115/300d electrical problem - PeachParts Mercedes-Benz Forum

kip Foss · #1 07-20-2004, 07:39 PM

I just put the finishing touches on a 1975 115 300D restoration but now it won't start. I drove it to the paint shop last month and drove it back. All was well. Now when I turn the key to start I get nothing, not even a click from the solinoid. I haven't checked the wire to the solinoid when the switch is turned to the start position but that will be the next thing.

I checked the 4 wire plug to the transmission and am getting fire on wires 2 and 3. I can jump the starter solinoid and it will start with no problems. Aside from the transmission switch is there a circuit that would keep the engine from starting? I checked the 2 relays (one of which I think is involved in the starting circuit) on the right firewall and both worked.

Originally I thought that there might be a connection through the seatbelt lock out but the local MB mechanic says that in this year model there isn't one.

Anyone have any suggestions? They would be greatly appreciated.

leathermang · #2 07-20-2004, 09:39 PM

Is that a manual which has the clutch pedal switch ?

whunter · #3 07-20-2004, 10:15 PM

There seems to be a rash of key switch failures going around.
Broken tumblers and switches.

Hope that is not your issue.

Palangi · #4 07-20-2004, 10:35 PM

I had a 75 240D. Memory is a bit fuzzy, but if I recall correctly, I think the owners manual did make mention of a seatbelt lockout. My car didn't require seatbelts connected to start, however I wasn't really sure if it had been disconnected previously. If I remember right, there may have been some sort of reset device underhood on the drivers side. There was some electrical stuff there that I could never quite identify, in fact I don't think I ever did figure out what those 2 fuses were for. See if Randy can have a shot at the owners manual. I will have a look at the old Chiltons book tomorrow and see if there's anything there (doubtful).

R Leo · #5 07-21-2004, 12:17 AM

Quote:

Originally posted by Palangi
See if Randy can have a shot at the owners manual. I will have a look at the old Chiltons book tomorrow and see if there's anything there (doubtful).

Been there, done that. No joy.
The FSM I got from you when I bought Nellybell has ZIP as far as whole car wiring diagrams. There was a 70s-era Chiltons in the stuff you gave me too. No diagrams there either. Bummer².

The FSM did have the info about the seatbelt interlocks. I pdf'ed the pages and sent them to Kip but apparently his 115 is a later model that doesn't have the ignition interlock feature.

I'm out of resources.

kip Foss · #6 07-21-2004, 02:45 PM

The car is an automatic and I have checked the 4 wire connector to the trans. to see it I am getting fire. The ignition switch does not feel as if there is anything wrong with it. It turns smoothly and other electrical connections to it work ok.

Back to the drawing board or c-cad as it may be.

Thanks for the help.

Palangi · #7 07-23-2004, 12:27 PM

I don't know if this will apply, but here's some interesting verbage from the old Chilton's manual:

STARTER INTERLOCK SYSTEM

SYSTEM DESCRIPTION

Beginning in August of 1973 (1974 models) all Mercedes Benz cars conformed to the regulation requiring a starter interlock system that prevented starting the engine if the seat belts were not buckled. To eliminate the possibility of defeating the system by permanently buckling the seatbelts, the system required that buckling the belts and starting the car take place in a preset sequence. Each front seat contains a contact switch that closes when the seat is occupied. The buckle on the front seat belts also contains a switch that closes if the belt is unbuckled. 1974 cars can be started by reaching in through the open window and starting the car with the key.

1975 models are equipped with the same basic system, but with an additional override switch, located in the engine compartment. In case the engine cannot be started due to a malfunction in the seat belt warning system, the starter interlock can be bypassed for ONE starting attempt, by pushing on the button on the switch with the ignition ON and the transmission in N or P. As soon as the transmission is shifted out of N or P, or the ignition switch is turned OFF, the relay in the override switch is opened. To repeat the process, the switch must be depressed again.

DISABLING THE INTERLOCK SYSTEM

As a result of Federal legislation, the starter interlock systemm used on 1975-1975 cars was replaced with a light and buzzer reminder system. The new law, which took effect 12/26/74, permitted the disconnection of the starter interlock system (but not the warning light) and Mercedes Benz does not advocate that this be done.

To bypass the interlock feature on all models so equipped (with or without an override switch), replace the seatbelt logic relay (Part No. 000 545 69 32 or 000 545 68 32) with a new relay (Part No 001 545 00 32). This will disable the interlock feature, but still allow the warning buzzer and buckle-up sequence to remain in effect.

Under no circumstances should the interlock system be bypassed by bridging either the override switch relay or the logic relay, since this could allow the car to be started in gear.

So, there Ya go. I bet even if your car is later than the seat belt interlock nonsense, the updated seatbelt logic relay circuitry and possibly the push button resetable relay may still exist. If you can locate those components you might be able to track down the problem.

The book doesn't have a picture showing the location, nor a wiring diagram which corresponds to the description, however, if I recall, on my 240D the pushbutton relay was mounted on a bracket with some other electrical components up top, in front of the brakes resevior and behind the fuel filter area.

Good luck!

kip Foss · #8 07-23-2004, 05:34 PM

Palangi,

Tks. for the write up. The system seems as complicated as one would expect. I have never had much luck with auto electrics on a good day. The MB schematics are particularly vague at best so I don't hold out much hope.

There is a 'start' circuit that is hot when the switch is turned to the no. 2 position. I jumped this circuit at the fuse and heard a relay click. It sounded like it was near the injection pump but I couldn't see it. This may be the bypass relay you mentioned. I hate to 'southern engineer' the system by installing a starter switch straight off the starter solinoid but it may come to that.

What gets me is that the car started and ran perfectly when I took it to be painted a month ago. Now nothing. This makes me think it is something like a non-operating relay. I am going to do somemore jumping and circuit testing this afternoon and hope for the best.