Glow Plug Wire colors on the square Connector and where they go. 84 300D
Here is a description of the numbered sockets:

1) rd/bk (Black) ignition switch (Voltage goes thorugh this to turn on the Glow Plug Relay, if you cut the Wire and wired a push button Switch to it and your GP realy is still good you could use the button Switch to turn the Glow Plugs on and retain the feature that tuns the Glow Plugs on while the Engine is cranking bay way of the Purple Wire on a W123. I also believe the Glow Plug Light would still work if you do this.)

2) vi (Violet/Purple) On turbo models goes to a Wire Terminal/Junction block and mates with a White Wire coming up from the Starter Solenoid. When you Crank the Starter + Voltages goes through the White Wire to the Violet one and turns the Glow Plug Relay On. (You could disconnect this wire at the Terminal Block or the small Terminal at the Starter Solenoid and wire it to a push button Switch.)

3) blu/wt (Blue with white strip)glow indicator Light

4) br (Brown)ground

The earlier models with the glow plug relay on the fender well will have an additional wire that goes to a temp sensor in the Cylinder head.

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84 300D, 82 Volvo 244Gl Diesel