Yet another oddity with 1974 coupes is the resistance wire in the ignition circuit. Has anyone come up with a work-around to eliminate the resistance wire? I'm getting 6 volts to the coil, have no idea if this is the correct voltage after 47 years...Thanks.
1974 Resistance Wire
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I have spares…
For most 12V points type ignitions, either a resistance wire or a ballast resistor is necessary to drop the voltage to the coil to around 6v. Most of these systems bypass the resistance wire or ballast resistor when the engine is cranking to provide a full 12v to the coil for a hotter spark for starting. BMW did both the resistance wire and the ballast resistor method in different years.
Some cars use a coil with internal resistance and in these cases the resistance wire or ballast resistor is eliminated but there is no way to provide 12v to the coil to assist starting.
All bets are off if your ignition system has been changed to some form of aftermarket system. In that case, follow the system manufacturer's instructions.
Some cars use a coil with internal resistance and in these cases the resistance wire or ballast resistor is eliminated but there is no way to provide 12v to the coil to assist starting.
All bets are off if your ignition system has been changed to some form of aftermarket system. In that case, follow the system manufacturer's instructions.
After 40-50 years of "resisting", those resistance wires are usually compromised by heat and corrosion. You can see this in the discoloration of the wire itself and perhaps melted sections of the insulation. I doubt that they are even close to providing the original resistance to the coil. Eliminating them on the '74 CS is as easy as finding the green wire in the harness behind the driver side headlight fixture, cutting and disposing of the clear insulation resistance wire to the coil at the soldered connection and then running a suitable gauge wire to the coil. Use a coil with a stated internal resistance appropriate for the ignition system you are using: points, 123 Tune, Pertronix, ... I always eliminate these on the '74 up 2002s as part of a electrical system checkup/update. If the vehicle is in good tune with a good battery, you shouldn't need any extra starter bypass boost to achieve ignition.
(edited to add the word "need" in the last sentence based on SFDon's sharp eye!)
(edited to add the word "need" in the last sentence based on SFDon's sharp eye!)
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Well, if you are intent on a work around, a voltage drop from 12v to 6v could be accomplished with a 100k ohm potentiometer like this one:
Provide a 12v input signal to terminal A and adjust the dial until the output from center terminal B measures 6v.
I just bought one of these in order to drop the coil signal from 12 v to 5v because the data logging device that I'm hoping to use to combine afr and rpm needs a 5v signal for rpm.
Provide a 12v input signal to terminal A and adjust the dial until the output from center terminal B measures 6v.
I just bought one of these in order to drop the coil signal from 12 v to 5v because the data logging device that I'm hoping to use to combine afr and rpm needs a 5v signal for rpm.
So 6 volts at the coil, engine running, is OK?
What kind of distributor are you using? Points? Pertronix? Other?
...and what coil are you using and what is its measured resistance?
Well, I would use the potentiometer to get to the desired value and operation, then measure the resistance value at that setting and replace with a fixed resistance.
Resistance is futile anyways.
Crane XR 700 with Bosch Red coil, CSi distributor (no capacitor with the Crane, of course.) Coil internal resistance is 6.8 ohms. Crane said ballast (wire) should not be bypassed. So far this set-up seems OK, coil is warm/hot, Crane black box does not get warm (if it does then Crane says more resistance is needed. I'm just wondering if 6 volts at the coil is too low given the 45 year old resistance wire...?
A Bosch red coil should read 1.8 ohms...
123ignitionusa.com
Here are instructions if you don't have a copy...
If I were me (
), I'd call Crane, tell them exactly what your setup is, and ask for their input.
Bosch Red Coil (Temporarily Unavailable taking Back Orders) 1.8 ohm recommended for 6 and 8cyl applications (Resin Filled)
Your US Distributor of electronic ignition distributors for classic cars 123ignition
Here are instructions if you don't have a copy...
If I were me (
), I'd call Crane, tell them exactly what your setup is, and ask for their input.
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The red coil requires a 1.8 ohm ballast resistor, the original black coil used a 1.1 ohm resistor. Crane says bypass for XR3000 only.