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#1
11-04-2013, 09:39 PM
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Failing coil?
Can a Bosch ignition coil change its primary resistance?
Now that I've got my timing correct, I'm investigating issue #2. In the final 1000 miles of my 7000 mile trip, the engine became harder to start each morning. The issue seemed to be the ballast resistor (1.8 ohm Bosch). Remove the resistor from the ignition circuit, and engine starts and runs perfectly. But today I decided to check the ignition coil. Bosch "red" coil; spec is 1.8 ohm primary, 12K ohm secondary. My coil's primary measures 2.5 ohms. How'd that happen? I wonder if this increased coil resistance, added to the ballast's resistance, causes voltage to drop below "sparking" level for the cylinders. Battery voltage: 13.5v. Voltage to coil without resistor: 13.5v. Voltage to coil *with* resistor: 5.1v. Note: for the entire 7000 mile trip I had the wires ignition and distributor wires attached backwards on the coil. Thanks. Tom 1960 190b nutmegflyer.wordpress.com 
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#2
11-09-2013, 10:39 PM
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Have you checked the gap or dwell? Have you measured the resistance of the resistor? I don't think the coil primary resistance is the issue. 5.1V seems really low. I will guess there is a loose connection, the resistor has gone bad, or the dwell is way too high (gap too small).
__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine) 1988 F150 144,000 miles (leaks all the colors of the rainbow) Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra
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#3
11-10-2013, 06:46 AM
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Turns out it was the points. Bosch points, only 7000 miles on them, but the follower was worn, and the contacts very pitted. Despite having that ballast resistor. Really surprising they deteriorated so quickly.
Replaced the points, set the gap to 0.017", started fine. Dwell was 50 so I left the points at 0.017. Engine starts immediately, idles smoothly at 800 rpm, and purrs like a kitten at speed. I'd never have thought it was a points problem - but you live and learn! Tom
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#4
11-10-2013, 10:21 AM
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FYI - Various gizmos are rarely the exact resistance specified. Usually a reading will be a little off one way or the other.
Actually go get some resistors of different values from Radio Shack. Then measure their resistance. Likely not a one of them is exactly the resistance specified on the package!
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