MSD 6AL #6421.....For some unknown reason, I have 12V at the negative (black) coil wire, and when it's hooked up to the coil, then I have 12V at BOTH terminals. Needless to say, the car won't fire. It was running fine yesterday...I've done the check by unplugging the distributor wires and bridging them...no spark. New coil...no spark. Did my MSD go south???
I have power, key power and ground is good.

are you getting a pulse from the distributor? To check this a digital VOM with a fairly fast sampling rate when hooked to the distributor and the engine turned over should show a momentary voltage. No plus would mean a dead pickup coil

The thing that baffles me more than anything, (besides not starting!), is that the black wire to the negative side of the coil now has 12V as soon as you connect the battery terminal. Am I not correct in thinking that there should NOT be power on that wire?? My logic would say that the positive(red) coil wire should get power with the key on...it does not. As I said, I disconnected the harness(2 wire) to the MSD distributor, bridged the terminals and continually broke that connection looking for spark from the coil wire...NOTHING.

Check to make sure you haven't lost ground to the MSD. The quick and dirty check would be a test light from the positive post on the battery to the MSD box. If you got ground then I would suggest sending it to TechWest

You can check the distributor magnetic pick up with a ohm meter, it should read between 300 and 500 ohms if it is good
As already mentioned a bad ground on the box can make it quit, I have had a race 7A box quit in the staging lanes with no warning

Well when the MSD unit puts 12 volts to the coil + side if it is not grounding the - side at that time you would see 12 volts on the - side also. Course it should go to 0 volts when the MSD unit grounds the coil primary. You may want to put a test light on the coil - side and see if it pulses the test light while cranking. But I am not sure with MSD if it will since they do put up to about 300 volts into the coil in pulses. I never tried it myself when cranking to see what it reads while cranking but I would look to see a change in the test light brightness. If no change at all then the coil primary is not being grounded and ungrounded and could just be a bad dist pick-up. I actually never tested an MSD like that as it may not ground the coil - until the eng is cranked but I am not sure. And if you dont have a good ground for the MSD unit the - coil side would also read 12 volts all the time. Good luck , Ron

Copy and paste from MSD troubleshooting page...


"Next, lets review the coil wiring. The ONLY two wires that should be connected to the coil are from the MSD. The orange wire connects to coil positive while the black wire connects to the negative terminal. No other wires should be connected to the coil terminals! Also, do not touch or connect any tools to the coil. With the MSD connected there will NOT be voltage on the coil terminals unless the engine is running – and in that case there will be over 450 volts due to the CD technology of the ignition. Again, the only two wires connected to the coil are the MSD Orange, positive, and the MSD Black, negative."

Magnetic Pickup Trigger:
If you are using the 2-Pin Magnetic Pickup of the MSD to trigger the ignition, follow these steps.
Make sure the ignition switch is in the Off position.
Remove the coil wire from the distributor cap and position the terminal so it is approximately 1/2" from a good ground.
Disconnect the MSD Magnetic Pickup connector from the distributor.
Turn the ignition to the On position. DO NOT CRANK THE ENGINE.
With a small jumper wire, short the Green and Violet magnetic pickup wires together then pull the jumper off. Each time the short is removed a spark should jump If spark is present, the ignition is working properly.
If there is no spark:
Inspect all of the wiring.
Substitute another coil and test again. If there is now spark, the coil is at fault.
If there is still no spark, check to make sure there is 12 volts on the small Red wire from the MSD when the key is in the On position. If 12 volts are not present, find another 12 volt source and repeat the test.
After inspecting the test procedures and inspecting all of the wiring, there is still no spark, the Ignition is at fault.

The following test will determine if your MSD is producing a spark.
Checking for Spark

No spark when I do what you're showing....got a new unit coming. Can't be anything else going by all the tests and checks. Thanks y'all.

New MSD unit solved the problem.