Try seeing what the Voltage Drop is with the Glow Plug Relay unplugged (disconnect the Connector that would hook the Relay to the Ignition switch; I believe it is sort of a six sided connector on the Relay).
When you are actually cranking the Engine your Glow Plugs are also to be on.
The solid Violet colored Wire goes to the Glow Plug Relay.

The Violet Wire with a White Strip comes from the Ignition Switch down to the Starter Solenoid and is where the solid Violet colored Wire gets Voltage from when the Key is turned in the start position and turns the Glow Plug Relay on during Cranking.

Your Battery Voltage dropping as low as it is could also be a tired Battery. If this was a Gas Engine and your Battery Voltage dropped to 10.5 Volts even if the Starter was Cranking OK your Engine would not start due to not enough Voltage for the Coil to work OK.

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84 300D, 82 Volvo 244Gl Diesel