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Old 12-05-2007, 11:52 PM
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Join Date: Nov 2007
Posts: 43
'74 240D HELP! Seat belt buzzing; Now Car Won't Start!!

Hi Folks. This is very weird.

My '74 240D had the weighted seat switches and Fasten Seat Belt light disconnected, apparently: that is, I could start the car without using the seat belts, no problem. The only buzzer is when the door opens and when the key is not all the way switched on (one click back). There's a new battery in the car, it always starts strong even in the relative NC cold.

A couple days ago while trying to figure out the heater blower, I took off the center console and looked around. Realizing, with the help of some of you, that it was the A/C blower, I put it back. I found the 2-pin weighted seat/buzzer connector unplugged under the driver's seat; not knowing what it was, I plugged it in, realized it was annoying, and unplugged it again. Everything was back to the way it was when I got it.

Today out of nowhere - while driving - the seat belt buzzer began to buzz, constantly, just going down the road. It didn't buzz while in neutral (it's a 4 speed), only when in gear, which of course is almost all the time.

I drove to my pal's motorcycle shop - it was him I bought the car from two months ago - and we just pulled the 3-pin buzzer unit out. No problem.

Then the car wouldn't start.

We tried everything. Put the buzzer back in. It continued it's annoying buzzing, constantly, but not a peep out of the starter. We tried connecting the seat belts. Nothing. Tried every combo of seat belts, no seat belts, buzzer, no buzzer. The damn thing just, all of a sudden, WILL NOT START. Will not even try to start. Makes absolutely no sound.

I assume there's a seat belt interlock relay. The Chilton's guide says as much, I think, but of course they don't say where it is. Is that the problem? Where the hell is the thing (the relay)? Does anyone have any wiring diagrams or just plain advice?

I've been driving this car constantly for two months, and it is a dream. Why would it completely stop starting all of a sudden? Apparently the interlock was overridden; what could I have done to un-override it?

Exasperated here; and we're both stumped. ANY advice is greatly appreciated.

Thank you,

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Old 12-05-2007, 11:56 PM
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Join Date: Jan 2002
Posts: 18,355
Can't help on how to find the electrical problem, but if you still need to use the car while trying to trace down the problem, you should be able to start it from under the hood by jumping the starter wires and the glow plugs too if necessary.
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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Old 12-06-2007, 12:20 AM
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Join Date: Nov 2007
Posts: 43
Thank you.

Does anyone know where the interlock relay is located, exactly?
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Old 12-06-2007, 01:25 AM
Posts: n/a
Is it possible that the connector you unplugged again is now touching something metal under the seat and grounded (like the seat belt is unbuckled)? Just a guess.
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