First, never trust the 'no serviceable parts inside' sign, it's a conspiracy by the auto manufacturers to keep you poor and dum.
L-jetronic is a Bosch-manufactured electronic fuel injection featured on many early 80's cars. The basic L-jetronic consists of a fuel pump, fuel filter, fuel lines, fuel regulator, fuel injectors, cold-start injector (could be omitted on some L's), thermo-time switch, coolant temperature sensor, auxiliary air valve, throttle plate and an air-flow sensor (flap type in analog L's discussed here).

Also included may be an assortment of different sensors.


Sensors:

Gadgets:

The bread and butter:

This is all common knownledge, move to the mods section