Another cold start question
 

My 1980 300SD has lately been bashful about starting even after heating the glow plugs multiple times. This, when my Rabbit diesel pickup starts immediately.Both in the garage side by side. Removed the plug from the relay and measure about 1 ohm to ground from the connector to each glowplug. Measured the voltage from the male pins that connect the relay to the glowplugs and find battery voltage at each pin. (no load) Reconnected the plug and measured the voltage at the glowplug terminals and find a bit more than 9 volts. At this time the battery voltage drops to something more than 11.5 volts. From the above I conclude the glowplugs are likely OK,the battery is fine, the relay action is OK-----but ,is this a normal voltage drop?
Anyone have any indication as to what voltage would normally appear at the glowplug terminals? Poor Soul would be grateful to benefit from your experience. :o :o