Diagnosis Time - what's failing that causing this?

[steved]

I got an odd one, and only those with aftermarket two-way radios may know the answer. I have a very clean CB installed and until today the receive was not affected by anything in the truck. This AM I started to go to work and I have IGNITION NOISE in my receive... not alternator, IGNITION. If you ever heard non-resistor plug wires through a radio in a gasser, you know what I'm talking about.



It was very clearly tied to the accelerator pedal position/load and not RPM. The farther the accelerator was depressed, the louder the feedback got. It did this very pronounced until things got warm, and it is still there just very faint.



The truck runs absolutely fine. Thoughts? FWIW, I've had a radio in this truck since I brought it home new in 2004... this is the first time I remember hearing this.

 

[B.G. Smith]

Check the coax cableial at both ends to make sure the shielding is intact and grounded. I had similar problem on 99 but my cable ran through emgine compartment, had to re-route. Might also try ground to the radio chassis. bg

 

[steved]

I know what will cause you to get receive noise, but I want to know what is causing it in the first place since I don't have spark plugs!



It started suddenly, which leads me to believe I got something bad going on somewhere!

 

[RDeFayette]

Do you have a choke coil installed in the power line to the CB?

Almost has to be alternator noise you hear as there is no "ignition" in a diesel.

Could be a diode going out in the diode trio giving you feedback.

 

[steved]

That's what I tried to explain... the noise is not linear with engine RPM (as would be the alternator noise). It is directly connected to the accelerator pedal... the further its pushed (regardless of RPM) the louder the noise in the receive. Keep in mind, with the manual I can stuff the pedal to the carpet and there is no converter to slip to allow RPMs to rise quickly.

I'm fully aware there is no spark ignition that could be causing this... I'm at a loss to what could have started causing it.

 

[B.G. Smith]

The noise in mine was the VP-44. No ignition in the diesel? What about the electrical pulses that activate the injectors? bg

 

[RDeFayette]

noise

The noise in mine was the VP-44. No ignition in the diesel? What about the electrical pulses that activate the injectors? bg

I meant high voltage (spark ignition) vs low voltage injector signal (compression ignition).

Sorry, I wasn't more specific

 

[steved]

My thoughts exactly... the ignition isn't a high energy "spark" discharge, so therefore there is nothing to make the noise. While the injectors are electronic, they are a solenoid, and they aren't even a solonoid making an electrical connection that could cause noise.



My concern is it started suddenly, and it worries me that I could have a broken wire somewhere. I'm not concerned about the noise in the radio, I'm concerned the source of a noise that I shouldn't have could be a problem.

 

[RDeFayette]

noise

My thoughts exactly... the ignition isn't a high energy "spark" discharge, so therefore there is nothing to make the noise. While the injectors are electronic, they are a solenoid, and they aren't even a solonoid making an electrical connection that could cause noise.



My concern is it started suddenly, and it worries me that I could have a broken wire somewhere. I'm not concerned about the noise in the radio, I'm concerned the source of a noise that I shouldn't have could be a problem.

Like I said before throw it on the scope and look for AC voltage in the charging system. feedback/bad diode.

 

[steved]

Can this be measured with a multimeter? I don't have access to a scope.

 

[RDeFayette]

No, not that I've ever seen, you need to see the trace pattern on the scope.

 

[steved]

And where would one take a vehicle to that would have this equipment??



My problem (if it is the alternator) is that its not OE for this model year truck, so you can see my dilemma. Taking it to a Dodge dealer will result in the (pointing at alternator) "you don't have the correct alternator" speech... regardless of the problem.

 

[RDeFayette]

Pretty much any auto shop that has an oscilliscope sould be able to do this for you.

 

[Alan Reagan]

If there is a tech school in your area, sometimes they will allow the students to do it for the experience. They usually have the scopes.

 

[TLane]

Disconnect the coax from the back of the radio. If the radio goes dead (no lights, etc. ) then the ground to the radio is missing/broken (radio is grounding thru the coax == bad). If the radio still works, notice if the noise goes away. If the noise is still present, then it's coming down the power line, not the antenna. If the noise goes away, then it's coming down the antenna - look for bad connections (try another antenna if you have it). If noise is coming down the power line, it's possible the electrolytic caps that should filter the power line have gone bad (radio still works but incoming noise isn't filtered out). Let us know what you find.

 

[Prairie Dog]

Someone posted of this same issue a while back. Problem ended up being a bad diode in his alternator which makes perfect sense.

 

[steved]

Do you remember where you saw that?

 

[Prairie Dog]

Remember what? Oh yea, no but I seem to remember the person taking the alt to a parts place and having it tested.

 

[steved]

I found part of it, it was partially a grounding issue... its very faint now, and only when cold. I will have the alt tested when I get home... I have a trip to Florida to deal with between now and then.



Oh, it only affects the CB it does not affect the AM (figured I would see if I got the same thing through a similar AM radio).

 

[Wingate]

If you think its the alternator, why not just turn the key on, engine off, and see if the noise goes away. .