A glow plug is just an electrical resistor.  Imagine a very heavy
duty light bulb without the glass globe.  When you pass electricity
through it, it heats up.  To check, remove the bolts holding the
electric wire to the glow plugs.  Put an ohmmeter between the
terminal (where the wire was) and the engine block.  If you
get an open circuit (infinite resistance), the glow plug's
filament (like a light bulb) is broken and the plug needs to be
replaced.  If you get a number like 10 or 30 ohms, (in that range-
don't know exactly what it should be) the glow plug is OK.

The very first check of the system is to check for voltage at
the glow plugs for a few seconds when you turn the key to "on"
and the glow plug light goes on.

tabe johnson/87 westy