Car engine RF interference

If it doesn't happen when you use the radio it's very likely not radio frequency interference -- if it were, it'd interfere with the radio. It's most likely a baseband signal that's coupling directly into the audio, either capacitively through the signal leads, because you've got a ground loop, or because the power you're putting into the MP3 player is dirty.

Are you powering the MP3 player from the car? If you are, and you use the player's battery, does the problem go away?

Are you connecting the audio cable shield to the radio's ground? Are you connecting _any other_ grounds to the MP3 player (like, by powering it from the car)?

Is the audio cable from the MP3 player well grounded?

What tim said, and additionally the radio's signal ground may not be the negative battery terminal however it will probably correspond to the quiet end of the volume potentiometer

Hi Tim (and Jasen),

thank you for your input thus far. Here is more detail.

I am not using car battery power to operate the mp3 player; I am using its own battery.

The audio cable may not be well grounded, after all. I measure 2.1 ohms resistance between the audio cable ground and the negative terminal of the car battery. I measure 0.0 ohms for the radio case ground, to the negative terminal of the car battery.

I soldered the ground wire of the audio cable to the potentiometer ground wire connection at the circuit board in the radio. At the time I thought I measured 0.0 ohms resistance between that site and the radio case; perhaps I did not.

Is the audio cable shield the "tin foil" type material that was wrapping around the three conductors in the audio cable? If so, it does not appear to contact the audio cable ground wire in the splice that I made, but I suppose it could be touching. Yes, I am a newbie at this...

This is a vintage car with the old fashioned plugs/points/condenser (capacitor) ignition. I know there are tricks for suppression with these components; perhaps I should consider them? Although, again because the radio is NOT affected by interference it would make me think these suppression maneuvers would be low yield...

Again, thank you for your time, and any further thoughts you can provide.

Regards, Dan

it's the ground wire, the one that connects to the sleeve of the audio plug,

if you connect short the contacts on the plug that goes into the mp3 player that should not cause noise with the engine going (while the short circuit is in place)

if it does cause engine noise you've got atleast one of the wires hooked up wrong, or maybe the cable is acting as a antenna ?

if it doesn't your mp3 player is picking up noise from the engine.

clean and/or replace the ignition system components.

Sounds like there's no braided shield on your audio cable. If that's the case, do yourself a favor and get cable with a braided shield. Something like CAT# MC-2 from allelectronics

Also: what is the splice you mention? (You want to avoid splices that break the shield.) Are you using a switch as was recommended in the earlier thread? If so, is it wired with shielded (braided shield) cable?

Ed

Hi Ed,

thank you for your input. I purchased a cable that would allow you to plug headphone output of an mp3 player into an aux port on a radio/stereo receiver. I cut off one end of the cable, and freed up the three conductor wires, which all were wrapped around a foil material, which I would presume to be shielding. To both the stereo channel input wires I added a 100 ohm resistor, summed them together, and then soldered the resulting single wire to the audio (AM) signal input to the trimmer potentiometer, on the circuit board in the radio. The remaining ground wire from the cut cable I soldered to the trimmer pot ground on the radio circuit board. Interestingly, there is about a 2 ohm resistance between the pot 'ground' and the chassis ground. The chassis ground measures 0.0 ohms to the battery negative terminal. The 'splices' I was referring to were the treatments described above.

The distributor wires in the car are silicone, RF suppression types, so there is no more I can improve, there. I have read recipes about attaching capacitors to the + side of the ignition coil, but I have not heard much good about that, from car stereo buddies of mine.

Thank you in advance for your time, and any ideas you can give me.

Regards, Dan

Ok, that "shielding" is poor, at best. Replace the cable with one that uses braided shielding. You want the signal carrying conductors inside the shield, not wrapped around it. Aside from that, it is very difficult to get good electrical contact to the foil. You want the shield to be electrically connected to chassis ground.

Ed