I am looking at a parts site and they show a Glow Plug Controller "Glow Plug Controller; Glow Time Output Stage; For E320 CDI models only. Unit is located below the intake manifold" and a "Glow Plug Relay/Controller; Pre-Glow Time Relay;
This unit is the Pre-Glow Time Relay located in fenderwell area, this is NOT the glow plug controller."

So if the Glow Plugs themselves Ohm out OK check to see if all Glow Plugs are getting the proper voltage as there is 2 other components that can cause issues.

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84 300D, 82 Volvo 244Gl Diesel