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Old 05-05-2017, 08:47 AM   #1
oldpurple
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Join Date: Feb 2005
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RPM related AM radio interference?

I have a '79 with an AM radio. I get an annoying buzzing that is dependent on RPM. I can't remember exactly when it started, but I don't recall really changing anything where it started immediately after. I had changed plugs/wires/cap/rotor but I don't think it started right after that... I feel like it was just random. I have had it stored for the last 6 months though so my memory may be wrong.

Any ideas? My thoughts are ignition or alternator but I'm not quite sure where to start.
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Old 05-05-2017, 10:33 AM   #2
wilkin250r
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Re: RPM related AM radio interference?

It could be you have noisy spark plugs. Your fix might be as simple as changing to a resistor spark plug.

It could also be noise on the line, which is often caused by a bad ground. Cleaning ground connections, making sure your radio's metal case has a good connection to chassis ground. Sometimes, unfortunately, the bad ground is INSIDE the radio, from it's own circuit board to chassis connection. The only fix is to crack it open, or replace it. A $20 DC noise filter also might give you acceptable results.

It could be coming in through the antenna, and is still often a grounding issue. Clean antenna connections.

A cheap, battery-powered radio is often a good tool to diagnose noise sources, the smaller the better, you run it around your engine compartment looking for the source of noise.
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Old 05-05-2017, 11:17 AM   #3
Dead Parrot
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Re: RPM related AM radio interference?

At idle, a single noisy plug should have a frequency of about 6 pops per second.
Math: Assume 700rpm idle. Divide by 2 since plugs fire every other rev. (350) Divide by 60 since RPM is minutes and sound freq is seconds.(5.8 round to 6).
8 noisy plugs will be about 50/sec. Very similar to the 60hz power hum you sometimes get from stereo equipment when connecting stuff while it is powered on.

A noisy alternator will likely be around 3~6khz at idle. There is an internal cap in the alt that is for noise reduction. Brushes near EOL might be arcing more then normal.

Could also be bad or non-rf suppression wires. Think the solid copper 'racing full power' wires.
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