Radio cuts out at idle - alternator problem?

[perthling]

So a while back I had a problem with my '74 spider dying at idle. Long story short I eventually traced the problem to a dicky ignition barrel.

It's been 6 months or so since I replaced the ignition barrel, but now I have another issue. When idling, after 30-45 seconds, the radio will start to crackle and then cuts out. I'm guessing it's getting insufficient voltage at idle.

So I'm looking for potential fixes - replace alternator? Install relays for radio and other items routed through the ignition? Brown wire fix?

Any and all thoughts welcomed!

[perthling]

Wow! From the lack of responses I must be the first ever Spider owner to find this issue!

Always exciting being the trailblazer...

[spider2081]

I am not familiar with 74 wiring and yes I would say you do have an unusual problem.
Most radios have been owner installed so that wiring would not be in a Fiat diagram.
Most radios draw very little current and are not real voltage sensitive. It could be low voltage but I would suspect something in the area of less than 10.5 volts causing the problem.
Is your alternator warning light coming on at idle? Are your headlights real dim at idle?
Can you measure the voltage at the fuse panel??
I work on small aircraft and have seen mechanical voltage regulators internal contacts make noise in radios.
Also ignition noise is most notable at idle because the repetition rate is the slowest.
A bad ground on your antenna (antenna not tight to body) could vibrate at idle and make noise.

JUst some thoughts

[perthling]

Headlights aren't particularly dim, alternator light isn't going on. I haven't noticed any difference whether i have the headlights on or not, in regard to it being more or less prevalent.

I'm contemplating changing the radio anyway so that may determine if it's a radio issue or a wiring/earthing/relay issue.

[majicwrench]

SUper easy to check voltage at battery and at radio at idle, then yo are not guessing.
I do hear that phrase "the radio cut out" from customers w alternator issues, but usually by the time radio cuts out voltage is getting pretty dang low. You should be able to see lights dimming etc. Does radio instantly improve when you pick RPMS up?? If so, is your idle way low?? Have an aftermarket alternator?? Some sort of amp for radio??

[perthling]

I'll have another check of the lights/alternator light etc.

Radio picks up straight away when revved.

Idle is standard (around 850-900 rpm).

Alternator is standard 124 item.

No amp or other heavy drain item in the radio circuit.

[majicwrench]

You need some voltage checks at idle.

[racydave]

I would think if it was the alternator, you would be having other problems. You may measure battery voltage at the radio. I like to load a circuit to check, so I use a twin filament bulb (1157) and power both fillaments then backprobe the voltage. This also applies to the ground side. You probably just have a bad connection somewhere.

[perthling]

So an update; it appears I may have an issue with my main beam headlight circuit as the issue is certainly apparent when the headlights are on - but I suspect it's not just a drain issue as I also have an intermittent issue which is that I can't get the starter motor to engage if the headlights are switched on. When I switch off the headlight switch the car then starts up as usual.

Additionally my dash illumination is no longer working so I need to start poring over some wiring diagrams.

[spider2081]

This really sounds like a poor connection causing a voltage drop.
It can be a bad battery positive connection or a bad ground. I would suspect it to be in the cars primary circuits. Battery terminals, starter connections, alternator connection, ignition switch connector something like that on the positive side. On the negative side check the primary chassis ground under the car connecting the engine bell housing to the bottom of the driver side floor. Also the batteries negative terminal, cable and grounding point.

[perthling]

I'll start checking. Suspect it may be related to the ignition switch again as that is the only main change made to the electrical circuit recently. I have another ignition barrel so will swap it out and see if the problem remains.