When listening to AM broadcast stations and I am not within 5 miles of the station I get very annoying static. However - and here’s my question - when I press the brake pedal the static goes away and the station comes in clearly. I only have to put very light pressure on the pedal… not enough to actually slow the car’s speed. This does not happen on FM. Is there a switch beneath the pedal that might be arching or causing this static? Any help would be appreciated.
2 things happen when you touch the brake petal. #1 you are no longer on the gas petal and the car and motor are coasting. There are spark plugs and ignition components that give off “static” radio signals and interfere with AM reception. If someone changed your spark plugs and put in the wrong type you’d get more static. When you coast the plugs are firing but the speed of the motor goes down and there is less interference. A fliter in the radio could have gone bad with the same end result.
#2 stepping on the brakes turns on the brake lights. That creates an active electrical circut. I have no clue why that would affect the radio AM reception, but perhaps it does.
The reason you hear noise pulses in AM signals compared to FM signals has to do with the type of modulation the signal carriers are using. AM, is amplitude modulation so changes in the detected amplitude of the signal are heard when noise pulses are added to the original signal. FM uses frequency modulation (changes in the carrier frequency) to modulate the carrier signal, so when the amplitude changes from the noise pulses are added to the original signal they aren’t heard (they get clipped out in the detector circuit), as like they are when they are detected in a AM receiver. Ok, radio class is over with, back to car discussions.
As for the radio trouble, I suspect you may have a faulty antenna ground causing this problem. The easiest way for a radio to pick up noise is to have a bad antenna ground. I think what is happening is due to static electricity building up from your tires as you move down the road. If the noise doesn’t happen while you are stopped then I would say that is very likely what is going on. When you press on the brakes the static is getting discharged through the brakes and the noise goes away. Hopefully you will find a dirty connection to the antenna ground between the antenna and the radio connection and clear the trouble.
hey Cougar - Thanks for your remarks. I do understand the modulation aspect as I have been a ham radio operator since 1960 and have taught many classes on radio theory including modulation principles. Any man-made noise is of an amplitude modulation type. However, the antenna ground is one thing I didn’t think of - but should have. Like the books say… ground, ground, ground.
As for whether the vehicle is moving or not, the static is present when the vehicle is, or is not in motion. Some of it is due to my location from the transmitting tower itself, which I expect to hear, but pressing the brake pedal actually eliminates some of the static even when the car is not moving but the engine is running. Turning off the engine lessens the static. So I’m thinking there’s definitely something electrical sneaking around in the wiring somewhere, which made me wonder if there’s a switch that works when the brake pedal is pressed. If a switch is arching and is in close proximity to the radio’s power wires - well, just a guess… But it fit’s the amplitude modulation aspect.
I’m definitely not an auto mechanic and don’t pretend to be one. So I do appreciate your comments.
Thanks again for responding to my querry.
I handeled all the radio/entertainment system concerns for my Chevrolet Dealer for many years. One of the best pieces of test equipment I ever bought (for $10.00) was a bare bones but known good AM test radio. I used it to eliminate the radio itself as not being the cause of the static but not being able to "block"it anymore.
You’re welcome for the comments but it looks like my first thought is wrong about what is causing the source of the noise.
The brake light circuit does use a simple switch to turn on the lights but I doubt the switch is the cause of the noise. The noise may be riding in the power to the radio and when you apply the brakes the noise level drops down in amplitude due to the load on the power. Does the noise level drop when you turn on the headlights? If it does then I suspect that is what is going on then.
The noise you hear may be coming from the alternator or some other electrical system in the car. Using a portable radio like Oldschool suggested is one way to track the source down. The closer you get to the problem the louder the noise gets. By checking for a bad antenna ground hopefully you will clear the trouble. You could check to see if the noise drops out by disconnecting the antenna plug to the radio. If the noise does drop considerably then the antenna is picking up the noise. If the noise doesn’t drop much then the noise is most likely coming in through the power lead to the radio.
Rochester was able to fix the same problem in Jack Benny’s Maxwell by adjusting the position where the cat whisker made contact with the surface of the crystal. In your case, you might take a portable transistor radio in the car and see if you can duplicate the condition of removing the static when you step on the brake pedal. This would confirm that the problem is generated someplace in the car.
Does the noise (largely) go away when you turn the headlights on? If so, I’d suspect that your antenna isn’t connected or the antenna wire is shorted to ground somewhere. That’d be because the noise is coming from the power line and is only audible because your antenna signal is very weak. Applying the brakes, or turning on the headlights shunts much of the noise on the power lines to vehicle ground.
And yes, there is a switch “under” the brake pedal that turns the brake lights on. I suppose it could somehow be generating noise when it is open, but it doesn’t seem likely.
