Expert: Cleggsan - 6/10/2010

Question
I have a 1970 ford am-fm stereo radio with an original 3 wire speaker output plug coming out of the radio, I want to hook up the speakers but am unsure of the wiring and do not want to blow the radio, the connecter is like the trailer connecters, one pin shielded in plastic the other 2 uncovered, common sense tells me the covered pin would be the common however I want to be sure. I need to know which is pos, neg, front or rear. The colors are orange, violet, and white. The white is shielded. Please help, thanks in advance.

Answer
What Ford do you have?  Mustang?  Pickup?  If you google around there are a few sites that have this kind of information for legacy cars.  

Also, if you have a VOM or ohmmeter you can trace out the wires to make sure you know which is which.  If you have an oscilloscope you could very easily look at the audio signal coming out when the radio is ON.

With a vom connected to the connections you think are the right ones you can set the VOM for reading AC volts and the meter needle will fluctuate according to the music or voice.

If there are two speakers it could be the ground wire is common and the other two are for each speaker; the ground would be connected to the - terminal on both speakers; the other two wires would be connected to the respective + terminals.

Happy Hunting. Let me know if you need more.

PS:  Connecting the vom to the outputs of the radio unit will not cause any harm because of its high resistance.  Just be careful in making the connections.  You can purchase a very suitable vom for doing this kind of testing at Home Depot or Walmart for under $10.