The Glow Plug Light Bulb in mine must have burned out but I always start OK so I just don't worry about it.

You did not give your Year and Model.

On 300Ds the older models have a Temp Sensor in the Cylinder Head and a wire to the Glow Plug Relay.
On the new ones up to 1985 there is a temp sensor built inside of the Glow Plug Relay so there is no need for on on the Cylinder Head and the Extra Wire.

In both cases as far as I know the Temp Sensor only controls the Glow Plug Light and has some troubleshooting ability by having the Glow Plug Light acting abnormally to idicate there is some issue with the Glow Plugs or the Relay.

My best guess is you have an issue with the Temp Sensor itself (or that circuit) or you really have some issue with the Glow Plugs or the Realy that has not gotten bad enough to effect your starts.

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84 300D, 82 Volvo 244Gl Diesel