W123 Starting Circuit Diagram? - PeachParts Mercedes-Benz Forum

Jetmugg · #1 08-19-2008, 11:00 AM

I have an acquaintance with a W123 300D which will not crank when she turns the key. Apparently this condition started out as a once-in-a-while problem, but now the car will not crank at all (it has a fresh battery).

I have rebuilt and done some in-depth electrical work on several diesel benzes, but never a W123. (check my sig line).

Does anyone have a link to the starting circuit diagram for a W123? (I think it's an '84 300D sedan). I'd like to find out the function of each of the wires on the plastic plug which engages the "ignition" lock cylinder. I have the Electrical Troubleshooting Manual for W201 cars, but I don't feel safe assuming that the W123 will be "close enough" for me to diagnose each circuit properly.

Also, any tips on removing this connector from the back of the lock cylinder? I have done this on several W201's, and know how to do it, but have never messed with a W123. Any advice would be appreciated.

Steve.

rkohut · #2 08-19-2008, 11:06 AM

I think this is for a 1980 300D. It may be the same.

Ignition
http://i177.photobucket.com/albums/w237/rkohut/1980%20ETM/IgnitionSwitch.jpg

Power distribution
http://i177.photobucket.com/albums/w237/rkohut/1980%20ETM/PowerDistribution.jpg

Jetmugg · #3 08-19-2008, 12:01 PM

That's perfect - exactly what I needed. Thank you very much.

Can anyone tell me if it's possible to separate the plug from the back of the ignition lock cylinder without removing the dash? (is it possible to lay underneath or reach under the dash and remove the plug?)

SteveM.

Johnt49 · #4 08-19-2008, 01:06 PM

Turn the key to the on position, then the plug can be removed for the ign switch

Jetmugg · #5 08-19-2008, 03:29 PM

Thanks John, is it possible to reach behind the dash and unplug the connector without dis-assembling the dash (after turning the key to the run position)?

Steve.

bgkast · #6 08-19-2008, 04:20 PM

I think you can do it with the dash in place.

Oracle12345 · #7 08-19-2008, 05:22 PM

get yourself a w123 service disc if you plan to do some of the repairs yourself

rrgrassi · #8 08-19-2008, 05:28 PM

Has anyone checked the Nuetral safety switch of the tranny shifter bushings? Can you crank the car by jumping the the electrical lugs located in the small black plastic housing on the passenger side fender well?

winmutt · #9 08-19-2008, 05:39 PM

rrgrassi is certainly on the right track....

Jetmugg · #10 08-19-2008, 06:56 PM

All good ideas, and ones I will use. Like I said, this is not my car, I'm just going to help out a friend. My first attempt will be to turn the ignition switch to the run position, wait 15-20 seconds, then jumper across the the terminals on the fenderwell with a remote starter switch in order to "hotwire" the starter. My suspicion is that the car will start instantaneously.

Then I will begin the rest of the troubleshooting with a voltmeter, working backwards from the starter until I find 12 volts while in the "start" position. Likely trouble spots will be the neutral safety switch, starter relay (if equipped), and the back side of the ignition switch itself.

If there are any common areas for the starting circuit to have problems, I'd like to hear about any more items to check.

My goal is not just to "make" the car start and run. My goal is to "allow" the car to start and run the way that MB intended.

SteveM.

tangofox007 · #11 08-19-2008, 07:08 PM

Quote:

Originally Posted by Jetmugg

If there are any common areas for the starting circuit to have problems, I'd like to hear about any more items to check.

The starter lock-out switch would be high on the list of suspects. Have you tried to engage the starter with the transmission in neutral?

Quote:

Originally Posted by Jetmugg

My goal is not just to "make" the car start and run. My goal is to "allow" the car to start and run the way that MB intended.

If you can get the starter to engage by bypassing the switch, for example, you can narrow down the list of probable suspects.

Jetmugg · #12 08-19-2008, 11:08 PM

I have never even laid eyes on this car, much less my hands (yet).

I'm going over to attempt to get it figured out, and then fixed on Thursday afternoon.

I will definitely try the starting in neutral trick. I had already thought about that as a way to test the safety switch. Are the safety lockout switches external to the transmission on the W123 cars? I assume they use the same long plastic electrical connector for the backup lights and the safety switch.

Thanks again,

Steve.

winmutt · #13 08-20-2008, 04:15 PM

Oh also give the starter a good whack or two if it doesnt start at the firewall.

Jetmugg · #14 08-20-2008, 04:30 PM

Yep, I'm an experienced starter whacker. After I have to whack one once or twice, then I pull it out and have it rebuilt.

SteveM.