'74 240D: Seat Belt Buzzer, Now Won't Start - PeachParts Mercedes-Benz Forum

sides · #1 12-05-2007, 10:50 PM

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,

Brian

OldPokey · #2 12-05-2007, 10:58 PM

In 1974 all cars were set up (by law) to disable the starter if the seatbelt was unfastened while someone was sitting in the front seat. It caused so much grief that it was discontinued by 1975. Just about every 1974 model car still around has had that brainless system defeated, but yours seems to be an exception.

You get to figure out what that relay does and bypass the connection it has opened between the ignition switch and the starter solenoid. It should be something as simple as a wire grafted in where the relay (the buzzer you just removed) was.

sides · #3 12-05-2007, 11:15 PM

Thanks. Do you know exactly where the seat belt interlock relay is located? There are no diagrams to be found anywhere.

The system was defeated - like I said, I could start the car without using the seat belts, no problem.

I did nothing to change anything in the car, so how could it all of a sudden start buzzing, the Fasten Seat Belt light starts coming on, and the car won't start?

Palangi · #4 12-05-2007, 11:50 PM

I seem to remember that it was among a row of mystery components on the drivers side of the engine compartment. The Chiltons manual had a sentence or two on it, but no circuit diagram or anything.

Palangi · #5 12-06-2007, 12:09 AM

Look for a bracket with about 5 or 6 relays in it, in the vicinity of the battery. I am pretty sure it is one of those.

Mike D · #6 12-06-2007, 07:25 AM

Before you drive yourself crazy, do the proper troubleshooting first! Use a test light to determine if you are getting current to the starter solenoid during the cranking position of the ignition switch. If not, THEN start tracing it back. Neutral safety switch first then the interlock. It may well be the interlock but don't jump ahead of yourself.

There is a three wire connection strip on the passenger (RH) inner fenderwell. The outer two terminals will activate the starter. Do a jumper wire to test the starter and solenoid.

nhdoc · #7 12-06-2007, 07:27 AM

IIRC the control relay is actually mounted under the dash, driver's side, by the E-Brake pedal. It's a silver can and looks like a cruise control amp. You should be able to isolate it by unplugging it and seeing if the buzzer stops. I am going from memory - back some 30 years, so if I am wrong forgive me, but I am pretty sure I remember that is where it is.

You'll either need to replace it or get a wiring diagram to figure out what wires to jumper to bypass it. I recall in the late 70's our 74 240D had this fail and we had to replace it, at that time it was an expensive part...probably about $100. Even though the seat sensors were disconnected it prevented the car from starting...just like what you are seeing. After I replaced it I found the circuit board had cracked and it probably could have been repaired by jumpering the broken traces but since I had already purchased a replacement I just threw it away.

If you need to temporarily bypass it to be able to drive the car you can use one of those starter switches they sell for firing off the starter directly from the solenoid until you fix the interlock box. Your car is a 4 speed so you could even get it going by rolling it. Since there is no ignition the car will run fine even with the failed interlock - it just prevents the starter from engaging.

nhdoc · #8 12-06-2007, 12:44 PM

I actually have the manual here and forgot that I did. Once you find the relay module, which is near the E-Brake pedal, you can unplug it and jumper from pins 1 to 3 which should bypass the entire system and allow the car to start. I believe this will also bypass any neutral safety switch the car has too so be advised, the car will start in gear if you bypass the module instead of repairing it.

If you want the entire diagnosis section I can email it to you, just PM me.

sides · #9 12-06-2007, 07:57 PM

Thanks to you I got the car to start. We took the relay out and wired the contacts closed, so it starts fine. Oh, and we initially jumped the pin connector like you mentioned and it worked great, too.

Now the flasher doesn't work. I assume the Hazard lights and, apparently, the left and right standing lights work through the flasher too, since they aren't working either.

Thanks again.

nhdoc · #10 12-06-2007, 10:00 PM

The circuits for the flasher and the interlock should not be connected so maybe you just knocked a connector off while you were working under the dash...check there first.