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MG MGB Technical - Diagnosing a bad Pertronix

What is the flow of power for the ign. system? Power from the white ign wire to the + terminal of coil, then from red wire to Ignitor II, then pulsed signal from black wire to the coil, and pulsed signal from coil to dizzy cap? Have I got that right? What about grounding the top plate inside the dizzy? What exactly is grounded? Thanks for any help at all. pertronix people are not much help.

Tyler: I sent a Pertronix troubleshooting PDF to your e-mail. When I was havingtrouble with my Pertronix system, it turned out being a battery that was going bad so that the voltage the Petronix unit was seeing was around 9.5V when the starter was cranking. When I repalced the battery, the Pertronix worked great again.
John Perkins

Tyler. Yes, you have the basic flow correct. Power from the white wire, which is switched power (from the ignition switch) and unfused. I do not know at what point the ignition relay was installed. All of my RB cars have one and I have been working on them lately. Hence, you have to make sure it is working properly and providing stable power to the white wire circuit if you have one.

The normal flow of current, with a points type system, is from the white wire to the positive terminal of the coil (negative ground system), through the coil's primary windings, to the negative terminal of the coil, and down to the points. The points receive their power from the coil wire and ground through the points plate of the distributor, a two piece item. The points plate has a thin wire attached which grounds the upper of the two plates to the distributor body at one of the two cross-point screws holding the lower points plate to the distributor body. Thus, current flows through the points and to the engine block when the points are closed. When they open, the current flow is broken, the magnetic field in the primary coil windings collapses and a current is induced in the secondary windings of the coil. Current flows from the center terminal of the coil, high tension circuit, to the distributor cap, through the carbon bushing in the center to the rotor, from the rotor to the spark plug terminal, through the wire, to the spark plug and grounds to the cylinder head when sufficient voltage has built up to jump the spark plug gap.

Hope this helps.

Les Bengtson

Current flow for triggering the coil is the same with a Pertronix as with points, the extra wire to the coil +ve is to feed 12v to the Pertronix electronics. I find the prospect of them not triggering at 9.5v a little alarming, a good battery is only about 10v at best when cranking. One thing to be aware of is that on an RB car the coil has a resistor in the white wire to drop the running voltage to the coil to about 6v. There is supposed to be a boost contact on the solenoid that bypasses this during cranking (one of the white/light-green wires, the other is from the resistor in the white) to connect full battery voltage to the coil (and hence Pertronix) during cranking. This results in about 10v at best as previously described, but if the boost circuit is faulty the voltage will drop to 5v or less. I can imagine the Pertronix having problems with this, which is why I have seen recommendations to connect the red wire of the Pertronix to a full battery supply i.e. the white at the fusebox, rather than the coil +ve, on rubber bumper cars.

The ignition relay was fitted from about September 76, but has little effect on ignition voltage.
Paul Hunt 2

This thread was discussed between 23/02/2007 and 26/02/2007

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