Polarity Reversing

by Dave Grundis

Originally published in the Austin-Healey Magazine, April-May 1987


Want to put one of those high-tech high-power car stereos in your Healey? How about a compact disc player? A pleasant thought until theft considerations and speaker limitations bring one back to earth which is the subject of this article.


Any type of radio installation, whether it be your Healey's first or a major upgrade, will require the changing of the "earth", or ground, from positive to negative. Two alternatives to this will be discussed and summarily dismissed.


The first is to find an old, positive ground radio (tough to do) and hook it up. Then you will have the sweetest music this side of the street; that is, until you try to do something crazy like start the engine. Those old radios don't put out much sound through their stock 4" by 8" speaker!


The other choice we'll explore before coming back to earth is the purchase of an inverter. This electronic black box reverses the polarity for a specific purpose, e.g., a stereo. The drawbacks of this alternative are: (1) finding one, (2) paying for something before you even buy the stereo, (3)mounting it in a car that has limited space to start with, and (4) very little power available for the stereo that you buy. This is because the inverter is limited in the amount of electricity that it can "reground" and so only a low-powered radio may be utilized. Also, the one inverter-run stereo I personally heard had a pronounced whine.


I have heard of one Healeyphile who put in a separate battery, complete with isolation charger, just to run the stereo. As if the boot wasn't crowded enough!


So that leaves us with reversing the polarity (changing the ground) to accept virtually any stereo on the market. What follows is a step by step process on how to do this.


First, turn off the battery at the "kill" switch in the trunk; also, ensure that the ignition is off. Reverse the cables on the battery. This is the toughest part of the undertaking and it may be done in a couple of ways. Lengthen the existing cables, turn the battery around, or buy a new battery with the positive and negative posts opposite from your existing one. Note that on most batteries, the positive post is larger than the negative one. Next, swap the wires on the ignition coil. If there are two on one side and one on the other, change all of them - just reverse the present situation.


You are now ready to reverse the polarity of the dynamos (generator's) field. To do this, take the small brown-with-green wire (not the brown-with-yellow one - that's the big one) from the generator. Follow it to the control box (voltage regulator.) Remove it from the regulator (it's the third from the inside and marked "F" on the regulator for "field").


Turn the battery kill switch on (but not the ignition) and touch the generator field wire to the new positive battery lead with a dragging motion to cause a spark. The easiest way to do this is to plug a jumper into the field wire and touch it to the now positive lead on the starter solenoid (the starter button) located on the lower right of the fire wall below the voltage regulator. It is very important that you achieve at least one spark because this signifies the reversing of the polarity of the generator's field. More than one spark is okay and it will not be harmful to touch the field wire to the other (negative) cable.


Now, although the car is ready, the tachometer must be removed and its polarity reversed. You may send it out to a speedometer shop and expect to pay about $65 to have it done, or you could do it yourself. It's not hard if you know how to use a soldering iron.


The tachometer, like the speedometer, is held in by two thumb nuts behind the unit. Undo these and slide the fixing straps off of the threaded shafts. Pull the tachometer out of the front of the fascia (dash) and remove the wires and bulbs. Remove the chrome ring and glass by loosening the tabs surrounding the unit. Twist the ring to line up the tabs with the notches and remove. Turn the unit over and release the two screws with lock washers, being careful not to let the unit fall out of the case. Do not remove the other two screws. Behind the "10" and "15" on the tach, you will find a red and a black wire. Carefully reverse these two fragile wires. On the opposite side behind the "60" and "65" you will find a green wire and a large tan resistor which is color coded brown-black-brown-silver. Remove the green wire from its post and one end of the resistor from the adjacent post. Trade and resolder. Reassemble the unit and replace in the dash.


Turn the ignition on, but do not start the engine.


If the polarity of the generator was not reversed, it will act as a motor and attempt to turn the engine over. (If this happens, turn off the key and attempt generator field repolarization again.) If this is not the case, start the engine. The generator light should go out and RPM should indicate normally. You can now go shopping for that new stereo. However, you inherited a new set of problems, like where do you put speakers in that car?


P.S. It took me longer to write this explanation than to do the actual process. Thanks to Ken Walsh of Walsh Motor Works and Jim Greco of Economy Imports for dispelling all the myths I had about how difficult and expensive this process would be.


Not what you were looking for? Don't forget you can check our back issues using the AHCUSA Magazine Index.

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