Hey guys. Haven't had an OM617 in a few years, ended up picking one up a couple of days ago for near scrap price. It's an 85 w123.

I had no brake lights. I unhooked the brake petal button, and there are two pins. When applying 12v to one pin, the brake lights light up as expected. But when feeding 12v to the other pin, it causes the glow plug relay to click. Any ideas before I start trying to figure it out?

Glad to be back in the community.

Aha! I figured it out. I was under the assumption that the brake lights worked when you pressed the brake petal, even if the ignition switch was off - just like every other car. This is not the case!!!

When I was feeding 12v to the other pin, it was feeding 12v to the IGN circuit, which was the same as turning the key to the "on" position, which caused the glow plugs to cycle.

D'OH!

I am looking at the wiring diagram and one wire comes off of the ingnition switch and goes to fuse 12 and it branches off to a RD/Blk wire going to the Glow Plug Relay and a Bu/RD wire going to places unknown.

If you fed that blue red wire positive current it would activate the glow plug relay.
The wires are the wrong color in the diagram but the are connected to fuse 12.

My guess is when you probe the connector with positive voltage you are sending that voltage actually the wrong direction to the Glow Plug Relay which is also connected to fuse 12.

That is my best guess.

__________________
84 300D, 82 Volvo 244Gl Diesel

I always assumed the brake switch worked on the earth side like the cars horn does

Go to one of those free manual sites and look up the wiring diagram and see what you think. I am making an guess from what I saw.

__________________
84 300D, 82 Volvo 244Gl Diesel