Normally, if the L terminal is for the light, then it will be positive when the engine is running and the alternator charging.
The other side of the charge warning light is also connected to battery positive so if the voltages are the same then the light will be out. If the alternator is not charging at all, or not enough, then the voltage from the battery ...
You start by isolating each component connected to that fuse / circuit and finding out which one is causing the drain.
A process of elimination and it helps if you approach this logically ie disconnect all and reconnect one component at a time.
First, have the battery LOAD TESTED. What you have described is one of the symptoms of a failed battery.
If that's not it, the next thing to check is a poor connection from the battery to one of its cables or a poor connection from one of those cables to their attached to your vehicle. Check those to ensure they are tight and corrosion free.
It sounds to ...
The very first thing to do with this is get your battery checked. Then if bad, replace it. Then when you have a working battery, check the charging system. Make sure all battery and charging system cables are cleaned and not defective. This can not be diagnosed until these steps are taken and done properly. It may even solve the problems.
When a Battery is disconnected from a running engine, there is a milliseconds high voltage spike that occurs the instant the terminal leaves the battery connector. Some researchers say it can be as much as 150v in your vehicles 12v circuitry. Remember that ECU operating voltage is 5v.
A customer brought a car to me with all the lamps blown.
I asked him what ...