View Full Version : OT: Possible causes of AM car radio static?

06-28-2004, 09:47 AM
Car AM radio sounds great when engine is not running. Turn on engine and get popping static that corresponds to engind rpm's. Happens when sitting still too so it's not static buildup from tires.

This car has a generator. (Brushes need replacing?)

What are some possible causes?


Ragarsed Raglan
06-28-2004, 09:52 AM
Put a capacitor across the generator output line to earth (watch the polarity!). I think your description of a 'popping' sound is more likely to be HT RFI, this will need suppression HT leads or suppressed plug caps (or both).

If the problem persists then Haggis forced into the speaker cones works well!


Alistair Hosie
06-28-2004, 09:57 AM
Dont listen to a word RR says he knows nothing about haggis whatsoever http://bbs.homeshopmachinist.net//biggrin.gif Alistair

[This message has been edited by Alistair Hosie (edited 06-28-2004).]

06-28-2004, 10:08 AM

Sorry. I'm drawing a blank this morning. HT = ??? (High Tension as in plug wires?)

If yes, then are you referring to resistor plugs and plug wires?

I'd try the haggis but the smell might be too much after a couple of weeks with high humidity and high temperatures. http://bbs.homeshopmachinist.net//smile.gif


Another fact that may enter into this is everything was crisp and clear as a bell then one day this started.

[This message has been edited by pgmrdan (edited 06-28-2004).]

John Stevenson
06-28-2004, 10:19 AM
Have you tried new wiper blades ?

Ragarsed Raglan
06-28-2004, 10:26 AM
Yes, HT = High Tension, sorry I did not explain. RFI = Radio Frequency Interference, the other oft quoted expression in this case can be EMI which = Electro Magnetic Interference.

Has the weather changed? Atmospheric conditions can drastically affect/effect things like RFI. High pressure, dry air will cause more problems.

Haggis, on the other hand, is not affected by anything. ......Although I have to admit I do like Haggis, especially if it has a smidgen of pure malt poured over it!


06-28-2004, 10:53 AM
I kid you not. Try spraying some WD40 on the spark plug wires.

[This message has been edited by Evan (edited 06-28-2004).]

Paul Alciatore
06-28-2004, 11:57 AM
Yes, it is RFI. Not necessairly HT. Possible cures include:

Replaceing spark plug wires

Adding capacitors from the 12 volt power leads to the radio to ground. Use both a 0.1uF ceramic and 50uF or 100uF electrolytic in parallel. Do watch out for the polarity on the electrolytic. Do this as close to the radio as possible.

Check that all connections under the hood are good ELECTRICAL connections. I watched an auto mechanic connect a groud wire once and it made me cringe. Wire was filthy and corroded. The metal he attached it to was painted and that was covered with grease and grime, and he didn't even tighten the screw well. He didn't have a clue about electricity. (I did redo it.)

Check the antenna lead and make sure it's tight at both ends. If necessary, replace the antenna.

Paul A.

06-28-2004, 12:21 PM
Something like this starting suddenly is usually a bad connection that has developed. Make sure the antenna mounting on the body is good. There should be a good ground connection there. Sometimes, the short ground lead (it's a 10 or 12 gauge wire) that goes from battery negative to the body metal is at fault. This is a good junction to clean up anyway, even if it doesn't affect the radio. Other than that, if the noise appeared suddenly, there could be a noise suppressor that has lost a connection due to vibration. Sounds to me like a connection has gone bad, possibly from having gotten wet, then dried. Any junction that is bolted or screwed to ground via body metal is suspect.

06-28-2004, 12:44 PM
I had a problem recently with my PT Cruiser. At first I put it down to the air conditioning compressor kicking in and sucking power. After a bit of testing it became clear that was not the problem. I took it to the dealer (yeah, I know, I could have saved money and done it myself BUT, have you looked under the hood on one of these?????).

The dealer quickly found the wires had been chewed by a varmit of some sort. He said it happens all the time. A similar situation could result in RFI on the radio.

Alan VanGorder
06-28-2004, 03:13 PM
Look for a small capacitor mounted on or near your voltage regulator or your ignition coil. If the capacitor is used with your ignition coil, it should be attached to the positive terminal of the coil.

Open circuit failure of a capacitor used at either location will cause AM static which increases in pitch as you increase engine RPM's. You can disconnect one end and do a crude functional check with an analog meter set on an Ohm scale.

Evan, I'm interested in the WD-40 remedy.
Any idea as to why it is effective?

06-28-2004, 03:59 PM
This car has new spark plugs, plug wires, points, condenser (that's 'old school' for capacitor http://bbs.homeshopmachinist.net//smile.gif ), rotor, and distributor cap. All high quality stuff from a place specializing in aircooled VW's. That's why I'm suspecting some older part first.

Alan, when you mentioned a capacitor near the ignition coil are you talking about the one on the distributor?

06-28-2004, 04:17 PM
Are you sure they are suppression or carbon core wires, as opposed to solid or copper core? Are the spark plugs the resistor type? If the answer to both questions is yes, carefully double check your connections on both ends of the wires to be sure you have good contact. Use silicone grease (dielectric tune-up grease) on both ends of the boots to be assured that there is no leakage from the wires. Also route them with looms so that they are not touching each other, and are spaced about ¼” from any metal. They will last much longer if you take the time to do this.
I am guessing that this problem started or worsened after the tune-up?

