On a 240 Diesel, it's rather complicated, or at least it can be depending on the year. On early cars, like the '82 I used to have, there are at least two units involved, one called a "blocking relay" under the dash that does most of the controlling, and then the actual glow plug relay which routes current to the plugs and is, IIRC, actuated by the blocking relay. I believe the blocking relay is responsible for turning the light on and off. On later cars, '84 and up, it uses one large relay/controller that is shaped like a brick and is the same one that is used on the 700 TD cars, which takes input from coolant temp sensor, ignition, and starter and then sends output to plugs and to light. The illumination of the light is *not* directly correlated to the operation of the plugs in this system, and I believe that may be the case in the earlier system as well.
On a 1983, I don't know what you are going to have. What does your GP relay look like? Is it a somewhat rounded-cornered one with only 4 terminals (2 big threaded posts and 2 spades), or a bricklike one with 2 big threaded posts and then a multi-pin harness connector?
Regardless, I believe the light should come on when you turn on the key, no matter how many glow plugs are dead. If that is not happening you may have issues with your relay, cluster, or wiring. However, as long as the plugs work and the engine starts.....
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86 745 D24T/ZF 345k lifted 2.5"
83 764 D24T/M46 155k
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