The Relay has the ability to say on for a total time about 30 seconds if you hold the Key in the Preglow Position before it times out and shuts off.

If the timer is shutting the Relay off a lot sooner when you do the above you have some issue with the Relay.
A good Glow Plug should get hottest at Tip and the Tip should remain the Hottest part. If they are getting hottest in the middle the Glow Plug is no good (this happend to me with one Glow Plug).

If the Relay is OK it is one of those instances where the Glow Plugs could be removed and tested on a Battery.
While the Glow Plugs are out Ream or otherwise clean the Carbon out of the Glow Plug Holes.



Have you ever checked the Glow Plug Light Bulb to see if it is burnt out?

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84 300D, 82 Volvo 244Gl Diesel