Help w/Crackling AM Radio

[float_6969]

OK, so I'm fixing a freinds car and here are the symptoms;He listens to a lot of AM radio and he's having problems. With the motor off, everything is fine and the station is clear. With the motor on and idling, it seem so to be fine as well. But as the engine speed increases, a crackling noise will become present and the severity and frequency increases with engine speed.

My first thought was that he had cheap/old plug wires. They had about 100K miles on them, so I replaced the plugs and plug wires as they were both in need of it regardless.

The problem hasn't changed whatsoever. He says that the problem isn't consistant either. Somedays it will be barely noticable, and sometimes, it's so bad he can't listen to the AM at all. It is only occuring with the AM, the FM is fine. I'm going to try double checking the grounds on everything, but i wanted some imput from others.

Thanks for the help,Ryan

[bryanmenard]

AM Radio is difficult to troubleshoot. Even without the car, it's going to be vulnerable to interference from a lot of areas...everything from sunspots to clouds, lightning, and a vehicle's engine and electronics. And, on the same token, AM has superior range to FM, but with lower sound quality.

I don't know what kind of car this friend has, but it's been my experience that AM works best in vehicles with an external antenna and not a window antenna.

Keep in mind, also that the Nissan Diversity System (power antenna and window antenna) works ONLY on FM...in AM, it pulls only from the power antenna...hence the name "FM DIVERSITY SYSTEM."

Did you consider that the motion of the vehicle could be causing the problem? A vehicle is an inherently poor location to receive any analog signal...moving constantly. AM is worse than FM on this.

If the car has just a window antenna, he's probably out of luck. If the guy is hardcore about shielding his AM signal, he should go with an aftermarket, external antenna, run directly to the radio with EMI shielded cable.

[zippitta]

OK is this a new headunit or is this one that had been in there for a while? If it is an old one tell him to go on down and buy another. Like that first guy said it is so hard to troubleshoot AM; it is old and just obsolete. But if you replaced all the wiring and it is still doing the same thing then I would say that the problem lies within the headunit itself. Shoot I cant even get the local AM stations and I have a $500 Alpine heady....lol

[bryanmenard]

He does bring up a point I failed to touch on. Electronic Components inside radios...transistors, resistors, capacitors, etc...they all go bad over time. It's a fact. AM radio reception is highly dependent on those components. I wouldn't be shocked if there was something at the board level that was on the fritz.

[xekushnr]

is it just one station or is it all? i remember one time when i was driving to a friends apartment and i was listening to a NBA finals game. my reception would change with my RPM, a/c, whenever i put my turn signal on...AM radio is very susceptible to interference.

[float_6969]

Well after much troubleshooting, I'm pretty sure it's the head unit. This is on a 1994 Chevy Lumina Van. It's the stock head unit. It doesn't have an external antenna. What I believe has happened though is that the rectifier in his alternator has gone bad/gotten weak. I belive that the 30VAC coming out of the alternator (should be 1.5VAC max) has wreaked havoc on the board. In particular the capicitors and diodes. I basically told him that he'd have to get the alterntor repaired/replaced and that the HU is shot and will need replaced. He laughed and said, "No thanks, I'll listen to FM!" Thanks for all the help and imput though!

[EW]

OK is this a new headunit or is this one that had been in there for a while? If it is an old one tell him to go on down and buy another. Like that first guy said it is so hard to troubleshoot AM; it is old and just obsolete. But if you replaced all the wiring and it is still doing the same thing then I would say that the problem lies within the headunit itself. Shoot I cant even get the local AM stations and I have a $500 Alpine heady....lol

Aftermarket radio manufacturers do not spend money on the AM tuner section. Usually, a factory deck has better AM reception.