If you have the original series glow system in the 1978 then the later model parallel plugs will not work....The thread sizes are different and the newer plugs are made to operate with 12 volts applied to them individually....the earlier series plugs operate in a string with the last plug attaching to ground and the first plug being attached to the glow circuitry...the series plugs are made to have just a few volts across them and it is true that if one is burned out (open)...the current path is broken and a no-glow situation happens........you could disconnect the plugs from each other and measure them individually with an ohmmeter to compare with each other.....this may help you decide which one is bad....ohmmeter test are done with no power applied to the circuit.....
As an (emergency measure) you could jumper across the open plug and use a shorter glow cycle to get it started....this is going to glow the remaining plugs hotter and faster so keep that in mind when starting....just do it quicker......This technique applies to an open plug not a shorted plug....
If your 240D has the parallel plug upgrade you could swap plugs from the newer car....hope this helps....

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)