jwog666

get a clamping diode like the one your ac circuit uses and wire that in, you can most likely get one out of the fuse box of any car at a junkyard. most look like fuses with the pic above stamped on it.

dkersten

You lost me there.. If you put Anode to the positive side of the coil (12v from the switch) and Cathode to ground, you are grounding the 12v that comes from the switch and you will blow a fuse when the button is pushed. Am I mis-interpreting what you are saying?

I haven't been an electrical engineering student for almost 20 years, so trying to process this is having to break through a lot of cobwebs. I was looking at this from a diode being a pure one way gate and the burst of voltage from releasing the charge on the coil in the solenoid as a larger voltage differential than the 12v power source (battery or alternator). In those terms, your power goes from the battery (fuse panel, etc), through the momentary switch (button for purge), to the positive side of the solenoid, through the coil, to ground. When you release the button, you have 12v on one end, an OPEN switch after that, then a high voltage (charged coil), then ground. That high voltage is going to take the path of least resistance, which would be ground. Thats my take on it, but I am not an electrical engineer, so I could very well be missing something. A diode is not a PURE one way gate, there is a threshold at which point it will allow voltage to flow backwards through it (Peak Inverse Voltage), and I don't know how that affects things.

The thing that has me hung up is if that charge drops to ground, the chassis on that vehicle is big enough of a ground plane that it should not affect the radio. It seems to me that what might be happening is that the charge is coming back through the switch side and jumping the contacts on the switch as the switch is released, in which case the surge of voltage would be on the same power feed the radio uses, and would easily cause a pop in the speakers. If that is the case then I would think a diode would stop that surge from backfeeding to the radio if put inline on the power side. Once again, I do not know if that surge is enough to break through the Peak Inverse Voltage of the diode and still come through.

It has me curious. I could be totally off base on this, so go ahead and try what 157db is saying, whats the worst that could happen?

157dB

I seem to have it backwards.
The diode always points to positive and the cathode designator line on
the package should go to positive.
Its not the surge but the spike thats actually transmitted thru the wiring
like a good ol fashion spark gap transmitter of days old.
The Diode snubs the PIV.
The pop could be from the arc produced from breaking the circuit with
the switch. Try a by-pass capacitor, like a points ignition uses, across the switch terminals to further reduce the unwanted pop.