Here is the MSD schematic for wiring the Ready-to-Run distributor along with an MSD-6A-style CD box. Note that the diagram requires straight 12-volt source to both the box and the distributor.
Crank Start Cures
John Kukura, Crystal Falls, MI: I have a strange problem on my '72 Chevelle with a 396ci big-block engine. It has an MSD Pro Billet distributor and a Blaster 2 coil. The engine won't start with the key in the "Start" mode. It will spin over forever but won't start until the key is released back to the "Run" mode. Then it runs perfectly. I've checked for spark at the plugs in "Start" mode and there is spark at the plugs while the starter is running, but the engine won't fire until the key is released. Is my ignition switch bad, or maybe a bad module?
I disconnected my Sun tach, thinking it was shorting out my ignition, because when the engine was spinning over, the tach needle would wind up as far as it could, then go back to zero. I've never run into this before and it's making me crazy. Thanks.
Jeff Smith: We called our friends at MSD for help on this one, John, and it appears there may be several issues conspiring to cause your problem. First of all, we're assuming you are using the MSD Ready-to-Run Pro Billet distributor model PN 8360. We're assuming this for several reasons, but mainly because you don't mention the use of a CD box like the MSD-6A. The Ready-to-Run distributors are designed as stand-alone ignition distributors that don't require a separate CD box and are easily installed with three simple wires. Here's where part of your problem comes in.
Back in the dark ages of points distributors, the factory employed an in-line resistor wire that reduced voltage to the positive side of the coil. The reason for this resistor was to limit the normal 12 to 14 volts from the charging system down to around roughly 6 volts to prevent cooking the points. To improve starting performance with this weak, low-primary voltage ignition, GM cars like yours used a separate wire feeding straight battery voltage from the solenoid directly to the coil only during cranking.
In the MSD installation manual for the Ready-to-Run distributors, you are instructed to connect a positive 12-volt wire from the ignition switch to the positive side of the coil. Using the existing factory resistor wire seriously limits voltage, especially under cranking, to a level that could drop well below 8.5 volts. This voltage threshold is important because this is the minimum voltage required at the coil to operate the ignition. In your case, this could create a situation during cranking where voltage drops to just above the 8.5-volt level. The ignition may actually have enough voltage to fire the plugs, but that very weak spark isn't nearly enough to start the engine. When you release the electrical load created by the starter, the voltage will increase, and this may be enough (because the engine is still spinning) to start the engine.
For a quick test, run a jumper wire directly from the positive post on the battery to the positive side of the coil and see if the engine starts immediately when cranked over. If it does, it's obvious you need to eliminate the resistor wire and run a full 12-volt switched ignition wire from the fuse box to the positive side of the coil. As a side note, this wouldn't have happened if you were using an MSD-6A or any other CD-style box because the CD box is powered by a large, four-gauge wire directly from the battery that supplies electrical power to the ignition system. The power wire to the coil is merely the switched wire that turns on the system.
It's also possible that you may have a poor ground between the battery and the engine or between the engine and the body. A poor ground circuit (compounded by high-resistance battery cables) can place a great load on the battery, which creates a major voltage drop through the circuit when the engine is cranking. This contributes to the hard-start situation because cranking voltage drops more than it should because of the high system resistance. Add a situation where you don't have a good ground between the body and the engine, and voltage drops again between the battery, ignition switch, and distributor. It doesn't take much of a voltage drop through a couple of these circuits combined with high-resistance battery cables to make it very difficult for the engine to start. We would suggest heavy-duty 0/1 copper multi-strand battery cables such as those available from MAD Enterprises. We use MAD's cables on every car we build or upgrade because of their minimal cranking resistance that allows maximum current flow to the starter. We would also add a minimum of two high-quality ground straps between the engine and the body to ensure a minimal voltage loss.
We also called Sun's techline and asked if a poor ground circuit could contribute to the erratic tachometer operation you described. The technician we spoke to calls this poor ground operation "fly back," where the electrical voltage is looking for a good ground path. When it can't find a decent ground between the tach and the battery (because the body isn't sufficiently grounded to the battery), it will back-feed through the positive side of the tach, creating a spike in the circuit, indicated by the wild needle fluctuation. The fix is to ensure you have several quality ground circuits between the body, the engine, and the battery. This will improve the entire electrical circuit and you'll probably notice that your headlights will get brighter as well, making it a brighter idea for all kinds of reasons.
More Info
Autotronic Controls Corp. (MSD); El Paso, TX; 915/857-5200; msdignition.com
MAD Enterprises; Springville, CA; 559/539-7128; madelectrical.com