A "DA Plug..." The Solution To A Common Problem

The often misunderstood Fifth Avenue "DA Plug" has been around since 1987.  It came about out of necessity, and while the job it has to do is simple, it is very important. Let's have a look....

When you install an alternator on an antique vehicle that originally had a generator charging system, good things happen. You will have a charging output at idle and low rpms, something the generator charging system could not provide.

That gives you brighter headlights and easier starting because the battery is now fully charged and remains that way.  The alternator now provides the current to power the headlights instead of the battery as it did with the generator charging system.

Having the alternator provide all of the electrical current makes is possible for the battery to only be used for starting, with the current quickly replaced in the battery from the alternator after starting.  The less times the battery is deeply discharged and then recharged, the longer it will last. That is why the batteries in modern vehicles last so much longer.

The one downside of installing an alternator on an antique vehicle is that your antique vehicle will no longer shut off using the original ignition key. That is because the alternator produces an electrical output at idle (whereas the generator did not) so even though you turn off the ignition key, the output current from the alternator at idle, will travel backwards thru the ignition switch keeping the ignition coil "hot" which allows the engine to keep running!

This same thing happened to the engineers in Detroit when they switched to an alternator type charging system. Their solution was a little more complicated. They developed the "accessory post" type of ignition switch with an extra "accessory" terminal that was isolated from the rest of the terminals on the ignition switch.

The alternator was wired to that post, and that allowed current to flow from the ignition switch to the alternator but not backwards from the alternator thru the ignition switch. So when the new design switch was turned off the current stopped at the accessory post and the ignition coil no longer received battery current. Without battery current...the engine died. Problem solved!

Many of the "experts" will tell you to do as Detroit did and go get, and install an "accessory" type ignition switch and life will be good. That is true, however...in many applications an "accessory type ignition switch will not fit in the same place as the original ignition switch, and in some cases you just don't want to loose the look and fit of the original switch.




Example of a typical aftermarket "accessory" type ignition switch.

That was the situation I was in. I decided their had to be an easier way to fix this problem. After many months of experimenting the DA Plug was born. The DA Plug simply snaps into the top of a standard GM style internal regulated alternator (the standard alternator on all types of 1973 thru 1985 GM vehicles). That style of alternator remains popular even today because of its easy mounting, the fact that it is internally regulated, and its bullet-proof reliability.

I build the DA Plugs using heavy duty ISO 9001 rated electrical diodes to be sure your antique vehicle will shut off using the original ignition switch with no modifications.  A diode is simply a one way electrical valve and only lets current flow in one direction. The DA Plug also "turns on" you alternator so it begins to charge as soon as you vehicle is started.

That is important because the idle speed of your antique vehicle is slower than the 1200 rpms idle speed of cars when the alternators were introduced in the late 1960's. So to make the alternator start charging sooner that it was designed to start charging the DA plug send a little battery current to the alternator when the ignition key is turned on which tells the alternator to "go to work!"

While I was at it I made the DA Plugs compatible with 6-volt, 12-volt, and 24-volt,  electrical systems, because I also convert a few antique military vehicles from generator to alternator that still have the 24-volt electrical systems.

Thirty plus years later... the DA Plugs are still as popular as ever. I still make them here in the Good Old USA, and the price has only changed once in 30 years, from $15.00 to $18.00.

The yellow exciter wire comes plenty long enough to reach from the alternator back to the ignition switch or ignition coil, so there is no wire splicing. The other short red wire connects to the battery stud on the alternator and tells the regulator inside of the ignition what to do. It is as simple as that. Instructions are included in the package.

To get a genuine DA Plug look for the red and yellow wiring like shown in the picture above. I sell DA Plugs on the Fifth Avenue website and thru vendors like Speedway Motors.  Typically they are NOT available at the local auto parts store. I have tried but they do not see the need for a DA Plug. Many counter men "get it" and send customers to me which is fine as long as the problem gets solved.

If you go to the local auto parts store and ask for an alternator wiring harness...this is what you typically get. It will NOT do you much good if you are installing an alternator on an antique vehicle. You will have no way to turn off the engine with the ignition switch...

A Note About 1-Wire Alternators -
A one wire alternator is designed to start charging about 1200 or greater engine rpms. One wire alternators were originally designed for racing applications where there was not much electrical load and the engine ran at higher engine rpms.

A one wire alternator stores up magnetism in the rotor from the last time it operated. When the rotor speed reaches high enough rpms the magnetism is released and the alternator begins to charge. If the rpms drop below the alternator's cut-in speed, the alternator will quit charging until the rpms are increased. If the vehicle sets for a month or longer the alternator can loose the magnetism in the rotor.

That is why a one wire alternator typically does not work well in an antique vehicle application. Most antique vehicles do not have a high enough idle speed to make the alternator start to charge.

The internal voltage regulators in a one-wire alternator are different than a conventional alternator so you CAN NOT just add a DA plug to fix your one-wire alternator charging problem at idle. You will need to change out the one wire internal regulator for a conventional two wire regulator and then use a DA Plug to live happily ever after.

How do you know if you have a one-wire alternator? You will see a black rubber plug where the DA Plug would normally go. That is your clue that you have a one-wire alternator.

Rubber plug identifying a one-wire alternator.

So now you know what a DA Plug is and what it does. It is a simple fix for a common problem and is part of what you need for a reliable charging system.