System 1 is a "Slow Glow" system controlled by a module (12 volt) System 2 is a "Fast Glow" system controlled by a module (6 volt) System 3 is a "Fast Glow" system controlled by a thermal controll (6 volt) System 5 is a "positive Temperature Coefficient (PTC) plug (6 volt) Selecting the correct plugs is VERY IMPORTANT!! For instance:.. *If you put a 12 volt plug in a 6 volt system, it wont heat enough to start the engine easily *If you put a 6 volt plug in a 12 volt system, it will most definatly burn up causing major problems...as in removing the heads to get out the broken glow plug...not very cool!! How Do I know which Glow Plugs to get, you ask?....well heres how: The Older 12 volt "Slow Glow" used from 1978 to 1979 is stamped 7G below the threads, and has a 1/4" terminal and a black insulator. The 6 volt "Fast Glow" used from some 1979 till 1983 is stamped 8G below the threads, has a 5/16" terminal and a black insulator The 6 volt PTC plugs used from 1984 till the end of production is stamped 12G below the threads, has a 1/4" terminal and a tan insulator
Heres an easy way to find out....face the engine and look at the left front corner (passenger side) of the intake manifold...there is a screw in thing with wires hanging out...kinda looks like a engine temperature sender...but thats on the right front corner near the thermostat housing, anyways, if it has a single wire conected, then its module controlled, if it has this big wire harness with 6 wires then its thermal controlled