Weird radio issue (brain teaser)

[johnb007]

Yes I am surprised AM reception is unaffected. And the FM is not just noisy, there is zero reception when the alternator is spinning. When the alternator is disconnected the FM is totally fine even when the engine is running. Very strange. Anyway I just got a fancy noise suspesssion device from Jaycar to put in series with the +ve power supply to the radio. How do I get the radio out of the dash to install this device?

[johnb007]

Quickest thing to do is to swap out the alternator and confirm its causing (or not) the issue.

I took the drive belt off the alternator so it was not turning, started the engine, and FM worked perfectly, so it is absolutely the alternator causing the problem. Funny thing is the alternatir is charging the battery OK with voltage 14.2V. Everyting else electrical is fine. Just no FM with the alternator running!

[johnb007]

I found a video on Youtube regarding removal of the radio.

[johnb007]

Well I think I give up. Got the radio out. It had 2 +ve inputs. A red and a red with white stripe. So I put the fancy noise reducing do-dad in series with the red input. No change. Then I put it is series with the red with white stripe input. No change. I did not want to put it in series with both red and red with white stripe simultaneously as I might damage something. What is the red with white stripe wire?

[vlad]

Hmm, red and red with white stripe sounds like speaker wires.

[ac1176]

Agreed. Pretty sure red/red+white is the front-right speaker output pair. No value in putting the filter on these two wires.

There are two power feeds to the radio. A permanent 12V supply from the battery on red/green (keeps the radio from knowing that it hasn't been stolen) and a switched supply from the ignition ACC position to turn the radio on when the keys are twisted in the ignition barrel (blue/red). I'm not sure if you'll need a filter on the ACC power feed, the permanent 12V feed or both to be a valid test of your fix.

[ac1176]

I took the drive belt off the alternator so it was not turning, started the engine, and FM worked perfectly, so it is absolutely the alternator causing the problem.

Also, don't discount that the radio could be at fault rather than the alternator. All that's really been proven is that FM reception is sensitive to the alternator running. Could still be something else - antenna connection loose/intermittent, bad ground on the radio chassis. Car electrical is a notoriously hostile environment for electronic components. Could be that the radio has become more sensitive to fluctuations in the battery supply when the alternator pushes the charging voltage up beyond the nominal 12V, where a new radio would simply shrug its shoulders and soldier on regardless.

[johnb007]

Great thanks. I just wrongly assumed red was +ve and black -ve. I will retest with the correct wires. Oh and your logic is correct. Maybe a new radio is on the cards which my friends say will double the value of the car! But I like the car and I have had it since new and it really has been reliable. Only very minor stuff has gone wring with it. I think it is great.

[johnb007]

Yipeeee. Putting the anti noise do-dad in series with the correct wire (red/blue) did the trick. Thank you very much for your sage advise. I really appreciate it. I now have FM with the alternator running.