Burning up coils (part 2)

[denson]

I burned up 2 coils in two months. One was a good older Bosch and the other was probably part of the bad run Bosch had. My repair shop tested the voltage regulator and says it runs perfect. I think he said 13.25 volts. He said that since I do not have any smelly "bad egg" odor, it is not the regulator as the regulator would be overcharging the batteries also.

Well, this was good and bad news as now I am driving around with a spare regulator and waiting for the next bomb. I know you have all been there!

I know little about regulators and electrical, so does anyone have any thoughts on this.

John

[304065]

Is your car otherwise completely stock? Bosch CDI and Bosch 001 black CDI coil? What burned up, a silver Bosch? When you say burned up, do you mean that the coil shorted out internally from the primary windings? Or did the tower catch fire? What were the operating conditions in which both failures occurred, around town, or high-rpm cruising, or track? Hot day or cold, etc?

[denson]

It was a low 80 degree day and I was at maybe 3,000 rpms. No, thank goodness, no fire. The car just went dead. Yes, it was a silver Bosch coil, which I sanded and sprayed black. The car is all stock.

Thankfully, I swapped out a spare and made it home.

[304065]

John,

I guess the first question is "why do things burn up" and the answer is because they get too hot. "Why do they get too hot" well Joule's first law tells us that the heating is proportional to the square of the current times the resistance.

Now, CDI ignitions are a bit different than their Kettering predecessors in that the coil is not used as the primary energy storage device-- in a CDI ignition, the energy is stored in the Capacitor Cs, which charges in series with the ignition coil. When the moment arrives, the polarity of the capacitor is reversed through a high-speed electronic switch known as a Thyristor or SCR, and the electrons stored in the capacitor having nowhere to go, flow through the coil, which steps up the voltage from 460V up to about 46,000V, resulting in a very hot but very short spark indeed.

Bosch cared enough to publish a simplified diagram of this as follows:



As you can see, the coil and CDI are in series. Unlike Kettering, where the primary resistance with points has to be about three ohms, the primary resistance on a CDI coil is 0.4-0.6 ohms, not quite a dead short.

Anyway, where I am going with this is as follows:

The silver brazilian-made coils have a bad reputation. I don't know why, I have never cut one open. Perhaps they used thin wire, copper being expensive, or perhaps the potting compound they put around the wires to prevent vibration and carry away heat didn't fully encapsulate the wires, which will cause them to get very hot and the vibration kills them. Perhaps also the resistance spec is different than the original Bosch 001 black coils, which were made in Germany.

Anyway, HEAT is the enemy of electrical components. Probably what happened is the coil shorted internally, which reduces the primary resistance, causing it to get even hotter (there's Ol' Joule again, doing his magic) and so on. Painting it probably didn't help although it probably didn't hurt too much. Did you have solid, metal-to-metal contact between the coil bracket and the studs on the fan housing? Porsche put it there for two reasons, first, the electrical console was getting crowded, and second, to put it in the cooling airflow (there is prodigious airflow past it and into the fan). (They did move it back in 1984, however, maybe to reduce vibration).

This is all I can think of given the facts. Are you now using a Black Bosch 001?

[Flieger]

Joule? I thought it was Ohm. V=IR, P=IV=I*IR=I^2R, P is power dissipated across a resistor in the form of heat. Maybe that electrical power to heat is where Joule comes in?

[304065]

Max, Ohm laid down the law in 1827, whereas Joule did his thing in 1841. Joule heating is also known as "Ohmic heating" so I think it's Germany 1, England nil. But you get the idea-- the more current flowing through a resistance the more it heats up.

[Flieger]

Perhaps Lucas uses Joule's Law and Bosch uses Ohm's Law? In that case I think it is plain to see who the winner is.

By the way, if you use Joule smoke in your Bosh parts, the German parts will reject said English smoke in a rapid, exothermic reaction. That is the cause of what you perceive as "fire". The Brazilian-German coils have a hotter temper and do not tolerate the wrong smoke as long as the Teutonic coils from the Fatherland.