for your efforts.
Then move on to the normally-open contacts, numbers 87 and 30. You'll need to manually force these to close by pushing gently on the armature so the card can do its thing. Close them just hard enough for the card to be gripped and slid in and out with a litle friction, keep doing it until the card comes out clean.
Re-install and enjoy a brighter headlight and reliable starting. The above advice stems from my recent experience when the starting became progressively worse over time until I decided to get off my arse after putting up with it for weeks and do something about it...
A couple of photos, the first shows the layout behind the left side panel. The red "emergency" button is part of a mod detailed on the thread below. I'm afraid the photos are gone but I've put new links to them on post 122 of that thread:
Pictures linked here:
I had to use it for the first time as eventually repeatedly pressing the start button brought no results. The extra pressure exerted by the emergency button on the contacts kicked the relay into action and got me going.
The second shows the internal details of the starter relay. The contact numbers shown are moulded on the plastic relay base next to the plug-in tabs.