Laptop - no charging light even when socket & charger works

[carnageX]

But if i do have a multimeter and detect an incorrect voltage, what should i do then to fix it?

Depends on where you detect the incorrect voltage at.

If it's coming out of your power brick...brick is probably bad.

If it's coming out of the point just after the new jack-plug you installed, it's probably something with the jack-plug.

If voltage is correct at both of those points...probably a bad mobo or some other circuit nearby.