Starting problem

[John D]

After filing the points, my 1945 AG-6 sputtered to life early this spring after sitting for about 10 months. I had to replace the clogged fuel filter, but it ran well as I pushed some gravel on the driveway.  The following weekend, it wouldn't start - no spark.

Now 3 weeks later, I finally got enough time to look at it today. It took all morning to go through the entire ignition system - I cleaned all the contacts, and made sure nothing was wrong anywhere - even removed and cleaned the bakelite block that allows the neg wire from the coil to connect to the points.  I installed a new set of points, condenser, swapped in a spare coil and wires.  I turned over the engine with the distributor cap off, and could now see the spark jump across the points as they opened up.   Since the distributor is grounded to the engine, there was no ground to go wrong.  I took out one plug, and still no spark.  

The only things I had not replaced were the cap & rotor, and they were the only possibilities at this point.  The cap looked fine, but there was a little corrosion on the contacts, and some spider webs in there too.  I cleaned it all out, and still nothing.  I had the ohm meter with me and for kicks I checked out the rotor.  The meter jumped as if the center contact didn't have a clean contact with the outer rotor contact.  I just figured it was a loose wire on the meter, but decided to go to the NAPA store and buy a cap & rotor anyway.  I put the new rotor in and it fired right up.

Have you EVER seen a rotor go bad?  In all the years I've been doing tuneups, I can't ever recall having one that did'nt work.

[Blake Malkamaki]

If you pull the coil wire out of the cap and crank it over, is there spark at that wire? If so, that should rule out everything upstream of the cap and rotor, including the coil and points.

If you are seeing a spark across the points, that may tell you the condenser is bad. Not sure 100% on this.

[John D]

If you pull the coil wire out of the cap and crank it over, is there spark at that wire? If so, that should rule out everything upstream of the cap and rotor, including the coil and points.

If you are seeing a spark across the points, that may tell you the condenser is bad. Not sure 100% on this.

I had tried checking spark at the coil wire - it was there.  Everything checked out except the rotor.  I want to further bench test that rotor - no apparent corrosion (and I had cleaned the outer edge); the center button was sitting high enough to contact the cap.  What puzzled me was the ohms reading fluctuating (measuring resistance from button to outer contact).  I suppose it could have some corrosion where the button is riveted to the rotor, but even after a careful visual inspection I would never have suspected that.

So that's another anomaly to chalk up to experience.

[Blake Malkamaki]

Usually caps and rotors fail because they get hairline cracks or carbon trails caused by a spark following moisture or fingerprints across the plastic and leaving a trail of carbon, which subsequent sparks can follow. Corrosion could be a problem, but the spark at the rotor is many thousands of volts, so it should jump a corroded contact. However, if the corrosion is causing resistance, the spark could take a path of lesser resistance and just travel across the rotor and ground itself back into the shaft.

[440roadrunner]

I would recommend that you tear apart (destroy)  that rotor.     If I read you correctly,  the rotor is OPEN  as opposed to shorted (carbon tracked) to ground?

The thing is, a rotor is a sort of "high impedance"  switch,  that is,  very low current and very high voltage,  so a little corrosion and resistance here and there should not matter--the spark will just jump across.   In fact, years ago there used to be two popular rotor types for the "back then"  GM  points distributors--a "short"  and a "long"  rotor.    The short rotor was supposed to be some sort of spark intensifier,  same idea as the whatever they were called  "gap"  spark plugs that had a gap inside the ceramic.


The only thing I can offer is years ago a friends  sister had a Dodge Colt.   The rotor had an obvious  gap between the center and outer tab, and had gone bad---there's a radio suppression resistor cast into the rotor....like this one here:

[Dale Barker]

My name is Dale.  I have restored 3 cars and 12 tractors.  Since you replaced the cap and rotor, you can't be sure that it was the rotor.  If you would have replaced them one at a time you would have been able to tell which one was defective.  Your original distributor cap may have had a microscopic crack you couldn't see with the naked eye.

Dale