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MG MGA - Possible pertronix problem

My car developed an interesting condition. The car is negative ground with a 5 main motor. It has a pertronix ignition and sport coil, both ten years old. Once in a while when I turn on the ignition, the car will make a little "bluh" sound and rock slightly. My first thought was that somehow the starter was firing but it lacks that distinctive starter sound. What I think now is that a plug is firing and igniting a bit of unburnt fuel in one of the cylinders. I can see this happening with a points distributor but didn't know if it was possible with the ignitors trigger. So I'm wondering if I'm starting to see signs of failure in an old ignitor or coil. Or maybe I have nothing to worry about. Anyone have any thoughts?
Mark J Michalak

I also have a 10 year old Pertronix in my MG with Pertronix coil. My car died on the road. The coil was so hot I couldn't touch it. After the coil cooled down my car would start up and run another 5 miles and die. Same think hot coil. I called my son and he came with another coil. We installed his coil and I drove home about 25 miles with no problem. I installed a new coil and took it for a test drive and the new coil got hot again. I removed the Pertronix and installed new points and condenser. I just did this and have not test drove it yet. If the coil still gets hot I don't know what to do next.

When I removed the Pertronix plate that holds the magnets, one of the magnets fell out. So I think the Pertronix is the problem. I guess nothing lasts for ever. My car ran fine until the coil failed so I don't know if it could have been the loose Pertronix magnet.

Lyle Jacobson

Mark--excellent diagnosis. I agree. The explosion is barely moving the piston and at the same time expanding out one of the partially open valves.

I think you ought to go ahead and replace the Pertronix ignition and coil with another Pertronix unit. You got 10 years out of the old one. That sounds pretty good to me.

JM Morris

Adding some new information/symptoms:

What I am perceiving as a plug igniting unburnt fuel is actually happening when I switch OFF the ignition. This is also accompanied by the tach needle jumping (early MGB induction tach). While I don't always get the cylinder ignition, I do always get the needle blip. Don't really know if that's normal or not since I have nothing to compare it to.

In addition, the ignition/exciter light comes high when the ignition is on, the car starts, revs up, light goes out, alternator (Lucas ACR) charging properly 13+VDC, everything acting normally. BUT, when I open the ignition switch, the light comes high again and will stay high for a few seconds after the engine is completely dead, then will go out. This will only happen after the car has been running.

I removed the ignition switch from the equation completely and am using spade connectors during troubleshooting. Same symtoms.

Referring to the print, one way that light could be getting voltage after the ignition switch has been opened, then going out on its own, might be through the alternator or collapsing field voltage from the ignition coil. Replaced alternator with brand new unit. Same symptoms.

Put meter on + ignition coil terminal:
0VDC with ignition off
+12VDC with ignition on/car running
When the car is switched off, the ignition light stays on, and I have +1.8VDC on the + coil terminal until the light goes out, then 0VDC.

Soooo, I'm assuming that there must be a way for the coil to suppress or disperse the voltage from the collapsing primary field to keep it from backfeeding, and that whatever it is has failed and the ignition light is now bleeding off that residual voltage.

Any takers?

At any rate, the coil gets replaced next.


Mark J Michalak

What you are seeing is caused by the coil creating a spark as the field collapses, just as it does when the points open. Shutting the switch off when the points are closed does the same thing. Engines don't normally stop in a position of points closed, unless they have very low compression in one or more cylinders, which is why you don't see this "fire by key switch" more. It was a common stunt at antique car shows to set the engine just past TDC and use the manual spark control to open the points and make a spark; big old low compression engines would start that way..

The electronic system may be triggering in some similar way. But, in order for any interruption of the circuit to cause a spark, there must be current flowing through the coil. The trick with the light and the residual IGN voltage indicates that something is feeding current after you shut the switch off. This would necessarily seem to mean a pretty big capacitor someplace. Any radio suppression caps anywhere? Power supplies of any kind wired into the (Ign/Green/White) system? Stuff like radios with a bad diode letting power caps backfeed. If none of the above, then I'd be looking at alternator faults.

I'd check for suppression caps, disconnect all Green accessory circuits, unplug alternator, and see what affects it. If those do not find it, it must be the Pertronix acting goofy. Problem here is that the Pertronix has no power supply after the switch is
OFF, so either there are caps in the Pertronix or something else is supplying power.

Actually, I just went out and checked a car here. The behavior of the IGN light is normal, and I suspect the residual voltage on IGN+ is too - result of the alt field decay through the lamp. That leaves the
Pertronix bleeding power through the coil.

FR Millmore

FR- yeah, that makes sense, if I understand you correctly. The light is the result of residual voltage from the collapsing alternator field, and the pertronix trigger is closed providing a path to ground and therefore a way for the bulb to light.

Just for grins I think I'll pop the dizzy cap and see where the engine's stopping. As for everything else you suggested, I have all my accessories on a separate fused circuit coming directly off +battery. The only other device on the white circuit is the fuel pump. Is a newer electronic SU, so I don't know exactly what the solid state circuit in there looks like but I suppose it could have a capacitor.

I'll try isolating the alternator, but I'll go ahead and order a new pertronix unit along with a coil. Can never have too many spares, right?
Mark J Michalak

Apparently the Ignitor has an internal cap- found this on the web:

"modern electronic ignition systems don't
necessarily have a wave form that matches the conventional breaker-points. With a breaker point system, the wave form present at the negative coil
terminal rises rather rapidly as the points close, reaching a sort of rounded peak as the points remain closed for a period and then have a tapered tail as the points open and the current is broken (combined with the effect of the condenser). In the case of the Pertronix Ignitor, the wave form is controlled by the electronics in concert with a capacitor. The wave form has a characteristic which includes a very sharp rise, a level plateau of 10 to 15 milliseconds (which duplicates the closed points) and a exponentially tapering tail. With the ignitor, the voltage never is
allowed to completely drop to zero. Instead it hits a low of around 1.6 volts and rises to a high or around 400 volts. This waveform combined with these voltages causes the coil primary windings to saturate quickly and fully... much more so than breaker-points. This is why the Ignitor is capable of assisting the coil in delivering much higher than normal voltage
output (if needed)."
Mark J Michalak


I went to install a new Ignitor and found that the black wire of the old unit had gotten pinched by the distributor cap exposing some bare wire. Popped in the new unit and all appears to be functioning normally.
Mark J Michalak

This thread was discussed between 12/06/2012 and 28/06/2012

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