Audio Systems: Car Audio, chevrolet monte carlo, chevrolet monte carlo ss

Question
I have a 1986 Chevrolet Monte Carlo SS.  When I turn the key forward and run the stereo directly off the battery, the system plays fine.  When I run the car (and the alternator is turning), there is lots of noise in the system almost like there is a bad ground somewhere.  I checked the battery and alternator connections and all seem fine.  A friend of mine told me that there is a main ground for the car's electrical that connects to the firewall that might have come loose.  Is this true?  Do you have any other suggestions about what might cause this?

Thanks,
Jim

Answer
Well, grounding may indeed be a part of the cause, but not usually the whole picture.  A central ground is commonly used, but if all the ground points are solid and secure it should not be a problem for car electronics to ground and any good point.

If your noise is ignition noise, ie, it goes up and down in frequency with engine rpm then you have ignition spark or alternator interference getting into the radio or audio.  This is a common problem on older cars. Newer car designs are carefully engineered by the makers to keep the interference emissions low.  And electronic device manufacturers are careful to have plenty of rf filtering so spark noise is not a problem.

Further, an alternator in trouble (open or shorted diode inside the alternator) will sometimes be a cause.  Or, the filter capacitor on the alternator is bad.

It will likely require a few tests to determine the cause.  I suggest you look at the crutchfield advisory and follow some of their ideas in search of the problem.

http://www.crutchfieldadvisor.com/ISEO-rgbtcspd/learningcenter/car/noise_suppres...

http://www.crutchfieldadvisor.com/ISEO-rgbtcspd/learningcenter/car/noise.html

Further, check to see if you are getting it only in radio am or fm or cd.  Knowing this can help you determine the problem.  For example, if you are getting it on am only and cd/tape and fm are fine, then you have an interference getting into the am tuner section - often caused by ignition spark noise and you may need spark arrestors on the ignition wires.  If you are getting noise on all program sources it may be getting into the audio amplifiers due to poor grounds or interference.  

So, you have some checking to do. In a tough case you may need to take it into a diagnostic repair/install shop.  The techs have great experience in hunting down these kinds of problems.

C