Compatible Coils for Use with Hot-Spark I Ignition

Applies to Hot-Spark I (blue, not black) ignition kits

Coil Warning: DO NOT use a low-resistance or an HEI-style coil. Use a coil with resistance in the primary circuit of at least 3.2 Ohms (Ω). Using a coil with less than 3.2 Ohms primary resistance can cause the ignition module to overheat and misfire or fail, and will void the Hot-Spark ignition warranty. If using a coil with less than 3.2 Ohms primary resistance, install the included 1.4 Ohm external ballast resistor to increase the total primary resistance to at least 3.2 Ohms. Use 18- or 20-gauge wire between the ballast resistor and the coil’s positive terminal. If in doubt about whether you need to use the ballast resistor or not, go ahead and install it - it can't hurt anything and it may prevent problems down the road.

DO NOT reverse the polarity of the red and black wires - it will destroy the ignition module (and void its warranty)! The Hot-Spark module’s red wire connects to positive ( + or 15 on Bosch coil). The black wire connects to negative ( - or 1 on Bosch coil). Remove the condenser and its wire from vehicle.  All other wires are connected to the coil in their original places. This module is designed for 12V negative ground applications only.

Make sure that the ignition wires have plenty of slack inside the distributor and are not rubbing on any moving parts. If you need to extend the length of the ignition wires, use 18- or 20-gauge (AWG) wire. Crimp tightly or solder all connections.

Remove points, condenser and condenser wire from distributor. Remove the condenser and its wire from the vehicle.

Check Charging System Voltage: If your vehicle’s charging system produces more than 13.5 volts at high RPM (about 3,500 RPM, measured at coil's positive terminal), we recommend installing an external ballast resistor with about 1.4 Ohms resistance between the hot wire from the ignition switch and the coil’s positive terminal. Or, alternatively, you can install a high-resistance coil (about 3.8 to 4.5 Ohms primary resistance). The extra primary resistance helps to keep the coil and ignition module cool, prolonging their lives and ensuring trouble-free operation. The ballast resistor is usually mounted on the firewall or on or near the coil. Because of the high heat produced by air-cooled engines, we recommend installing a 1.4 Ohm ballast resistor or a high-resistance coil when used with Volkswagen or Porsche. If your vehicle was equipped originally with a resistor wire between the ignition switch and the coil's positive terminal, do not remove it.

If the charging system voltage, measured at the coil's positive terminal, is more than 14 volts at 3,500+ RPM, the voltage regulator likely needs replacing. Too much voltage can damage the ignition module.

Make sure that engine oil level is on the full mark before revving engine (air-cooled VW, in particular)!

Calculating Amperage to the Distributor

Some vehicles have overactive electrical charging systems, sending from 13.5 to 14.8 volts or more to the coil's positive terminal, shortening the life of the points, coil or electronic ignition module. An air-cooled VW charging system's maximum electrical current (as much as 13.75 volts or more), combined with the added heat of the air-cooled engine, are sometimes enough to cause the ignition module to overheat and misfire until it cools down again. Usually the best fix for this problem is to either install a 1.3 Ohm external ballast resistor between the ignition switch and the coil's positive terminal (more on this below) or to install a high-resistance coil (3.8 to 4.2 Ohms primary resistance). Alternators typically produce more current than generators. If peak charging system voltage exceeds 14 volts at 3,500 RPM, the voltage regulator may need replacing. A modern, solid-state voltage regulator is a big improvement over an older mechanical voltage regulator. To test your ignition system’s compatibility with the Hot-Spark ignition module, you’ll need to run a couple of tests:

1. Determine the voltage leading to the coil’s positive terminal from the ignition switch and if a resistor wire is present: Begin with a fully charged battery. Start the engine and let it idle. Connect the black lead of a voltmeter to engine ground. Connect the voltmeter’s red lead to the positive ( + or 15) terminal of the coil. With the voltmeter in the DCV 20 mode, observe the voltage reading. If the voltmeter reads 12 volts or more, there’s little or no resistance in the wire going from the ignition switch to the positive terminal of the coil. If the wire leading from the ignition switch to the coil’s positive terminal is a resistor wire, the voltmeter will typically read 6 to 9 volts DC. Now, rev the engine until the voltage stops increasing (usually about 4,000 RPM). Write down the voltage reading.

2. Next, measure the resistance of the coil’s primary winding: Label and disconnect all wires going to the coil. Make a note of to which terminal (+ or -) each wire goes. Using an Ohmmeter in the 200 Ohm (Ω) mode, connect the black lead to the coil’s negative terminal and connect the red lead to the coil’s positive terminal. The Ohmmeter will display the resistance of the coil’s primary winding. Write down the coil’s primary resistance reading (in Ohms).

Ohmmeter Calibration: When the Ohmmeter’s red and black leads are connected to each other, the reading should be 0.00 or very close to zero. If, though, for example, with both Ohmmeter leads shorted together, the reading is 0.3 Ohms, you’ll need to subtract 0.3 Ohms from the reading you get when measuring the coil’s primary resistance. For example, if the coil’s resistance reading is 3.5 Ohms, but when the Ohmmeter leads are shorted together the reading is 0.3 Ohms, then the coil’s primary resistance is 3.2 Ohms:  3.5 Ohms - 0.3 Ohms = 3.2 Ohms. This calibration procedure applies only to resistance measurements made in the 200 Ω mode of a digital Ohmmeter. The inherent resistance of the leads is negligible in higher Ω modes of the Ohmmeter. If you’re using an analog Ohmmeter, touch the leads together while setting the needle on zero before measuring resistance.

3. Now, divide the voltage reading from Step1 by the coil’s primary resistance, in Ohms, from Step 2. For example, if the voltage is 12.75 volts and the coil’s primary resistance is 3.2 Ohms, the current running through the points or Hot-Spark module would be 3.98 amps:

Example: 12.75 volts / 3.2 Ohms = 3.98 amps (OK)

or

Example: 13.75 volts / 3.2 Ohms = 4.3 amps (too much current for the electronic ignition module)

More than 4.0 amps of current is too much for the ignition module when used in a four-cylinder distributor. More than 4.0 amps can cause the module and coil to run too hot and the module and/or coil to possibly misfire or fail.

External Ballast Resistor: If your coil’s primary resistance measures less than 3.2 Ohms, you’ll need to replace the coil with one of 3.2 or more Ohms resistance or to install the included external ballast resistor between the ignition switch and the coil’s positive terminal, increasing the total primary resistance to 3.2 Ohms or more.