Electrical issues, I'm stumped, UPDATE

[slowmobile]

Did it again today. A little warmer out. Coil and MSD box are hot to the touch. Coil moreso than the MSD box. Thoughts? Time for a new coil? Seems like idle quality is worse right after it cuts out and I get it started again...

[jmarkaudio]

Could be the pickup coil in the distributor. How old is it? Unfortunatly intermittant problems are trial and error. Try any parts you can borrow first.

[af2]

It only does this when warm. We have had cool fall days lately so driving 15-20 minutes is OK before it starts acting up. Letting the car sit for 30-45 minutes helps, but after everything warms up again it starts cutting ignition again.

I'll offer an opinion just because I had the exact thing happen to me a while ago. The internal lead in the coil post to the internal winding can fail due to engine vibration, it maintains continuity until the heat builds and opens up and quits until it cools off again. To check this, run it until it quits, then jump out and pull your primary wire and see if when you crank it over you have spark. If not, just plug in another coil (does not have to be MSD for this test) and crank it again and see if you now have spark. Also had the same problem with the pickup in the distributor, but this is more rare than the coil problem. MSD used to sell a HD coil specifically made with an internal service loop for high vibration endurance applications (NASCAR, Off Road, etc) which mostly fixed this problem. Don't think it's related to the alt, but...
Hope you find the problem, electrical gremlin stuff can drive you to drink for sure.

I feel the question was answered here. Get another coil to try and you might be surprised. Been there myself!

I had to edit thinking about a mechanic I worked with that told me a coil never goes bad and look at the carburetor! I changed the coil and sent it home to never come back!

[steveD]

Have you checked the voltage with the new alternator? If it's high ( >14.2volts) then the ignition might overheat and malfunction. The coil will run hot as well.

[Tuner]

Use your DVM and check for AC volts at the alternator output terminal. With all the lights and accessories on and the engine at about 2500 there shouldn’t be over .1V (one-tenth volt) AC.

[slowmobile]

Tested voltage right at the coil and it was 3-4 just before it dies. I am just guessing here but that can't be right...but again I am guessing. I have it wired exactly as MSD calls for in their instructions using the white wire as the trigger since the distributer is a "Ready-to-Run" version. Orange out of the box goes to + and black to - on the coil.

All other switches and the battery have 12+ volts at idle. Even the lead off of the battery switch that powers the ignition box and coil. Shouldn't I be getting 12 volts out of the MSD box to the coil?

Any thoughts?

Thanks guys.

[jmarkaudio]

Are you not using the pickup coil in the distributor? If you are using a module to trigger it you are adding potential problems into the picture. Try connecting the pickup coil to the mag trigger wires bypassing the other electronics. I have never measured voltage at the coil with an MSD, but realize it is a capacitive discharge, probably not a stable voltage reading. Make sure the large red and block wires go DIRECTLY to the battery. No switches.

[slowmobile]

Are you not using the pickup coil in the distributor? If you are using a module to trigger it you are adding potential problems into the picture. It's triggered by the distributor. Orange from the distributor to the white trigger wire. Make sure the large red and block wires go DIRECTLY to the battery. No switches. Will re-do those tonight.

Thanks!

[vinmann]

Be careful measuring voltage at the coil with an MSD. The voltage would be measured in AC and could be as high as 400 volts (peak) depending on plug gap. Enough to to make you jump and hit your head on the hood if you touch it. Because the waveform is not sinusoidal and is only present for 20* (out of 90*) or 22% and your meter measures AC in RMS or roots mean squared and your plug gap is tight you likely wouldn't measure even 200 volts. I've never measured voltage here.

If your distributor is "ready to run" then you could wire in a switch that would bypass the MSD box.

The MSD box uses its power in spikes just like it sends power out. That is why the wires are big and shouldn't connect through a switch other than a master disconnect.

You could put a test light on the orange and white junction. ( The other lead to v+) The light would keep flashing as the motor dies telling you the distributor is fine.

[slowmobile]

The MSD box uses its power in spikes just like it sends power out. That is why the wires are big and shouldn't connect through a switch other than a master disconnect.

Rewired it. I DID have the power from a switch, I swore I didn't but I am glad I checked. No problems now.

Thanks guys.