Grounds and AC ripple

["Warren R Anderson" <wranderson@xxxx>]

>But now I have a noise that changes pitch with the increase in engine
speed.
>This problem started when I installed the 89 Fifth Avenue alternator.
>This noise cannot be turned down with the volume control only by turning
the
>radio completely off.


The power side of the circuit can be dealt with as noted with feed through
noise filters and strap on capacitors. The ground side I would suggest
should also be checked carefully. This would be to check the battery
negative path from the batt (-) to the radio and the antenna mounting and
fender and hood.

Cars with computer controls require a clean voltage supply. Checking is done
with an oscilloscope or a volt meter set on a low millivolt AC scale. AC
ripple will always show up as a voltage (alternator is a rectified three
phase AC electrical machine) measured as maybe 20 MV and less is better. I
do not have a good alternator older Chrysler to check right now but I would
think 100MV AC + would be a good indication that an output diode is bad in
the alternator. A bad diode will also keep the output down.

Side note, most rebuilders will reuse old diodes if they pass testing and I
do not believe this practice works real well. I used to build alternators in
the shop because of this practice and way too many come backs. Late '80's up
applications today we install rebuilt units with the highest number of new
parts and this does work fairly well.

Warren Anderson
Sedona,AZ