Weird radio issue (brain teaser)

[johnb007]

I have a 4 cylinder injected manual 1993 Magna wagon with a very weird radio issue. AM and FM both work fine when the engine is NOT running with the ignition switch in either the "Acc" and "On" positions. However as soon as the engine starts, the FM stops working but the AM still works fine. I assume it is something with the alternator or ignition system. What else could it be? Any ideas?
Thanks
John

[mcs_xi]

I have a 4 cylinder injected manual 1993 Magna wagon with a very weird radio issue. AM and FM both work fine when the engine is NOT running with the ignition switch in either the "Acc" and "On" positions. However as soon as the engine starts, the FM stops working but the AM still works fine. I assume it is something with the alternator or ignition system. What else could it be? Any ideas?
Thanks
John

the radio internals are stuffed. They age and die.

[johnb007]

Update. I took alternator belt off, started the engine and FM is then working fine. So it is the alternator. No Alt warning light is on the dash and it is charging the battery no trouble. So it is some type of electrical noise suppression component attached to the alternator that has failed. Somehow it is cutting out the FM signal to the radio. Any ideas what that could be? I do not want to replace the alternator as it is working fine except that it cuts out the FM radio signal.

[magna buff]

on a 4 cyl alternator shell there might be that little black box held on with a screw

[johnb007]

the radio internals are stuffed. They age and die.

Radio works fine with engine off (AM and FM). It turns out it is some weird thing associated with the alternator when it is running. Weird. Or I guess it could be some radio component that stops the alternator charging circuit from interfering with the FM circuit.

[magna buff]

little black box held on with one screw

[johnb007]

Before I figured out it was the alternator I did change the voltage regulator on the ignition system (284K00707) which of course made no difference. So it is either in the alternator circuit or in the radio circuit which shields the FM signal from the alternator interference. With the alternator not running the FM is strong and clear, but with the alternator running it is zero, not even much static.

[johnb007]

I can see the regulator on the face of the alternator. Is that what you mean? I will go check the voltage that the alternator is putting out.

[magna buff]

nope ..rectangle box top left on alt body ..may be a suppressor

[johnb007]

Voltage at battery terminals with engine off 12.7V and 14.4V with engine on. Maybe it is some circuit in the radio that shields the FM signal from electrical interference or maybe the FM circuit has become ultra sensitive to voltage variations so when it is 12.7V it is fine but when the voltage gets up to 14.4V the FM circuit stops working? I will ask a friend who is into radios. He might know.