i have a 1997 Subaru Legacy gt 2.5 sedan. I have a factory am/FM cass. with CD player. When the car is off the radio plays fine but when the car is running it get interference in FM mode. i thought the radio was bad so i changed it with a nother factory radio with the same problem. The radion was fine untill the past week. Any clues on the problem?

 

thanks Joe

I got radio interference once because of an alternator going out. Any other weird electrical anomalies?

Bad Alt, chattering brushes in any electric motor in the car, a bad ground...

 

 

nipper

Is there any way to be sure it is the alt? It is not making noise.

 

thanks Joe

I checked the ground on the radio and it looks good will look at motor grounds.

Is there any way to be sure it is the alt? It is not making noise.

 

thanks Joe

 

remove the belt that drives the alternator.

You can check to see if the alternator is causing the trouble. While the car is not running you can pull the connector from the back side of the alternator, that will disable it. Then start it up and see what happens. After the check turn the car off and replace the connector.

 

Alternator noise usually appears as a whinning sound in the audio and you can hear the RPM changes in the whine as the alternator changes speed. A bad antenna ground can cause this thing also.

You don't need to fool around if you have an alternator going out. It is cheaper just to replace in your driveway than to get 50 miles away from home and it break down on you. Then you have to pay for the tow home, then do your alternator change. They are not hard and I can change them on a Subaru in 5 minutes.

Disconnected the alt. and that is not it. I'm going to check the ground on the antenna next.

Does it change with engine rpm?

Does it change with engine rpm?

 

Nope the sound is the same

The problem you are having doesn't appear to be typical. Usually it is the AM section that picks up noise, not the FM section. This is due to the way the modulation design works. I wonder if something is wrong inside the radio. I suggest you check and see if the noise goes away if you disconnect the antenna. If the noise is still present then it is most likely due to a problem inside the radio. There possibly could be noise feeding into it through the power lead I suppose but that is fairly remote I would say.

Does it change with engine rpm?

 

On Am it does Fm not really some times not at all on fm.

Spark plug wires? On old or damaged ones the current can cause interference if the insulation is bad. Also fits with the problem in that it only occurs when the engine is running.

 

Good luck!

When the blower motor was dieing I had that issue. When the motor stopped running I replaced it and no more noise.

 

Vary the speed on the blower and see it the noise changes.