In a 1979 300D (123) there is a black box on the drivers' side under the hood. It contains a strip fuse and the glow-plug relay. Behind the strip fuse and before the relay, mine has 4 pins. the left front is labeled 50 the front right is labeled 31 the back left is 15 and back right is 1A. there are three wires that should be attached to the pins. One is a ground to the body (black), there is a blue wire that goes to the block? or the plugs themselves? and a red wire which goes to the ignition? or vice-versa. Can anyone tell me which wire goes to which pin? I suspect they are hooked up wrong, and I'm afraid to experiment and cause damage. Thanks!

On Mercedes the Ground Wires are Brown.

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84 300D, 82 Volvo 244Gl Diesel

You can download 15-705 "Operation of Preglow System" from zShare.

There is a schematic in section B "Preglow system engine 617.950 1978-1980" showing connections 15, LA, 50, 31
Keep in mind rt is red, sw is black, bl is blue, ws is white so rt/sw would be a red/black wire probably solid red with a black trace.

15-705.pdf - 0.98MB

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1985 300D Turbo
"Evolution is God's way of giving upgrades" Francis Collins

God (or sentient spirit being of your choice) helps those who help themselves.

There's a post in the stickies section called TROUBLESHOOTING, Get the most from your Post! It has good advice on what to ask and how to ask it. If you are interested in working on your own vehicle, or even if you simply want to know what a mechanic is going to do to your vehicle, you should really get a copy of the shop manuals, either paper or digital. You can find both on ebay. The answer to your question is easily found in the schematic that raslaje linked to, and is also in the shop manual.

Personally, I like the electrical questions. I'd be happy to help you with whatever problem it is you are having. If you post your symptoms along with any pertinent troubleshooting you've attempted I'm sure you'd be swarmed with responses.

The numbers you refer to are electrical circuits numbers. 30 is a direct connection to the battery, 50 is energized when the key is in the starting position, 15 is energized when the key is in start or run, 31 is ground, 1A goes to the glow plug dash light, 6 (G?) goes to the strip fuse and on to the glow plugs. In order for the GP relay to be connected incorrectly, someone would have had to disassemble the connector and re-assemble it incorrectly. Highly unlikely but not impossible. It's more likely that you have a bad connection somewhere in your system. After you look at the schematics and test your system a bit, tell us what's going on and we'll be happy to help.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

Thanks guys!

I'm gonna check it out this afternoon after work!

just like the "ground"

I thought the relays on the early 123s were under the dash?? or maybe 79 was the transition year

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

well, this car has been "modified" by a previous owner. I've got the ground and dash light plugged into the right pins, but there is a red wire that has been soldered to one of the fuse holders in the fuse box and should go to one of the other two pins and the blue wire to the other pin (i.e. one goes to 50 and the other goes to 15 but I'm still not sure which is which). Also, the ground wire is black, but you are right, most of the time they are brown.