Check my work - dead ignition

Well in the joy that is trying to keep an old car on the road my old mercedes decided to loose spark.  Car is a 1978 Mercedes with gas engine.  Uses a transistorized ignition, no points.  Has ballast resistors.  

Started the car and it started right up as normal then stalled out.  I have been unable to get any spark.  I checked and I am not getting any 12vdc to the coil.  From what I gather there are two connections to the ignition coil, one is 12vdc from starter, other is power through the ballast resistors.  When the car is started, power from starter goes right to coil, good hot spark, when start is disengaged the power comes through the ballast.  That power comes through the ignition switch.  

I think given no power to the coil it means the ignition switch is dead.  Is there anything else I need to check before calling it, don't want to misdiagnose this.  I also have had an ignition switch die on my motorcycle before, same symptom, ran good then dead in the water.  

Any help or suggestions are appreciated, thanks folks. 

Did you check the ballast resistors for correct resistance/open circuit? Are you getting voltage to the input side of the ballast?

No power to the resistors when cranking, both resistors seem to have resistance, they are a low value, hard to measure so as long as they are still intact they should work.  

If you have a wiring diagram, see where the power comes from and backtrack until you find the culprit.  You're doing great so far.

And yes I know that it's likely that the ignition switch is really difficult to get to, but you can check the easy to get to things like fuses and relays first.

Run a jumper wire from the battery to the ignition coil.

Crank engine.

If it starts, thank me. Then, replace the iginition switch. 

In reply to earlybroncoguy1 :

It already starts and runs from the ballast bypass wire from the starter.  Which is a fine bit of diagnostication.

In reply to Pete. (l33t FS) :

Does that ballast bypass wire have 12v on it all the time, or only when cranking (ie, the starter is engaged and spinning over the engine)?

I'm guessing since it's a "ballast bypass wire", then it serves to bypass the ballast resistors while the starter is cranking over the engine (because a lot of times battery voltage drops a significant amount while the starter is engaged, often enough to cause a weak, or no, spark), which means it most likely does not have power on it once the ignition switch is released from the momentary "start" position, and returned to the constant "run" position.

With the ignition switch in the "run" position, there SHOULD be voltage from the switch, through the ballast resistors, to the coil - it won't be a full 12v, because the resistors...resist it. If there is NO voltage to the coil at all with the switch in the "run" position, then you either have a bad switch, or a wiring problem. My bet is the switch.

Running a jumper wire (a "hot" wire, if you will) from the battery to the coil bypasses everything - ignition switch, wiring, resistors, etc.    

Ok, jumped 12v to coil and car started right up, I am calling it for the switch, only has 170k miles and 45 years under its belt, guess it was living on a prayer. 

The switch isn't too bad to replace just gotta pull the steering wheel and cluster.  Mercedes was nice enough to use same switch on all their cars.  

Thanks for help.  

Well I replaced the ignition switch and it was still a no go.  Started messing around with the test light, I was looking at the wiring from the wrong side.  Well one ballast resistors had like 12.2 vdc in and 12.1vdc out.  The other resistor had 12.1vdc in and .600vdc out.  That shouldn't be the resistor is rated at .6ohm.  ok, it measures something like 60kohm.  Not good.  

I went through my junk and found a turn signal resistor with something like 1 ohm resistance.  Put it in place of bad ballast resistors, got 6vdc out.  Car started right up.  

I guess the .6ohm resistor will kill a car dead, sucks to have replaced the switch but it was hella old and I got to fix some stuff behind ins.  Cluster.  

Thanks for the help and hope this might help someone else.