Just acquired 993 C2S. Absolutely love it.

I've noticed when the car is running, what sounds like interference is coming out of the speakers (it has the Porsche Classic Radio installed). It's a steady "tone" sound, like a high-pitched dial tone. If I activate the window motors, or add more "load" to the electrical system, the tone drops to a lower note. Is it something with the wiring? A bad ground?

When the car is off and in accessory mode, no tone.

Adjusting the volume on the radio doesn't make any difference, and haven't looked into disconnecting it to see if it stops, but that's my next action. Just wondering if anyone has come across this.

 

Hi CMYKE,
If the sound is coming from the speakers it may be electrical noise from the ignition or another source in the car. To eliminate it purchase some chokes from eBay or Amazon and clip one to each of the two hot (+) wires that go to he radio and to any hot (+) wires that go to a power amp if present.


Another source is poor shielding or a grounding issue related to one of the signal wires to a power amp if present or poor grounding of the radio head. Test this by attaching a wire to the body (ground) and touching the other end to the radio head chassis or the outer barrel of one of the power amp outputs on the back of the radio even if no power amp is present. If the sound disappears when the wire is touched to something it is a poor ground or sheilding where a ground wire (-) needs to be checked or a ground wire can be added to the point that made the sound subside.

Andy

 

I would suggest that you have one of the following:-
a. Aerial problem.
b. !2v car DC power supply induced noise.

1. Check the aerial outer screening cable is in tact. (use meter in ohm to measure).
2. Check the inner car aerial for continuity and is not suffering from a partial short circuit
between aerial and screen. (use meter in ohm to measure).

3. If both are ok Check your aerial amplifier is connected and working.
measure with a volt meter +12v. Return is -12v is car chassis.
With FM the signal uses the capture effect ie: the loudest radiated noise wins.
————
4. Install an in line 12volt power filter, available from eBay or Amazon. if the interference is
suspected as coming from this source.
————
5. The car radio does not seem to be the problem.
———-
Hope my suggestions are of use.

From Joe.

 

Thanks, y'all! Will investigate today. I suspect it's the NOKIA amp based on the reading, but could possibly be the HID headlight ballast as well? Any issue pulling radio fuses while the car is running to see if it stops?

 

No problem removing fuses, load is approx 4.5 amps to 5 amps when operational.
Would suggest stop engine, pull fuse, start engine, observe. Don't think this is the problem.
--------
No comment on remote Nokia amplifier, Other than replace OR monitor the input from the radio head driver and the output of the amp with an oscilloscope but requires professional test equipment. (last resort)

regards,
Joe.

 

Hi CMYKE,
Now that you mentioned you have a Nokia amp and the car has an aftermarket radio it leads me to believe its the sheilding ground on the wires between the head and the power amp. Check the wiring to the amp from the back of the radio. Including any harness adapter. You can also just swap out the wires going to the power amp with new ones.
Here is information on the harness connections in the 993:
https://993servicerepair.blogspot.co...ire-color.html
I also saw this replacment for the connection to the power amp, You will need to look up if the radio head has the same socket as this cord.
https://www.suncoastparts.com/produc...3494CABLE.html
Andy

 

FYI, and I hope this doesn't apply to you (or to me for that matter), but I had the exact same issue in my 1988 Toyota 4x4 P/U (186k miles, original owner, yadayadayada...), and replacing the alternator in the truck solved this problem. The thing was on it's last legs. I tried chokes (I think that's what it was - those magnetic thingees you put on electrical lines to eliminate noise/interference), but nothing worked. Again, FYI....

 

Start here and report back

https://a.co/d/4qYsPlv

 

Hi OS,
In your case, it sounds like one or more of the diodes failed in the voltage regulator that converts the alternator's alternating current (AC) into direct current (DC). My understanding is that when one or more of the diodes fail the alternator pulses DC with each rotation into the electrical system creating quite a bit of noise that can't be choked off. It can take the form of a buzzing sound from the audio system that changes frequency with engine speed changes.
Andy

 

hi Andy, well said and well explained, thank you. All I know is replacing the alternator in my truck solved the buzz noise issue. Again I don’t know if this directly relates to 993s, but maybe a helpful datapoint. And as I recall, the noise did get worse with higher RPMs. Maybe something OP can check.

 

I (maybe mistakenly) assumed from the first post that "It's a steady tone sound", meant that frequency doesn't change with RPM, but OP should confirm this, because if it doesn't, it'll rule out anything engine-speed-related including: ignition, fuel injectors, alternator, etc.

 

correct, frequency does NOT change with RPM. If I activate window motors, it does though.

the next 10 days are pure rain, so I'll have some time to get in there and hopefully isolate it! Will report back

 

Good. That takes a lot of things off the table.