Author Topic: Mystery electrical problem  (Read 2168 times)


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Mystery electrical problem
« on: August 24, 2020, 23:58:15 »
Today I started my 1966  W111 Coupe I’ve owned since 1978, just to warm the engine, and possibly take it out for a drive but rain showers came in so I didn’t take it out and only warmed it up in the garage. I’ve had some electrical issues recently so I ran through all the lights possibilities, and with the headlights and running lights on with the engined at idle, it  shut down!  I turned all lights off and re-started the engine again; then with lights on as before, within seconds the engine quit again!.  Help!  And thanking you in advance for any ideas on what may be the problem.  Larry L
« Last Edit: August 25, 2020, 00:06:50 by larryled »


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Re: Mystery electrical problem
« Reply #1 on: August 25, 2020, 02:27:05 »
There are others that are way better at electrical systems than I am. My understanding is that the headlight switch serves not only as a light switch, but also a major electrical junction. So it is conceivable that your stalling issue could be related to the headlight switch. Look at a wiring diagram for a W113 on this site. They are different in some respects, but the concepts are the same.
'68 280SE W111 coupe
'60 220SE W128 coupe
'70 Plymouth Roadrunner 440+6


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Re: Mystery electrical problem
« Reply #2 on: August 25, 2020, 14:11:30 »
I would start by removing both battery cables, and cleaning the battery posts, and the cable clamp ends.  Follow the negative cable  and remove its connection(s) to the body and transmission/engine, and clean those as well.  There may also be a 2nd ground cable that goes from the transmission to the body, so do the same with that.  While the battery cables are off, remove all of the fuses and clean their ends, and clean the terminals that they set against.  Once everything is cleaned up, re-assemble, re-start the engine, and try the lights again. 

You haven't mentioned whether the battery is in good condition, or if you had to jump start the car each time you started it.  So of course, replace the battery with a good quality battery (not something from wall mart) if you have a battery that's old, or in poor condition, and do a charging system voltage test using a digital volt/ohm meter.  You should see 13.2-14.2 volts DC with the engine running higher than idle speed.


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Re: Mystery electrical problem
« Reply #3 on: August 26, 2020, 11:38:47 »
Dear Friend

I agreee with ideas stayed by wwheeler and austinado16.  The Main light switch is a convergence point where power is distributed to many circuits prior to arriving to fuse box.

I will suggest to start by checking your battery and charging system condition.  It seems to me that there is not much power (available  Amps) to feed the ignition coil and keep the car running.

As you say your car starts, we can presume that your battery is Ok for the moment. Nevertheless confirm that voltage with car off is close to 12.6 volts. If not charge it.

With car running battery voltage should be above 13.7 volts to 14.1

There is a cable from ignition tumbler in color red/black which is the feed wire to fuse port. No.2 (at fuse box) see how it looks. If not melted or damaged any way.  This cable feeds all T15 circuits, which are all the things which have current with ignition on.

The cable going to coil resistance also is red and black and comes from exactly the same point where the other red/black coming from ignition tumbler meets in fuse no.2 entry port.   Both cables are on the UN-fused side .

Make someone help you for doing some Multiimeter test to measure voltage at the ignition resistance input in the following conditions:

1. Ignition tumbler in the RUN position prior to cranking the engine, voltage must read close to the same voltage of battery.  Measure voltage at battery and then at this point, write the values.. a value close to 12.6 volts is perfect, if it is 12v is not enough

2. Crank engine and warm it and let it Iddle.  Without turning light on, measure voltage again at the battery, and later at the coil resistance input side. Values should be now close to 13V or more as the alternator, generator should be working.  The value should be higher than then ones noted on point 1)

3. Make a friend turn lights on while you measure voltage at ignition coil resistance. Note values in your mind, see if there is a significant voltage drop that is not corrected by the charging system in a while. You should expect that voltage drops for a few seconds and then you should see a recovery as voltage regulator is doing is work (the part outside alternator. Or if alternator has internal voltage regulator, the same).

If we observe that voltage keeps dropping our charging system is defective.  Battery may also be damaged if it can not keep voltage for 30 minutes or more to drive you safely home.

You have not mentioned in your post if the charging light on the instrument cluster comes on at any moment. Charge light should come on prior to starting engine and turn off after engine is running.  If it does not come on at any moment the bulb is burned in which case we have found the culprit.  Remove central instrument cluster and replace The charge light bulb (red) with one of the same size a wattage (power). It should be 4W.  Without that bulb the charging system does not operate correctly.

Nevertheless, you have to check your alternator connections to see if they are clean and tight. The same for the connections at external voltage regulator.  The ground connection of the external voltage regulator, and the list goes on and on....

If you have experienced strange electrical behavior in the past I think is time to service your wiring harness.  Cables are prone to aging and there are many Things to change and service in an old wiring loom to eliminate those erratic behaviors, and bring it back to a trouble free mint condition.

I do that for friends and customers. Contact me per PM if you need help on that.

Best regards
« Last Edit: August 26, 2020, 17:31:47 by lpeterssen »