I have a 1970 Z/28 Camaro with a strong, small-block Chevy engine that that I’ve owned since it was new. Mostly I autocross race the car and recently I had a problem with an electric fuel pump. After replacing the fuel pump and readjusting the flow levels, I took the car out for a test drive and about 5 miles from my house, the engine died. I’m back after a tow truck ride and the engine has fuel, so it appears that my problem is with the ignition. I cleaned all the terminals and replaced the cap and rotor but it still won’t start. The distributor is an MSD Billet with vacuum advance, and an MSD-6A CD box and matching coil. Is there a way to determine if it is the distributor, the 6A, or the coil? I’m thinking about just buying a new distributor and trying that. Any ideas?
Gerald L. Los Angeles, CA
Jeff Smith: Before you spend any money on new parts, let’s do some simple diagnostics first to determine exactly what failed. Too often, guys just jump to conclusions about what has failed and keep buying parts until the problem is repaired. If you take the time to perform a few simple tests, often you can identify the problem. Let’s get started.
In your case, the MSD distributor uses a magnetic pickup that triggers the 6A CD box to fire the coil. The best way to do any diagnostics is to eliminate the most obvious issues first. We know that fuel is not your problem, so we can safely assume the engine is not getting any spark. You can check this by removing a spark plug wire and crank the engine over. Assuming no spark, the quickest way to narrow down the variables is by testing the CD box. MSD supplies a white wire from the 6A box that triggers the box when using a points-triggered distributor. In your application, the white wire is not used. Simply remove the coil wire from the distributor and position it so that it will show a spark from the coil wire to ground on the engine. Don’t touch the coil wire during this test or you’ll get a nasty shock. Next turn the ignition to the on position but do not try and start the engine. Now temporarily ground the white wire to the engine. If there is a spark between the coil wire and the engine, then both the 6A box and the coil are working, which means the problem lies with the ignition pickup in the distributor. You can also perform this test by disconnecting the two-prong lead from the MSD distributor to the 6A box. Using a paper clip, jump the two wires (green and violet) coming from the 6A box. If both the coil and 6A box are good, there will be a big healthy spark.
If after double-checking your test, the system does not create a spark, then we’ve narrowed the possibilities to either the 6A box or the coil. The next step is to substitute a known good coil for the one in your car and perform the same test again. If there is a spark, then you know the coil is the culprit. If there still is no spark, then it could be that the 6A box is the source of the problem. But you should check one more item before making that assumption. First you should check to see if there is actually 12 volts on the small red wire from switched ignition to the 6A box with the key on. It’s possible that this wire has failed and that the MSD box is not switched on. If there is 12 volts to the 6A box, then the 6A amplifier is the culprit.
If you think the pickup is at fault, you can quickly test it with an hand-held multimeter. These meters are very inexpensive and will test for voltage, resistance (ohms), continuity, engine rpm, and many other features. Summit Racing offers several that are priced under $50. Set the meter to test for ohms. When you connect the two leads together, the meter should read 0. Now connect the two multi-meter leads to the purple and orange wire that comes from the MSD distributor. The meter should read between 500 and 700 ohms. If the reading is outside of this range, then it’s likely the pickup is bad. A new pickup is available as a replacement piece (and it’s an easy piece to replace.
Running through these tests should get you closer to locating the problem. Often the solution is really simple. For example, a friend brought his carbureted, 302-powered ’68 Mustang over one afternoon complaining of an intermittent issue where the engine would just quit. I discovered the ignition lead from the firewall to the positive side of the coil had four poorly-executed crimp connectors spliced into the wire. I replaced that abused wire with a new one, and the car has run great ever since.
Often, the solution is simple once the problem is clearly identified.