How to EASILY provide full battery voltage to a HEI ignition module. 

Some of you may know that I have a HEI conversion ignition system installed in my '67 GTO.  This is a good performing, stock looking system that replaces the old points ignition with a GM HEI module.  The module is triggered by a magnetic pickup and it's all fitted in the original Delco points distributor.   The HEI conversion has been installed on my car for over five years now and it has been working great but just like with a factory HEI, it needs full battery voltage to operate at an optimum level.

Recently, a
modification I made to the goat was to install a Bosch 30 amp relay to route full voltage to the coil and HEI module instead of relying on the ignition switch and under dash harness only.  This modification will work on any GM car that once had  points type ignition where the user is installing a HEI ignition system or any ignition conversion that requires full battery (14.2v) voltage.   

In testing voltages under the hood I found voltage at the coil (and module) was down by over one volt compared to battery voltage at idle, even though I had replaced the resistor wire in the factory harness!   Just replacing the resistor wire does not address other problem areas in the factory wiring than can contribute to a voltage drop.  This voltage drop most likely would be even greater once more demands were placed on the ignition system with the engine at higher RPM and under load but I admit I have not driven around with a voltmeter connected to the module.  

So, since I just happened to have a spare relay sitting around I used it to route battery voltage from the main power distribution source to the coil (and HEI module).  

Above:  The ignition relay wired up.  It could have been positioned lower on the firewall to make it less obvious but I already had a mounting hole here.  The coil is an American made Accel 8140 painted black.


















Wiring for the relay goes as follows:  the IGN 1 & 2 wires that normally go to supply the coil are rerouted to the relay terminal 86 and are used to simply switch the relay on and off.   IGN 1 is the “run” wire that is also the resistor wire.   IGN 2 is the non-resistor wire that is energized only when the ignition key is in “start” (cranking) position.   The resistor wire can be used for the relay on/off terminal since the relay consumes very little power to keep the contacts closed (0.08amp). A 14ga wire runs from an always hot power distribution point (main splice, battery + or post) to  relay terminal 30.   Another 14ga wire runs from terminal 87 to the coil and/or module.   Terminal 85 is ground and terminal 87a is not used.  Terminal 86 receives the signal (voltage) from the ignition switch.   14 gauge wire is all that is really needed for the short underhood distances involved and will be able to carry more current than the HEI module and coil can use.   

HEI/Relay Schematic


Right: The main power distribution post on my car.  This post replaces the factory splice and provides a convenient way to hook up accessories provided what you need is a constant (not switched) 12v source.  What is wired up to this post is: ignition relay (blue),  A/C relay (yellow),  horn relay (black with red stripe), dash power-up (black fusible link), alternator voltage sensing (14ga red wire), battery charging (10ga red wire) and the 8ga red is power feed from the alternator.  The other two blue fusible links are power for the headlight relays.

Now, with the relay installed and when the ignition switch is turned to run or start, the relay is energized, it closes and routes full power from the main splice to the coil/module.   The result, full power to the ignition system and no voltage drop! 

What causes the voltage drop as seen at the coil with a factory harness?   Since the dash harness is fed by only one wire and dash board accessories have to share the power feed with the ignition, something less than the input voltage will be available once power leaves the ignition switch.   Keep in mind that the voltage to the old points system was stepped down with a resistor wire anyway so having a little less voltage was not a concern.    


All the ignition switch does is send a signal to the relay telling it to complete the circuit for ignition power, the relay does the rest.  Most ignition boxes that take power directly from a battery source have this type of power distribution already built in, one of the reasons for their effectiveness.














Engine Run-on:
If your engine continues to run even with the ignition key turned OFF your are experiencing run-on.   This usually occurs in older vehicles with an external voltage regulator but can also occur with internally regulated alternators.   The relay requires only a small amount of current to stay closed and provide power to the module and coil.   So, if you are experiencing run on it is due to this small current going through the charging lamp and feeding the terminal that keeps the relay closed, even if the key is turned off.  

Early Ford and GM.  To solve the run-on problem install a diode in-line with the wire that goes to the charging indicator (see diagram at right).  Diodes allow current to flow only one way so be sure the stripe on the diode is pointing in the correct direction.  See the diagram on the right column of this page.

Ford: Install the diode inline with the wire going to the #1 terminal #1
GM: Install the diode inline with the wire going to the #4 terminal


GM 1973 - 1983 with Delcotron Alternators:

GM Delcotron alternators use an internal voltage regulator.  Install the diode inline on the smallest wire exiting the alternator.  This is usually a brown wire.