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Old 11-15-2004, 05:32 PM
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MSD diode gone bad?

Hey guys,
Like most rodders I run a MSD billet distributor, superconductor wires, and a blaster 2 coil on my 78 firebird with a small block chevy. When I installed it the car would run on for exactly twenty seconds after it was shut off. I read the instructions and found that when used with a delcotron alternator a diode (supplied) must be installed in the wire to soak up extra energy once the ignition has been cut. Problem solved...I thought.

All was good for a few months to a year but all of a sudden one day it started running on when the ignition had already been cut. The engine runs as if it were still in the on position (no missing or anything) but stays on for exactly 20 seconds every time.

Sometimes, however, the engine dies promptly when the key is turned off, like it should. Other times it does the 20 second run on. I was wondering if the diode can possibly go bad or if it is a bad connection. I soldered it into the wire and covered it with shrink tube to make sure it was perfectly snug.

As stated before, it worked fine for a long time but stopped working all of a sudden, so i figured i connected it right initially. Are diodes one way as some suggest? If it did go bad, how are they rated (ohms) and what would be the right size for a replacement. Is it easier to just reconnect the wire to the alternator without the diode and run a switch? Without the diode though it still might run on i assume.

Thanks all.

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Old 11-15-2004, 07:10 PM
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Diode

[size=3]Doc here:

To Check your Diode, Just get your DMM out, set it for Rx1 scale and put the BLACK probe on the lead on the diode that has the silver line painted on the diode body, and the RED lead to the other side... The Reading should be 0.00 if it hasn't blown open..

Next reverse the leads and check again..the reading should be infinite...no continuity...

Direction of conductivity:
Test leads:
--------->I----------
Red..........Black
=0.00 ohms in Rx1

---------->I----------
Black.........Red
Infinite (no reading)


If you get readings in Both directions, or readings in neither direction it's pretty much a done deal...get a new one!

Be Sure your Diode PIV is higher than your load...(Peak Inverse Voltage) or it will back-feed the diode and make it into a "wire" instead of a one~way street...

If you want a "Positive " shut off..you might consider the install of a relay where the diode is now controlled by the key switch, so when you shut it off the relay contacts open to the alternator wire... No diode to burn up!

As For replacement size, look on the diode, should have a number on it , like, 1N4001 or the like, take that number to the local electronics store and give it to them...get that or the next highest PIV rating... (usually, 1, 5, 10, 50 PIV ) and should cost you about $1.50 (probably about $19.50 cheaper than MSD..)

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Old 11-16-2004, 01:19 AM
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Great Deal

Thanks for the great info. That is why I love this site. I had no idea that there was so much information inolved in something that looked so simple. Unfortunately for me the car is back in chicago and I won't be able to do that work until I go home for winter break in a few weeks. I will save this post though and check it out when I get there. I am not a huge wiring guy and this is a great help. Thanks again.

Say the one I have tests good, but the problem remains, would it be a wise idea to get a new one one size up and rewire it to make sure? Just trying to make sure there isn't something else I am missing, but I am sure this is it.
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Old 11-16-2004, 01:52 AM
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Diode

[size=3]Doc here:

Yes, It is very possible for a diode to forward / backward bias check OK but break down under current...

And Yes, I'd go with a 50 volt PIV diode if you can find one..should be a standard "Rat Shack item for like a pack of 2 for $1.49..or so..be sure the diode body is about the same diameter as the new or larger (for current capabilities) Larger is always better!

The Relay provides a mechanically "Positive" way to shut it down, but is a few more wires to add...like relay coil to the ign side of the fuse buss, and relay coil to ground...then the normally open contact to the alternator side of the wire, and the center wiper (movable contact) to the ignition side of the wire..

The diode is easier to use, and if you "Oversize " this one should last forever...

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