If you were getting 10.5 volts at the Glow Plugs the Relay is working. You were also hearing the Relay Click off.

Could it be that the Bulb in the Glow plug Light is no good?

Also does you 81 300SD have the Glow Plug Relay that is connected to a Temp Sensor?

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84 300D, 82 Volvo 244Gl Diesel