I know that the Bosch blue coil is supposed to come with internal ballast resistance: nominally 3 ohms but accepted range, according to Bosch technical literature, of 2.4-3.3 ohms. That circumscribes its choice for applications. There is a lot of discussion in various internet fora about the blue coil(s)--(s) as seemingly there are several variations of the blue coil currently on the market.

The Bosch black coil comes with a sticker on it calling for an external ballast resistor of 0.9 ohms. Bosch technical literature in fact calls for external ballast resistance of between 0.6 and 1.0 ohms. Bosch TA tells me that the black coil is supposed to have zero internal reistance, but on another forum I read that someone had actually measured its internal resistance at 1 ohm. Unless the person got the wrong measuring result, I don't know what to make of this.

The red coil is also supposed to come with zero internal reistance. It comes with a sticker calling for external reistance of 1.8 ohms. Bosch technical specifications indicate a range of 1.4 to 1.8 ohms.

The Service Manual for my ponton Mercedes does not mention the phrase "ballast resistance/resistor." Where it discusses coils it does have a table called "series adjustment" and gives various values in ohms. Are these one and the same?

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Douglas
1959 M-B 220S cabriolet