Every time I press the horn, the seat belt warning light illuminates and the horn will not work. It is a 79 300td. Is there a common link between the two. When I turn the ignition I still get the seat belt buzzer and light. Any help would be most welcome!!

Welcome to my world.

Pressing my brake pedal causes the seat belt buzzer to go off, at half volume, if and only if the drivers door is open. Also makes the dome light surge.
Cruise control works every other Wednesday.

I've asked my question a time or two, and the first answer I generally get is "Check for a ground problem somewhere" - don't know if yours has a "Grounding Strap" but that's one place to start looking. Also, crossed or damaged wires back in the console somewhere... I hate to try to guess where.

Good luck! Some of the electrical experts will have other ideas. Most gremlins I learn to live with, but the horn is considerably more important than other problems (esp since some states require it for inspection).

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

For what it's worth, it looks like both the horn and the seat belt warning light are on the #14 circuit.

That circuit runs (at least according to the Haynes wiring diagram for the "79 300td) power to: the Horn(s), the horn contact, the "Solenoid Valve, automatic transmission", the Kickdown Switch, the Preglow Time Relay, and to part of the Warning Device (which powers the Driver's belt buckle switch, the Seat Belt Warning Light, the license plate lamps (?!???), the warning buzzer contact, and the left and right front door contact switches).

It's a busy little circuit...and a little bit overextended, if you ask me.

The horn wire from the fuse should be black and yellow, and from the horn to the horn contact black and yellow and pink.

The seat belt warning light wire should be black and green, and emerge from the "Warning Device". The wire that connects the warning device to the fuse is not marked with a color in my diagram.

I don't know the physical path of the wires in the '79 300TD, so don't have anything to offer there...

Someone with more experience will probably have a better idea...

A guess might be to look behind the instrument cluster, follow the black and green warning light wire, and see if there are black and yellow or Black/yellow/pink wires getting a little too familiar with it.

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1981 300D non-turbo

If you haven't already changed out the fuses, the first thing I'd do when attempting to troubleshoot an electrical problem is to replace ALL the fuses, if they are original, as they corrode and are the cause of many cursed little electrical gremlins!

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1983 300D (Hans)

um, have you tried disconnecting the horn? it is a very high amp device, and they do fail. it is the round thing beside the aux fan. just pull the wire off it, and see if the brake light still comes on. then try to put positive voltage to the connector on the horn and see if it honks. if it honks, it's not the problem. that is where I would start.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Thanks to everyone for their input, the number 14 fuse was a little corroded and not making good contact. I cleaned it up and now I have a horn
Thanks again for the help!