My '77 300D has a separate fuse box located on the firewall for the glow plugs with a 50 amp fuse strip. Or, perhaps your glow plug fuse is in the same fuse box with the other fuses. If not, look at your glow plugs and follow the wire attached to the last glow plug to the fuse box. This fuse went out once, the dash light did not come one, and I nearly did not get it started. Slim chance that say the first or second glow plug from the fuse box has gone completely bad and thus preventing the other glow plugs from getting power.
I'm sure others will have some knowledge about your problem.
Hope this helps.
America: Land of the Free!
1977 300D: 300,000+ miles
American Honda: Factory Trained Technician & Honor Grad.
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