Apply a volt meter to your battery and see what the voltage is. If it is under the voltage of a fully charged battery disconnect the electrical connectors at the glow plug relay.

After that if the voltage goes up that is a good indication that the relay was still on.

However there is another test. On the Glow Plug Relay is a flat copper Wire that is always connected to the Battery Voltage. Remove the screw and bend the wire away from the terminal and that cuts off all of the voltage to the relay with the exception of the Ignition Switch Voltage when it is in pre-glow.

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84 300D, 82 Volvo 244Gl Diesel