If somewhere on the Wiring Harness there is a Steel Bracket with a Rubber sleeve or coating on it the rubber typically deteriorates and can rub through the Insulation on the Wire.

Someone in another thread mentioned that if you disconnect the Connector going to the Glow Plugs themselves and you try to Preglow and the Fues Burns you have an issue in your Relay.

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84 300D, 82 Volvo 244Gl Diesel