SBC Starter Circuit Wiring

Simple Starter Wiring Diagram

Wiring up the starter motor circuit is rather simple. Based on the above wiring diagram, you need the following:

  • Ignition Switch - The starter is turned on by the ignition switch's Sol (solenoid) position. It is a momentary contact position usually on the key switch. It doesn't have to be on the key switch, you could make it a pushbutton switch instead mounted on the dash. This switch provides +12V to the starter solenoid.

  • Alarm Cutoff Relay Contacts - If you have a car alarm, the disable/cutoff relay contacts would go in series as shown in the wiring diagram. When the alarm is on, the contacts are open, preventing anyone from starting the engine.

  • Neutral Safety Switch (Clutch Engaged) - This is a switch that is attached to the shift lever (auto or manual). It prevents the engine from starting unless the transmission is in neutral or park for automatic transmissions. For manual transmissions, it is connected to the clutch pedal so that you can't start the engine unless the clutch is engaged.

  • Starter Solenoid - This is a connection on the starter or a separate soleniod that controls the starter motor. It is separate connection on the starter so that smaller gauge wire can be run to the starter to control it. The option is to use one big hunking switch and a battery cable to run to the dash to control the 100s of amps that the starter requires to turn over the engine. The starter solenoid is basically a big relay. It can be integrated within the starter or a separate device like the starter solenoid shown here.

  • Starter Motor - This is the motor that turns over the engine. It will have an unfused very large gauge wire going directly to the battery's +12V and often a smaller gauge wire going to the alternator or generator. The case is grounded and provides the return path the battery's negative terminal. The starter shown here has the starter solenoid attached to the top - it is the small cylinder on top of the starter.

Starter Problem

I ran into a strange problem when I shut off the engine after running my sbc 305, the starter momentarily engages as the engine dies. It makes a loud clanging sound. Both the starter solenoid and ignition are on separate relays from the 1954 ignition switch.

Here's why:

I had added two relays to the original circuit. One relay to control the starter and another to control the HEI ignition. I figure that the original ignition switch may not be able to handle the extra current that a modern car requires and the relays would lessen the current through the ignition switch dramatically.

When a relay coil is energized, there is energy stored in the coil while current is passing through. When I turn off the current, there is no place for the stored energy to go - it wants to return in the same direction it came from (collapsing magnetic field is the technical explanation). An ignition coil works on the same principle but it is also an autotransformer which bumps the 12V up to 40 kV which releases the energy through a spark through the air.

So my solution was to put a reversed biased diode (1N4001 - costs 5 cents) across the coil. The diode's positive lead (anode) is connected to ground and the negative lead (cathode) is connected to the coil's +12V source which is the ignition switch's starter position tab. A diode is a one way device, it only lets current flow in one direction from positive to negative (forward biased). During normal operation, the diode acts like its not even there (it's reversed biased - negative connected to +12V, positive connected to Gnd). But when the coil is de-energized (turned off), it allows the stored energy to be released in the opposite direction that it was stored in and return to ground

Why does the energy change direction? The stored energy, in the coil, changes polarity due to the change in the magnetic field generated by the coil. When a coil is first energized, the magnetic field is growing so its charge is +12V. When the coil is de-energized, the magnetic field is collapsing which reverses the voltage to -12V and causes a reverse current to flow. The diode indirectly becomes forward biased because now the negative voltage -12V is connected to its cathode (negative lead) and the positive lead (anode) is connected to Ground which happens to be more positive than -12V. This fits the criteria for the diode as a one way switch. The anode has to be more positive than the cathode.

It is common practise in electronic circuit design to add a reverse biased diode across the coils of relays to prevent voltage spikes and other bad things from happening.

I thought that it was odd that an automotive relay would require an external back emf bleedoff diode to be added to the circuit and did some further investigating. I found that there are two types of automotive relays, one has an internal 680 ohm resistor in parallel with the coil to bleed off the back emf and the other doesn't. I purchased mine on sale for dirt cheap and they didn't have the bleed off resistor. Normally a diode would be used but my guess is that since there is no polarity marked on the relay, the manufacturers use a resistor instead. This way you can hook up the coil either way and it will work - kind of foolproof. Also the resistor is 1/10 the cost of the diode so the manufacturer can save 5 cents per unit.

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Copyright July 2011 Eugene Blanchard