Proper grounding of stock car electronics is a critical to winning races. All ground connections should contact bare metal. Powder coating and anodizing both act as insulators preventing a good connection. Not only must the block be grounded, but the cylinder heads must be separately grounded as well. Whether you call it a brain, MSD, black box, amplifier, box, or ignition box, it must have a common ground with the rest of the car and electrical system. If the ignition coil is an E-core design, the coil bracket must be grounded. Make sure that the starter solenoid is grounded. Some solenoids have a dedicated ground wire, but most ground through the mounting bracket.
You should have a 1/4" ground stud welded to the chassis in an easily accessible location near the starter solenoid. This is a common ground junction point for the engine, cylinder heads, ignition, tach, driver coolers, and all blowers. It may be prettier to hide junction posts and the like, but when you are in a hurry trying to fix a problem, accessibility is essential. By making the ground stud 1/4" and using 1/4" ring connectors on all ground wires, you cannot mistakenly put a ground wire on a hot power source. Both the Master disconnect and the starter solenoid studs are 5/16" or larger.
The steering wheel stop switch should be connected in line from the ignition On-Off switch and the Ignition box. Some people who run MSD ignition boxes may use the points trigger wire and the stop switch to complete a circuit to ground. Their logic is "that if the switch fails, the car will still run." Logically, if a switch fails, it has a 50-50 chance of failure in either position. A quality switch assembly will prevent failure. The main reason from an electrical/electronic standpoint to interrupt the power from the ignition switch is to reduce radio interference. The wire from the ignition switch only carries 5-10 milliamps whereas the points wire carries many times that amount since its designed purpose was to carry to the points enough energy to burn through any oil film or moisture on the point surfaces and trigger the box. Always try to separate the PTT coiled cable and the Stop Switch coiled cable.
The master switch should be on the POSITIVE side of the battery circuit. Electronic devices including the tachometer, Ignition box, and alternator all take a very small electrical pulse and amplify it with transistors and other electronic components, to the point where the signal is strong enough to control the ignition output or other related functions. Electron flow needs to be shut off on the POSITIVE side to prevent a potentially harmful backup of electrical energy. Under a no load, condition when everything is shut off, it is acceptable to disconnect the NEGATIVE wire first. Interrupting the current flow by switching batteries via the ground cable, or otherwise shutting off the negative flow may cause electronic components to fail.
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