{"id":355,"date":"2025-02-08T21:55:25","date_gmt":"2025-02-08T21:55:25","guid":{"rendered":"https:\/\/www.whitedogbatteries.com\/wdblg\/?p=355"},"modified":"2025-02-08T21:56:46","modified_gmt":"2025-02-08T21:56:46","slug":"inside-a-motorcycle-ht-lead-and-plug-cap","status":"publish","type":"post","link":"https:\/\/www.whitedogbatteries.com\/wdblg\/inside-a-motorcycle-ht-lead-and-plug-cap\/","title":{"rendered":"Inside a motorcycle HT lead and plug cap…"},"content":{"rendered":"

An often overlooked but critical part of a motorcycle ignition system is the HT lead and plug cap, if either of these fail the bike won’t run.<\/p>\n

\"Motorbike<\/figure>\n

Note – The term ‘ignition system’ refers to the parts used to generate a spark at the spark plug. In modern motorcycles these are generally the spark plug, the coil and the CDI, older bikes used points (electronic contacts) instead of a CDI.
\nThe above explanation is a very over simplified version of the parts used to generate a spark at the right time!<\/p><\/blockquote>\n

What do the motorcycle HT lead and plug cap do?<\/strong>
\nThe job of the HT lead (high tension lead) and and spark plug cap is to transfer the charge generated by the ignition coil<\/a> (around 15 to 20,000 volts) and pass this to the spark plug<\/a> so the plug can provide a good, strong spark to ignite the fuel\/air mix.
\nThe 2 parts (HT lead and spark plug cap) are usually separate parts that can be disconnected from the coil and each other.<\/p>\n

Note – If checking for faults, always be careful when testing the spark with the plug out of the engine not to have your hands or fingers near the plug or touching the engine as you are likely to get one hell of a shock!<\/p><\/blockquote>\n

\"Coil,<\/figure>\n

The 3 parts of the motorcycle coil<\/a>, the ignition coil, HT (high tension) lead and spark plug cap<\/strong><\/p>\n

\"HT<\/figure>\n

Exposed HT lead showing the internal wiring<\/strong><\/p>\n

\"HT<\/figure>\n

The outer sleeve of an HT lead. This is usually fairly thick rubber or silicone to insulate it from the voltage traveling down the lead.<\/strong>HT leads usually contain a copper core coated in a tin or stainless steel outer covering which is why they often look silver (the outer coating acts as a resistor).<\/p>\n

\"Cross<\/figure>\n

Cross section of an HT lead showing the copper cored wire.<\/strong><\/p>\n

\"Motorbike<\/figure>\n

Motorcycle spark plug cap from a 400 or 600cc motorbike.<\/strong><\/p>\n

\"Cross<\/figure>\n

Cross section of a spark plug cap showing the internal parts. Normally there would be something joining the top and bottom parts of the coil together (it’s not there in the above pic).<\/strong><\/p>\n

\"Plug<\/figure>\n

The screw in the top of the plug cap that screws onto the HT lead.<\/strong><\/p>\n

Note – This join between the HT lead and the plug cap can cause erratic running problems (due to loss of spark at the plug) if there not firmly screwed together, so it’s always worth checking!<\/p><\/blockquote>\n

As with any other part, HT leads<\/a> and spark plug caps<\/a> can start to fail with age, symptoms of these parts starting to fail can be:<\/p>\n