I'm a 124 diesel guy, but I'm currently doing some restorative work on a '62 190D, and was hoping to upgrade the glow circuit. Has anyone with the older OM621 or OM615 engines upgraded to the 18mm pencil tip plugs and then added a regular OM616 relay? I have a used relay from an '82 240D, and was hoping to just wire it into the ignition switch, with a small LED indicator in the dash. Seems like this should be fairly straight forward, even though the temp sender would be deleted, not unlike the violet wire mod for afterglow. I just checked the resistance values of the pencil tips, and they're the same as those from an OM603, so this all seems rather doable. The starter and fuel shutoff will still be handled by the gorilla knob.

Anyone out there with direct experience going this route? Thanks

On mine the Violet Wire goes is used to keep the Glow Plug relay on during Starter Cranking. So thjat could be hooked to the Starter Solenoid circuit so it would work as it is intended to have Glow Plugs on during Cranking.
Use the other Pin that would go to the Ignition Switch for the normal Preglow.

Not sure about your system. Does it have a Position on the Ignition Switch for the Preglow, or did you have a Knob that you have to pull for the Prewlow?

If you have a separate Preglow Position on the Ignition Switch then you would have little trouble connecting it to the 240D Glow Plug Relay.
If you don't have as specific Preglow Position on the Ignition Switch I think the only position you could use would be the Accessory Position and then you could not use the Accessory Position for anything else because the Glow Plugs would always be on.

I used an Ford Type Starter Solenoid on My Volvo Diesel and there is no Light or any other indicator showing the Glow Plugs are on. I here the Relay thunk and hold the Switch down till I feel I have preglowed long enough and I start the Engine.
I don't think have a Light would be of any Help at all.

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84 300D, 82 Volvo 244Gl Diesel