Why does newly installed (vintage) radio hum???

[RSTarga]

I don't think 89's had CDI they had Motronic

[Rob G]

That may be why i cant find it. Thanks for the info.

Originally Posted by RSTarga

I don't think 89's had CDI they had Motronic

[TWJ]

Ingo,

I can turn the ignition on and not start the engine and there is no hum (high pitched whine!). It only appears when the engine is running. I do not have an amplifier, it is only the head unit and it is a 1972(?) Blaupunkt. The ground is connected to several other wires that then go to ground. Could this cause the "ground loop" or is there another problem?

Thanks much!

Originally Posted by ischmitz

CDI stands for Capacitive Discharge Ignition. It is the silver box with the cooling fins on the electric panel in the engine bay.



It uses a DC/DC converter that runs with 3.5kHz. To check if that is the culprit simply turn the ignition to ON but do not start the engine. If the hum appears it is likely the CDI unit. If the hum only appears when the engine is running it still could be.

The fact that the hum is NOT changing pitch with engine RPM makes me think you could have a ground loop. Are you using an amplifier or only the head unit driving the speakers?

Ingo

[ischmitz]

To answer TWJ: I would run a thick short wire with an eye straight to the chasis. Use a self-tapping screw into the firewall or find a machine screw that can take the ground cable. When you wire it into the brown wiring that runs behind the instruments you could end up with poor GND and that can contribute to the hum. These brown wires use spade connectors that corrode over time. And they power the instrument lights and could have several 100mV above GND.

To answer Rob: Your car has Motronic so there isn't any CDI unit. Still, make sure your head unit has a perfect GND. Same advise as above: Use a thick wire with an eye and find a chasis screw. That is the best way to get GND

Ingo

[Rob G]

Sounds good. I will give it a shot. Thank you foe everyone's help and Good LUCK to you
TWJ in locating your troublesome hum.

Rob G

[TWJ]

Thanks Grady, Ingo and others. I'm sure the answer is here. As always my knowledge of my car has expanded greatly because of you all!

I'll post the results.

Good luck to you as well Rob G, maybe post your results for others to learn from.

[renato]

TWJ,
Have you solved the problem? I have a similar problem in an 86 911

[HawgRyder]

You can also pick up noise...both ignition and alternator through the antenna.
To shield against this...make sure the antenna is properly grounded at the base...and also check for any resistance (no power...engine off) between the chassis or ground of the battery and the fender (where the antenna is).
Sometimes rust gets into the joint and creates a non-ground issue.
Hope this helps.
Bob

[dshepp806]

grounds,..grounds,..grounds..........only use one, never add them...if this doesn't help,..then I'd suspect what Ingo has suggested about alt integrity.


Doyle

[renato]

I reread the thread and I have exactly the same problem Rob G had. Just like for him, it started one day* and now it's always there the moment I turn on the engine. There is no hum on the radio when the engine is off. The pitch changes as my RPM change.
I looked at the ground on the radio itself and the one in the front on the passenger side. It doesn't appear to be along the hood shock going to the hood but on the inside of the fender - at least if I found the right one. I looked also at the alternator but the cables I saw looked ok.
The ground on the radio could be the culprit. Instead of a shielded cable, it has a piece of metal in an L shape with multiple holes in it. On one side it is attached to the radio's ground cable entry via a shrew through one of the holes and on the other side it is shrewd to what appears to be the ground on the chassis under the dashboard. The metal is not shielded and lots of cable run around it. I wonder if that causes it.
Didn't get to do it today but I'll get a shielded cable with an eye opening on both side and replace the metal.

* If I recall correctly that "one day" might have been the day I took out the radio to check the connections since I had a problem with the speakers and not proper channel mapping. It could be that I did change something with the ground that now causes this sound.

[renato]

Just as an fyi: I found the cause for my problem: It was the ground cable attached to the wiring harness adapter plugged into the radio. I did check all cables and couldn't find the problem. I decided to switch the receiver for a new one that had 4 channels + sub output to the amplifier since the old one had only 2 channels output and ... the noise was gone.
So as a tip for others, check the cables for the adapter first.