06-28-2004, 04:34 PM

I'm about 99% sure I have resistor plugs and plug wires. I'll double check.

The problem arose about 4 or 5 months after the tune up with very, very few miles on the components (still new, IMHO).

I've checked connections and they're all good. Usually, if plug wires are arcing I can see and hear them in the dark but that's not happening here.

I'll try moving the plug wires around (with a stick http://bbs.homeshopmachinist.net//smile.gif ) while one of my kids is listening to the radio and see if the popping goes away. Unfortunately the looms are missing.

After that I'll try putting the old condenser on to see if that helps.

No one mentioned that the generator brushes could be causing it so I'll ignore that.

Can a voltage regulator do this when it's beginning to go bad?

06-28-2004, 04:35 PM
WD40 is Water Displacer. Water can get trapped under the boots and cause corrosion and high voltage leakage. It says right on the can "To restore wet or flooded equipment such as: Engines, Spark plugs...."

Dan, a problem with the generator will create a whine at a much higher frequency than the ignition. It will of course change with engine speed. One question: If this is an old vehicle without a high energy ignition system you can safely pull a plug wire off, one at a time and short it to ground (with a spare plug). Does doing so change the noise?

[This message has been edited by Evan (edited 06-28-2004).]

Alistair Hosie
06-28-2004, 04:50 PM
why not try wd 40 on the wiper blades http://bbs.homeshopmachinist.net//biggrin.gif as they attract moisture.Alistair

06-28-2004, 04:56 PM
Evan; Yep, I can do that. It's a '73 so it shouldn't hurt anything.

I was also wondering about the ignition coil. I'm sure it's the original. Can they arc between windings or arc between a winding and inside the case as they get old?

[This message has been edited by pgmrdan (edited 06-28-2004).]

06-28-2004, 05:03 PM
The only way I have ever seen a coil fail is stone dead. Come to think of it I do recall one that would fail when it got warm but worked when cold. Also check your plug gaps. On a lot of new vehicles the plug gaps are pretty wide because of the high energy systems. If you changed plugs the new ones may have been gapped around .060". Older vehicles usually require around .035 or so. Increasing the gap drives up the voltage and will increase interference and leakage.

J Tiers
06-28-2004, 05:24 PM
Check that the engine ground strap is good, and battery connections are good.

Check radio ground, antenna wire and antenna wire connector and its ground. The connectors tend to get a corrosion coating on them and make bad contact.

If the antenna cable center wire isn't connected well to the radio, signal strength will be down and static will be more audible. Of course between stations, that won't matter as much.

Also see if the hood hinges are rusty and not maybe making good contact. Ham radio folks used to put a strap to ground the hood positively to the chassis.

Alan VanGorder
06-28-2004, 05:37 PM

I was referring to capacitor(s) external to the distrubutor as used from the late 70's to early 90's, on many Ford vehicles.

The primary purpose of the "condenser" in your application is to limit pitting of the points, thereby maintaining the correct gap over a reasonable service interval. I don't know whether the condenser limits radio interference when it is effectively on the negative coil terminal, as it is in your application.

Most of the replacement components you listed could contribute to AM noise. Did the noise start after any component replacement?

My memory of the two VW's I once owned has faded somewhat. Is your voltage regulator mounted on the generator? Any capacator there? Also, can you run the engine without the generator drive belt to eliminate your suspicions about bad brushes?

06-28-2004, 06:06 PM
Good idea about eliminating the generator without pulling the brushes Alan!

The voltage regulator is under the back seat. I don't see an external capacitor.

The noise did start after component replacement but it was several months after.

Evan, the plugs are gapped to spec. I don't remember off the top of my head what I set it at but I know it's not as wide as .060". (I looked it up in one of my references. I set them between .025" and .028".)

[This message has been edited by pgmrdan (edited 06-28-2004).]

06-28-2004, 06:19 PM
I gap the plugs on my rail buggy to .026. If you have upgraded to electronic ignition, go wider. A wire can die quickly if it is bouncing into the exhaust, and slowly if contacting a ground. If you can hear a wire, it is badly shorted! Other then that, check the connections and grounds as has been suggested.

06-28-2004, 06:52 PM
Go to Radio shack and get a noise filter for the radio it is a cheap fix. The filter goes in line to your power supply for the radio.

06-28-2004, 07:01 PM
As J Tiers already said I'd check the hinges on the hood especially if the vehicle has sat for a while. Rust in the hinges will stop the earth and result in exactly the problem you are having had the unpleasant experience myself. Easy to check just put an earth lead from the hood to a good ground also antenna earth. I have a tractor playing up on that one at the moment.

06-28-2004, 07:32 PM
One of two things,a cracked coil case,or a broken hood to body ground strap,both will do it.

J. Randall
06-28-2004, 10:40 PM
Dan, you mentioned not having the looms for the plug wires, check to see that none of the plug wires or coil wire is too close to any of the wiring harness that goes back into the cab. Noise can also carry in on a speedo cable or throttle cable if routed to close to high tension wires. James