Interference in the AM band

[Tim]

Ok, so I guess I'm one of the 5 people left who actually listen to the AM band (they have a really good oldies station out here), but I have a problem.

There is so much interference from the engine that it drowns out the radio. It's a kind of buzzing that varies with engine RPMs and goes away when the car's not running. It isn't the radio because both the original and new one have this problem.

I suspect there may be an ignition filter capacitor or something that went bad. Can anyone point me in the right direction?

My car is an '89 Auto with the funky fuel injection.


Thanks!

Tim

[JoDan]

My auto tech teacher brought this up before in class, I'm not exactly sure what he said about it, but I think he said it had to something with the plug wires or something else in that general area... but don't hold me on that

[bravekozak]

AM noise

Question 2-- my radio in this jeep suffers from terrible ignition
> noise while listening to the AM band. FM is ok. I suspect the plug
> wires may be contributing to or causing this noise. Is there any other
> culprit I should be thinking about if that doesn't provide the full cure ??
>
> Thanks for any helpful comments !!

Good quality cap and rotor with brass fittings, not the lousy
aluminum looking ones. Good resistor style spark plugs, and a
good set of wires. They make RF supression core wires, those
should be what you look for.

I use Bosch cap/rotor/wires and Autolite plugs. Works for me.

Does your 80 have points? Prolly not but just in case, there
is a noise supression cap that goes in there. Looks like a little
metal can with one wire on it.

Bad ground on the engine will cause lots of noise. Check your
engine ground strap to see that it hasn't corroded away.

[equaboi]

what your experiencing is called engine noise its when the voltage spike that occurs when you rev your engine is heard thru the speakers. their are many reasons u might get this but from what i read you both don't have amps or any high current systems so for your problem i would suggest to reground the antenna and the radio. to do this tape a speaker wire around the exposed part of the antenna and ground it to the chasis then ground the radio most radios have a small screw hole in the back for backstraps you can use this location to ground the radio to chasis if that doesn't work repost i'll suscribe and see if i can help you thru it

[Mike McKown]

If it's a rapid popping noise, it's probably the spark plug wires. A pop every time a cylinder fires. If it's more of a buzz, it's likely the alternator. Older cars used to have a capacitor in the charging circuit to stop the buzz.

Cars used to have capacitors on the heater blower, turn signal circuit, brake lights and the voltage regulator points to prevent interference of the radio.

[FB71]

Timmah...

What you are experiencing is called RFI, Radio Frequency Interference. RFI is generated by two primary sources, secondary (high voltage side) ignition and the alternator/AC generator. There is an RFI suppressor capacitor on the black wire (positive side) of the ignition coil. Having played with old tvs and radios, you already know that caps don't last forever. That could be the source, but because of the severity of interference you describe, I'm suspecting seconday ignition more. IIRC, the RFI cap is 2.2 microfarads, if you decide to replace it. Just grab a $0.99 mylar Radio shack unit, or grab one off of a coil pack in a newer car at the boneyard. But again, I still you have other secondary ignition issues. Check the ground strap from the block/head to the strut tower. If it's at all corroded/oxidized, replace it. Also check for voltage drop between the distributo body and the head. The dizzy body, when properly grounded, acts as an RFI drain, and supresses the interference. Check the resistance of the plug and coil wires. It should be around 2k ohms per foot, IIRC. The resistance supresses the RFI. Less than 2k will allow a lot of radiation. Make sure you have the correct spark plugs as well. Plugs are available in resistor and non-resistor. Non-resistor plugs will cause RFI generation.

The other possible source is the alt. If a diode has failed or gotten weak, there will be excessive A/C waveform generation. A/C on the power source to the radio will cause RFI.

The keys to determine whaqt the source of the concern may be is the actual sound over the speakers. If, at idle, you can hear what sounds like a 'ticking' sound at the speed of a single cylinder (sounds about the speed of a valve tick), you have a single plug wire at fault. If sounds like a very low frequency/pitch whirr, it may be a bad coil wire, a bad RFI cap, bad ground strap, etc. If it sounds like a moderate frequency/pitch whirr, and gets to be a very high pitch noise with only a little throttle, I'd suspect the charging system/alt.

Hope that helps.

[FB71]

If it's a rapid popping noise, it's probably the spark plug wires. A pop every time a cylinder fires. If it's more of a buzz, it's likely the alternator. Older cars used to have a capacitor in the charging circuit to stop the buzz.

Cars used to have capacitors on the heater blower, turn signal circuit, brake lights and the voltage regulator points to prevent interference of the radio.

the voice of experience!!!