I once encountered an after market radio with an antenna trimmer capacitor. I don’t expect ever to see another, but if your radio has one and it is misadjusted, that might cause your problem.
If you have access to an oscilloscope, you may be able to see the static pattern superimposed on what should be a straight dc voltage. Probe the power connection to the radio, and with the engine running, see if this static pattern goes away or is diminshed when you press the brake pedal. You might even probe the positive terminal of the battery while the engine is running to see if you observe the same pattern. My guess is that this static is produced somewhere in the alternator. I would also bet that the A.M. radio reception is o.k. when the engine is off.
The shield around the antenna lead is supposed to help block noise from the radio if its ground connection to the chassis is good. However, the shield from the antenna lead can not stop noise coming in through the power supplied by the battery and alternator. If you can briefly disconnect the alternator and you don’t get the static while the car is running, you’ve isolated the source of the interference. There may be a capacitor at the alternator to block interference that is not doing its job. If so, replace the capacitor.
As an interesting side note to electrical interference, I teach a computer architecture and harware course. I was talking about the dc power to the computer and how the ac power must be converted. I was using the 'scope to demonstrate the difference between ac and dc current. When I probed the ac outlet in the classroom, I did get the ac sine wave, but it had superimposed interference on the wave pattern. Later, we began having problems with the computers in the building. My uninterruptable back up power kept going on and off. It turned out that the voltage fluctuation in the ac line was going between 70 and 150 volts and that the power supply to a big mainframe computer was backfeeding harmonics to the ac lines and causing these voltage variatons. The interference we call static can cause a great many problems.
My test radio was not a portable it was a standard GM radio designed for automobiles. 90% have the same modular plug.
I agree that ant ground OR “hot” connection may be faulty. However, last time I chased down one of these the ant was a telescoping whip mounted on the fender (also licensed in 1960). Seems that all the new ones are built into the rear window or the windshield.
The brake pedal switch MIGHT be completing a “sneak” ground path (RF). I doubt that the switch itself is creating any noise, but it should be easy to test: (with the car parked but engine running) just unhook the “hot” wire from the switch.
IF you need to chase down the noise source, recall from the 1960’s what ignition noise sounds like: raspy low-pitched buzz (like a spark gap, duh!) whose pitch varies with engine speed. Also recall from the 1970’s “alternator whine”: a slightly rough tone in the middle registers; pitch also varies with engine speed. Other than those two, modern cars have too many computers that could be putting out some RF noise. SNR is usually high enough to cover the normal computer noise, so you’re back to checking the ant connections again.
I have one more thought. This is far out, but I wonder if something in the cruise control might be causing the problem. When you touch the brake, you disengage the cruise control. Even if you have the cruise control switch off, I suppose something in the circuitry could be causing the static. In the interest of science, why don’t you remove the fuse for the cruise control and see if this makes a difference.
The cat whisker days! Your dating yourself now Triedaq.
Is the static random or does it increase and decrease in with engine rpm’s? If it increases and decreases in frequency with engine rpms then an ignition component would be suspect. Bad Plug wires would be one source, In the olden days a bad condenser in the ignition could also cause a problem, I do not know if condensers are used in your system or not. As stated previously spark plugs could be a cause but would put that pretty far down the list.
If I see a oppurtunity I will take it and I see one. I was reading on increasing L2 cache and the article stated that any thing over 2megs is a waste due to limitations on the Front Side Bus. It seems the only way to beat FSB limitations is to go to 64 bit but this is kind of a nowhere solution due to limited applications written for 64 bit,could you comment?
Yes, it dates me on two counts:
1. Listening to the Jack Benny show when it was on A.M. radio 2. Building crystal radio sets--no tubes, no batteries or other outside power source
LOL. Well I’m most likely right there with you Triedaq (getting towards the 60 mark). I remember using a small tunable coil with a ferrite slug, a germanium diode, and earphone to listen to stations. It was shaped in the form of a rocket ship and the nose cone slid up and down for the tuning. I wish I had that thing today.
Sorry about side discussion here folks, please forgive me. No more reminiscing.
Just a word of thanks to all you fine folks for your suggestions. I now have several possible trouble shooting areas to investigate. Unfortunately, I injured my back so twisting and bending under the dash will have to wait until I am up and around better. But your comments point me in the direction(s) to search and destroy.
As for you dating yourself, Triedag, if you can honestly say you remember CK-722s then you’re old.
Thanks again guys.
The CK-722 transistors were made by Raytheon. I put together a 1 trnasistor radio from a kit that I bought from Allied Radio Corporation–later incorporated into Radio Shack–that also featured a printed circuit board. I don’t remember whether or not this trnasistor was the CK-722. This was about 1956.
I wonder what the cost of that transistor back then would be in today’s money. You could probably purchase a fancy calculator now with the inflated money.