David Butcher's Datsun Roadster Ignition Enhancements

Sat Oct 25 18:41:38 PDT 1997

With the onset of the cold weather (and an apology to my friends in Australia, who will have to wait a season or two before you can use this information), my attention is now focused on the morning ritual of starting the car.

My car (a '67 1600) is in reasonably good shape. It has a rebuilt cylinder head, new battery, and good compression. When it is cold (from being left out of the garage overnight), it takes 5-10 seconds of cranking to start the car. Of course, that is unacceptable ;-)

Here are the facts:

This car has no excuse. It should start right up. I checked to be sure that everything was tight, and I applied a liberal amount of "Automotive Electronics Cleaner" to the distributor cap and rotor, and to the coil tower. This has historically been my cure for backfiring, hesitation on drive-off, and rough idle. The carbon dust from the center contact of the distributor cap may create crossfire problems as it slowly builds up on the cap and rotor. It is a good idea to clean your cap and rotor at the end of summer every year. However, this time that was not the problem.

I decided to think things through, and do some measurements. One little- known fact about this year of Roadster is that there is no ballast resistor shunt circuit. Most modern cars have a special circuit which is used only when starting the car (in other words, only when the ignition switch is causing the starter motor to turn). The purpose of this circuit is to bypass the ballast resistor and apply full battery voltage to the coil while the starter is running, because the starter pulls the battery voltage down considerably.

The other avenue of research I embarked on was to test the voltage at the coil. Here was what I found:

Note the very low voltage when the starter is cranking. The coil has only 6 volts to work with, which is not much. I made one further measurement. I measured the AC (alternating current) component of the voltage at the coil at idle, and found it was 1.7 volts. That means the coil would pull the voltage up and down by that amount as the ignition system switched it between building magnetic field ("charging") and delivering spark ("discharging").

I decided two enhancements were in order. First, I needed a ballast resistor shunt that was active only during starting, and second, I needed to provide smoother voltage input to the coil. To solve the first problem, I bought a Hella headlight relay, and mounted it right by the coil (actually on the coil clamp). This relay has four connections, and the circuit diagram looks like this:

      | ------------|    Ballast Resistor  |-------o Coil o--DIST--- Ground
      |           | |______________________|  |
      |           |                           |
      |           |                           |
      |            ---------|====|------------
      |                     |SWCH|
     __                     |COIL|
    |__|--------------------|WWWW|------- Ground

Ignition Switch            Hella Relay

The secret is to find the wire on your ignition switch which goes to the starter solenoid and tap into it. Run the new wire to the relay, which is connected to the two ends of the ballast resistor. The relay simply shorts across the resistor, removing it from the circuit, but only when the starter is cranking. You may wonder why I did not simply hook the wire to the coil side of the ballast resistor. The reason is that a circuit would be formed from there back through the starter solenoid, which would kill the ignition.

To solve the second problem, I purchased a surplus Mallory capacitor, I chose a 50,000 MFD 16v capacitor, which is about the size of a small water glass. I hooked the capacitor up between the input side of the coil and ground. I mounted the capacitor above the heater plenum inside the car where it would be sheltered from the elements, and ran a wire out to the ballast resistor, connecting it to the end which went to the coil. This capacitor is large enough to smooth the voltage going to the coil, producing a .2 volt ripple where there was 1.7 volts before. While the coil is being charged, the capacitor pitches in and provides extra energy that the ballast resistor would have otherwise blocked. When the coil is nearly charged, the capacitor continues to draw through the resistor until it is charged as well. The principle here is that the current through the ballast resistor is limited, causing a voltage drop when the coil "wants" power, and the capacitor provides extra power during that time. The overall effect is that the coil "sees" less of a restriction on amperage when it needs power, but there is no change in the peak voltage when it does not.

I am sure by now you want to know the most important thing - did it work? It Sure Did!! The car starts better, idles better, and drives better.

WARNING: I do not recommend these modifications to anyone. There are so many ways you could make mistakes and make your car less reliable, or create dangerous conditions with potential for sparks and fire. It is also possible that these modification could damage parts of your ignition system. If you attempt to duplicate my enhancements, please do so at your own risk, and be careful! Finally, do not expect these enhancements to help if you have other problems with your car, such as mechanical problems with the starter, bad timing, or faulty ignition components. Carburetor adjustments must be correct as well.

Note: The Allison Ignition System is now marketed by Crane Cams. Here is the installation guide. See the Crane Cams Site for more info.


This information is provided in good faith but no warranty can be made for its accuracy. If you notice something incorrect or have any comments, or information to add to these pages, feel free to mail me.
First and Last Name:
Email (Optional):

Please enter any questions or suggestions you have in the area below:

To send your comments to David Butcher, press this button: .

David Butcher
davidbu at www.los-gatos.ca.us
Tel (408) 978-5495
Los Gatos, California 95030
Back to: [ David Butcher ]
[ SU Carburetors | Heater | Cooling System ]
* Copyright 1994-1997 All Rights Reserved