I have serious interference through my radio that is related to engine speed (varies with rpm, not present when engine is off). It is very prevalent in AM band, and actually prevents the reception of many stations unless you are under the transmitter!
I have checked the stereo ground, installed a new antenna, and there is a resistor on the coil. the new Taylor wires are SUPPOSED to be resistor wires, and now I am stumped. Any ideas? Can those resistors go bad? Any way to check them to see if they are good?

Thanks-

 

Ok I am not a Radio Technician, but I do know a little about electricity, so here is my theory.
The noise could be an AC signal riding on the DC voltage to the radio. It is usually "removed" by a capacitor or filter that is on the voltage regulator and/or coil and/or alternator. Could be all three could be less, depends on the vehicle.
The capacitor/filter allows AC to pass thru it to ground, but blocks DC voltage. So you could replace the capacitor/filters on speculation, as yes they do go bad, and see if that helps. At least they aren't very expensive.
That said, it also could be a bad alternator, as I understand it a bad diode will "leak" AC voltage which can cause radio interference.

Sparky

 

Is it a regular popping noise or a whining noise on RPM travel?

Dennis
78 F-150 429CJ Silver Ex.Pkge
641/2 Mustang,Pre-World's Fair car #8092
64 Fairlane S/C waiting for a 390
68-Mustang.Sunlit Gold 80,000 miles

 

in my radio installation kit it says there are a few things that can cause this.
1)some older vehicles require the use of a dielectric compound on the distributor rotor to prevent interference.
2)gotta have suppression spark plug/coil wires.
3)gotta have the resistors installed on the spark plugs.
4)the radio power wire has to be seperated from the the speaker wires, ie:not touching them, even the insulation
5)most vehicles have a radio interference protector in the voltage regulator, it needs to work.

if the sound gets worse when you turn on the headlights and all accessories than it is related to the alternator/charging circuit.
if not, than it is the ignition circuit.
p.s: the resistor in the primary circuit for the coil would not cause it.

 

Thanks for the responses!
It is a buzzing whining sound that rises and falls with the RPM.

I may have misused the term resistor- I mean the capacitor/filter that attaches to the coil clip (Duraspark II) on the coil. It is there, but does it work... Any way to test it with a multimeter before I waste 30 min of my life with the buffoon behind the parts counter who wants to sell me a condenser???

Appreciate the help, guys-

 

Hey, fordsnharleys,
Thanks for the input- by resistors on plugs you mean resistor plugs... OK I guess I answered my own question. I think I have everything needed, but may change plug wires to be sure...
Thanks

 

>Thanks for the responses!
> It is a buzzing whining sound that rises and falls with the
>RPM.
>
>I may have misused the term resistor- I mean the
>capacitor/filter that attaches to the coil clip (Duraspark
>II) on the coil. It is there, but does it work... Any way to
>test it with a multimeter before I waste 30 min of my life
>with the buffoon behind the parts counter who wants to sell
>me a condenser???
>
>Appreciate the help, guys-

Actually I believe that a condenser is another term for a capacitor

Sparky

 

Condenser and capacitor are the same term.

You can be getting interference from one of two sources:
1) Direct conduction from the 12v power line that feeds the stereo or
2) Radiation from the high voltage ignition into the antenna.

If you only have the interference on the radio and not on the stereo you can rule out the latter.

Some suggestions:
If it is an older vehicle with a divorced regulator, make sure the condensor at the regulator is good. Or just replace it.

Make sure you are pulling power to the unit from the battery and not from the IGN terminal on the switch.

Make sure the unit and antenna shiels are thoroughly and properly grounded.

Listen to the guy at the store. You may need an inline filter for your 12v supply line, which is what he is probably trying to sell you.

These things can be maddening. Good luck.









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ok, Yes the little condenser on the coil is there for that particular reason, to supress the primary signal interupptions, I believe it is called the R.F.I capacitor, I had to replace mine, instead of testing it, just replace it they only cost 2 or 3 bucks, and then you can rule that out. I thought by the term "resistor" you meant the inline resistor used on external resistor, points type coils, I did not know how old a vehicle you were working on.

On the spark plug resistors, some real old vehicles came with non resistor plugs, some high performance plugs come without the resistor, but if you have normal every day plugs, they have the resisitor.

if you can isolate it to charging or ignition it would be fairly easy to figure out, if it is charging you may have to install an inline filter like cougarjhon reccommended, its easy.

now, on the duraspark 2 system, you should have a dab of dielectric compound on the distributor rotor, it flings off eventually and you need to put more on. this stuff suppresses radio interference. just see if its there or not, it will look like a clear type grease on the rotor tip. you can get it at any parts store.
Justin

 

Justin, Cougarjohn, sparky,
Thanks for all the help. The subject vehicle is a 79 Bronco with 400, Duraspark II, etc. Come to think of it, the static and interference is present when playing tapes as well as radio- this would indicate interference from the power supply?
I will add some dielectric grease to the rotor; check capacitor at v. regulator ( I believe it is internal so may just replace it); and replace the RFI capacitor at the coil.
I am using the power supply directly from the factory harness which originally went to the stock AM radio. I prefer the switched ignition and accessory power this provides over a direct battery connection. Would a inline filter be necessary when using this power source? What are the disadvantages of using this stock radio power supply?
My comment about the parts counter guy was hypothetical and based on past experiences; I was implying the guy would try to sell me a (points type distributor) condenser when I showed him the RFI capacitor... I guess my weak wit doesn't transmit well through this medium!
Again, I appreciate your advice and troubleshooting help!
AL
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If the problem persists > Exchange the power wire for a piece of Coaxial Cable used for TV hookup. This cable is well shielded and will do a great job to eliminate engine electrical noise.


Dennis

78 F-150 429CJ Silver Ex.Pkge
641/2 Mustang,Pre-World's Fair car #8092
64 Fairlane S/C waiting for a 390
68-Mustang.Sunlit Gold 80,000 miles

 

From the information you are giving, it is pretty clear that the noise you hear is coming in on the 12v supply line. It should not be hard to eliminate.

Go to any stereo installation shop and tell them the problem. They will sell you a line filter to stick on the wire that feed your stereo. The thing is designed to pass dc (the steady 12v that feeds the unit) and shut to ground any ac fluctuations (alternator hash and primary ignition noise) that are now passing into the unit. Make sure that you have a good ground on your unit, and make sure that you get a good ground on the filter.

The idea of using coax on the feed line is good. But make sure you ground the outside braided conductor well. Good grounds are everything.