1988 XJS - New sound system, strange hum
 

Hello, I just helped my son put in a new sound system in his 1988 XJS and noticed that when the electric fan is on the back speakers hum really loud, it stops when the fan stops. Only the two rear car speakers, not the front ones or subs.
We replaced the stereo, all four speakers and ran all new speaker wire. The speakers connect directly to an amp in the trunk, RCA cables then connect to the stereo from the amp. There are two amps, one for the speakers and one for the subs. Both amps are in the trunk with positive and negative right off the battery. The wire for the power antenna is being used for the remote to tell the amps when to turn off and on. So far it all seems to be working well (using the ground on the drivers side rear for one of the subs caused problems with the drivers side rear brake and signal light so we ground straight from the batter rather).
Any thoughts or ideas? I'm guessing it has to do with using the power antenna lead for subs remote but cant figure out why just two/six speakers are acting up.
Thanks - Mark

hmmm idk if this could be related, but how is your fan circuit diode?

Thank you sidescrollin, at least that gives me a start :-). I will take a look.

Not familiar with that circuit, but obviously something is shared since the radio and fan can both come on with the key on accessory. All I can really think of is the diode on the left fender. If its acting up, maybe its allowing some feedback

This only happens when the key if off and out of the ignition. When the car is running it doesn't happen. I'll try with the key in the run position, motor off, and see if it does it then as well.

So the radio isn't on or playing music, its off and the car is off but its making noise? I am pretty sure the fan is allowed to run with the car off until the temp goes down, so it always has access to power.

Try a better or add additional ground on the amps. If not try a electronic noise suppressor.

It sounds like a bad ground Try switching your speaker input. If the noise follows the speakers you will know where the problem is. If it doesn't you know where the problem is .. After you shut the car off, try pulling the power lead to the amp. Bt the first thing I would do is insure the fan is properly grounded!