MSD and Mallory Unilite

Here's a hint for anyone that's using a mallory unilite distributor and an msd box. The mallory unit needs to have between 7 and 9 volts to work correctly. If you wire it the way the msd instructions show, you'll actually get whatever your battery voltage is to the unit. (Not good for it) The problem is, the ballast resistor or ignition resistance wire is designed to step down the voltage with the unilite AND a coil drawing current from it. When you hook up an msd box, it handles the coil voltage/current itself, leaving the "switch" for the msd and the unilite as the only things feeding from the ignition wire. This causes a problem, because the unilite draws ~0.02A, which when hooked up to the mallory ballast resistor (1 ohm) only drops the voltage by 0.02V. I ended up wiring a 270 ohm resistor in series with the unilite to get the correct voltage drop, ~5 volts. This puts it down into the 9 volt range when the engine is running. If you run a wire from the I terminal on the solenoid to a point between the distributor and the resistor, you will get enough voltage while cranking to start the engine. Sorry for the long post, but this problem drove me nuts, and those unilite modules aren't cheap (and don't need any help breaking[img]/phpBB/images/smiles/icon_biggrin.gif[/img]) Also, msd doesn't seem to address the issue in their instructions.

Arrrrrgggggggggggh........he said Mallory..............[img]/phpBB/images/smiles/icon_eek.gif[/img][img]/phpBB/images/smiles/icon_eek.gif[/img][img]/phpBB/images/smiles/icon_frown.gif[/img]
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I've been running the Unilite since '86 without issue. I run it with switched battery power: i.e. 12V with key on. That was the requirement when using their Pro coil.
Currently running it with the Holley Strip Avenger(?) ignition with no issues.

There are a couple of things to understand with ignition which makes the "7-9V" rule difficult to understand.

The unilite "grounds" the green wire when the shutter is open. The current is fed through the pos wire on the coil to gnd via the unilite. The wire that feeds the coil needs to have a ballast to prevent what is called "saturation" from occurring in the coil. This is when the current swaps the coil. Two problems occur: energy is wasted. Heat is generated.

When you convert to MSD what they are looking for is a "trigger" signal. They want to see effectively what is called a square-wave input. What they do is control their box when they see an external voltage swing between 0v and say a minimum of 7V to insure they "see" the distributor trigger.
There is effectively very little current flowing in this case and you can direct connect the unilite to 12V.

Hope this helped...

joeyalex: Were you actually having problems burning up the Unilite modules, or are you just trying to play it safe by wiring it this way? According to the Unilite installation manual on Mallory's web site, the wiring diagram shows connecting the distributor directly to the +12v side of the ballast resistor, and the coil is the only thing on the low side of the resistor. Then in the document it says:
"NOTE: THe purpose of an ignition ballast resistor between the ignition switch (12V) and the ignition coil positive terminal is to restrict current flow through the ignition coil. Failure to use an ignition ballast resistor will eventually destroy the Ignition Module."
Then it says:
"EXCEPTION: If your vehicle is equipped with a HYFIRE Electronic Ignition Control or similar aftermarket ignition control, use the wiring procedures stated in the instructions included with the ignition control"
I know you've got an MSD box and not a HYFIRE, but they both probably operate similarly...
Here's the URL to the document:
http://www.mrgasket.com/pdf/mallory_...niliteDist.pdf
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GREAT STUFF!!

But I still wasted a unilite while using the ballast resistor. It only happened once thought in 5years. Still don't know why? Too much heat maybe?

That's odd...maybe I had a different problem. The truck would be driving along the street, and the ignition would just start cutting out. Felt almost like it was running out of gas, but the tach needle would dive from 2000 to 0 and then go back to 2000. It felt like it wasn't sparking for a second and then started sparking again. It hasn't given me any trouble since I added the resistor. Hopefully it won't. Ah well, I'll see how it goes...