The glow plug relay unit contains a small circuit board in addition to the
actual relay.   My best GUESS at this time is that the circuit looks at engine
temperature and then either enables or not a timer circuit that energizes the
relay.

What I have done that I believe is more informative, is run a wire direct from
the glow plug bus (with small fuse in the line right at the connecting point
(terminal of #4 glow plug) up to the dash through a seperate LED and current
limiting resistor.  This way, whenever, and for whatever reason the glow plugs
are powered, I have positive indication on the dash.

This tells me:

1)  if glow plug circuit is enabled, but the glow plug fuse has broken (stock
glow plug indicator dash light on, but my  glow plug light not on).

2)  normal glow plug operation (both stock and my LEDs on)

3)  If the glow plug relay has arced it's contacts and isn't shutting the glow
plugs off. (I have had this happen twice to me in 171K miles and 13 years -
not a lot but unattended can be a major problem).   This one is important!  My
(now retired) mechanic showed me a partially melted cylinder head
pre-combustion chamber/glow plug and piston where the relay had not opened and
the owner had driven merrily on his way down the freeway until the parts
actually distructed.  $$$$$$$$$ !

Harvey

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