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Coil ballast resistor combo?

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Guest Anonymous

Helping a friend track down an elusive high speed miss. His modified tii was running the ubiquitous blue coil along with a CD unit. The ballast resistor, which appears to be original, was bypassed. I decided to swap coils with the original black coil from my 72 and reconnected the ballast. (Engine ran the same except high speed miss disappeared.) Since I pulled out several coils (just in case) and I had my fluke handy, I decided to measure the various coils and ballasts for their resistance and my results were a bit different then what I expected.


Two "black" coils (from a stock 02 and a 73 e9) each coil measured 1.3 ohms.

Two Bosch "red" coils, each measured .8 ohms.

Two Bosch "blue 227" coils each measured 3 ohms.

One yellow accel 8401 coil measured 1.2 ohms.

Three ballast resistors (including the bosch ballast resistor mounted on the tii ALL measured .8 ohms.


I have read Zenon's comprehensive review of the various Bosch coil offerings Black, Blue and Red. http://www.bmw2002faq.com/content/view/69/32/ I have also read other's analysis and interpretation of what is supposed to work and what really works. http://www.ratwell.com/technical/BlueCoil.html and http://www.356registry.org/Tech/coil-blueblack.html http://rb-aa.bosch.com/boaa-pl/Product.jsp?prod_id=70&ccat_id=32&language=en-GB&publication=1 Now, I am not so certain there are any absolutes given the advanced age of these cars and their installed ignition components.

Zenon's post recognized that some of the early black coils and ballasts resistors were .9 ohms. This might account for the some of my measurements for the ballasts, but not the black coil measurements. Again, the black coils measured twice the resistance reported by Zenon, but maybe they were newer "post 72" coils. Zenon also suggested the red coils would need more resistance and might need two .9ohm resistors in series to prevent damage to the (red) coils.

Since the measured resistance of the red coil matched the ballast resistors on hand, it seemed the simplest and most logical route to use that formula. Are we missing something?

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