SBC Starter Circuit Wiring - 1954 Pontiac
Starter Problem
When I shut off the engine after running my sbc 305 for awhile, the starter momentarily engages as the engine dies. Makes a long clang. Any ideas on why this would happen? Both the starter solenoid and ignition are on separate relays from the 1954 ignition switch.
Here's my solution:
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.
Fusible Links
BTW you can test your fusible links by connecting your 12V battery up backwards. The link at the starter will smoke, catch fire and melt while you frantically try to unscrew those damn side screw battery terminals....... Fortunately, they do their job and nothing but a short piece of 14 ga wire is damaged....
a fusible link is a short piece of wire about 1" to 2" in length that is a small gauge of wire. It is inserted in series (in between) the battery and starter wiring which is quite a big larger in size. If a short occurs like in my case, the fusible link heats up and melts because its the bottleneck for the current to flow. It'll melt before the bigger gauge wire will. Most of the times, the fusible link is well hidden in the wiring harness and difficult to find.
I was installing speakers in the dash of my 1972 Corvette and had to drop the steering column. When I reinstalled the steering column, I squeezed a wire and shorted a hot +12V line to ground. All I saw was smoke coming out of the engine compartment. The battery is behind the driver's seat in a little storage compartment. You never saw anybody move that fast to disconnect a battery!
It took a week, to find the fusible link in the wiring harness in the engine compartment, repair it by adding another piece of wire and hook everything up again!