Car Stereos: am radio noise, Ford, quality radios, grand marquis

Question
83 Merc Grand Marquis, 84 Ford PU, same AM noise on  weaker stations. These are good quality radios as far as I can tell. Tried changing ign coil cap., changed sprkplg wires. disconnected alternator and ran engine, replaced radios too. Static always there when engine runs, changing in proportion to rpm. Old Hornet radio (not high quality) with old style ign picked up static only when I put it next to open-hooded Merc. What do you think?

Answer
Hi John,

Static/whine in proportion to engine RPM is usually a result of ground looping.  Unfortunately, fixing it can be very time consuming.

If you're really serious about getting rid of the noise, try disconnecting the ground from the factory harness, and regrounding it to the SHORTEST distance part of the frame of the car you can find that is not welded to the frame, and not painted.

Good luck,
Justin