Around a year ago a friend sent me a failed Pertronix module (and parts associated with it). At the time I deconstructed the main part, did not discover to my complete satisfaction what happened and today I decided to finally get to the root of why it failed.
Basically the unit itself is a very simple Hall Effect sensor coupled with a resistor of unknown value (it was imbedded in epoxy and was destroyed when I deconstructed it). That wasn't the problem although at the time I suspected it could have been. Not to bore anyone, but a Hall Effect sensor works more/less like a switch when exposed to a magnetic field...and it is important to understand that "switch" will vary in effectiveness depending on the magnetic strength. And what I mean is at a point it will still work okay and then with one hair's less magnetism it won't work any longer at all...that loss of magnetism can be the result of increased distance to the magnet or simply loss in the strength of the magnet. In this instance the distance is a constant, so we have to look at the strength for our answer. That is what I failed to do originally and as it turns out was a mistake on my part.
Pertronix relies on a plastic wheel that's mounted firmly on the distributor cam. Please see image 1 below. This 100% plastic magnetic wheel has 8 sections separated by slits (for a V8, that is). Magnetism from the outer side of each section of the wheel fires the spark plug for 1 specific cylinder and hence the need for slit-provided separation. Just like the simple ignition points approach but magnetically rather than mechanically. About 2mm from the top there is a spring steel ring to keep centrifugal force from deforming the slits in the plastic wheel.
Now please turn your attention to the second image. The red arrow shows where the Hall Effect sensor must receive its magnetic impulse...the narrow zone above and below that line. But unfortunately there is zero magnetism present on the outside of the wheel at that area...all the way around the wheel. I can detect faint magnetism on the back side of the wheel but not on the front side at all. And after removing the retaining ring I notice there is definitely an irregular magnetic field present on the ring...stronger at the ends of the ring. I don't believe it is strong enough to trip the Hall sensor but it is there. The wheel is plastic so I have to think they imbedded the plastic with ferromagnetic iron oxides. Those can retain magnetism for a good while given the right environment but can lose magnetism in the wrong situation and the answer to the question "why" can be complex. When you look lower...to the location of the green arrow you start picking up stronger magnetism on the outer side as well as the inside. In this case, the thing failed quickly. It had been working okay...and then one day the engine died and would not re-start no matter what (a very common complaint with this product). No spark at any plug. I have to think the entire thing was designed incorrectly and I also think the retaining ring somehow caused or helped cause deterioration of the magnetic field needed to make this work.
I think it is obvious loss of magnetism is the cause, and I suspect the engineers miscalculated and simply didn't build those wheels correctly. Either that or they intended them to fail. The wheel should have been designed to last virtually forever (which is why I didn't suspect it at first) but it wasn't. You can make up your own mind as to whether you want to buy one of these and whether you want to depend on it.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
