From what I have read, here and other places, even the modern 606 GP relay will post a GP light for a second or two in hot weather. if no light at all when the key is first turned, start searching for a bad plug. or a bad ground to the block. also, the GP fuse can have a crack that allows power sometimes and denies it others... so a new fuse strip would not be out of line.
good luck.
don't forget dieselgiant's site for GP testing procedures.
John

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!