Radio Static

rngr746 · October 6, 2014

My vehicle is an 07 Sierra Classic with the 4.3L and 56,000+ miles. I developed a noticeable static problem on the FM band only of my after market

Pioneer AM/FM/ CD player some time back. WhenI fianlly got it into the shop it was discoverd that my alternator was going out and that I had a

poor ground for my computer. The alternator has been replaced and the ground repaired. I still have a static on the FM band ,and

again, none on the AM band. I was told the radio needs to be replaced. Also, I have ancedotal information that the truck has an FM amplifier that

may or may not be the problem. Can anyone provide me with information on where this amp is located and can it be tested to check if it is good or

needs replacing?

It is the sound of my plugs firing, rapid stacatto in nature with the frequency rising with engine rpm's. Additionally the plug wires were changed

in April at 53,100 miles. The new plug wires are Taylor 8mm Pro Series. The mechanic and his assistant that did the work had both never heard of

or experienced the problem I described. At the time the alternator was diagnosed as going bad it was putting out 15 volts. This 15 volt output with

a bad ground led them to believe the problem was somewhere else besides the plug wires.

Can you suggest a solution?

Doug_Scott · October 7, 2014

I would make sure that there is a ground wire from the battery to engine block, and a ground wire from the battery to the body, and a ground wire from the alternator shell to the either the battery or to the same location as the battery to engine block ground location. You also should run ground wire from engine to chassis, using the same point at the connection from block to battery. As you can see there are lots of ground wires to make sure all parts are properly grounded.

Now, make sure antenna is tightly mounted. The easiest way to test the amplifier is to simply disconnect the existing antenna lead from back of radio and plug in a test antenna into the radio and see if the static stops or at least changes a lot.

To me, this sounds like the FM amplifier is not grounded properly, or has a cap burnt out inside the amp. The lack of static in the am band actually goes against RFI noise that you are describing. Due to this, I think you will find the FM amp is bad, or the antenna is not attached and isolated properly. It is also possible the radio head unit has a bad capacitor on the power line in.

If you want to be sure it is not the alternator, either disconnect the wiring from the alternator (not really a good thing, even without power it can generate power and have no where to go) or better yet, just pull the belt off and run the engine to see if the noise is still there. Be careful with the length of time you run without the belt, don't go driving if the same belt runs the water pump.

rngr746 · October 7, 2014

Doug - Thanks for all the good info. It gives me a good start on chasing down the problem. Can you tell where the FM amp is located? Is it part of the radio? Also the alternator is new, the original was putting out 15 v. The mech's think that is what "fried" something.

Doug_Scott · October 8, 2014

Doug - Thanks for all the good info. It gives me a good start on chasing down the problem. Can you tell where the FM amp is located? Is it part of the radio? Also the alternator is new, the original was putting out 15 v. The mech's think that is what "fried" something.

The FM amp is likely an add-on and would likely be either at the antenna or at the radio head unit. Since it requires power, I would check the back of the radio head unit and follow the antenna cable. If you are lucky, it will be right there, and dead simple to unhook and connect antenna direct to radio head unit.

Do you have issues with tuning in FM stations in your area that required the antenna to be added?

How did the mechanic test the alternator output voltage? 15v is actually within specs, provided the battery was requiring a charge. Your truck should have that new system that came out in 05 or 06 where the charging system is control by either PCM or BCM. To help with fuel economy and battery life, your truck will actually reduce alternator output if the battery is fully charged, and will increase voltage when battery is starting to discharge. This is different to the standard way we have had since alternators were invented. Standard way is to simply limit voltage to 14.2 to 14.7. Under the new system, 15v is quite possible, and can be normal. Before you say the mechanic should have known this, keep in mind these are the same guys that had never heard of RFI impacting the radio. Ignition noise in the radio has been around since the first car was running near the house while the mother was listening to her soaps on the RCA.

rngr746 · October 8, 2014

The antenna is the original that came with the truck. The radio is an add on that was installed very shortly after I got the truck in 07. It has never had any problems until the static issue. And yes, I do have problems tuning in a couple of the FM stations due to their distance from me. This was also the case when I had another truck. The alternator was checked with a multi meter of good quality and the check was made at the alternator. Out put with the new alternator is 14.2-14.7 volts. When the original alternator was checked, they disconnected the ground from the back of the case and the other connection (sorry, I don't recall the name of this connection). Both were done seperately and did not make any difference int the static noise. Again, thank ou for the assistance.

rngr746 · May 18, 2015

This is an update to my original post just in case somebody else has this problem . After much trial and tribulation, the problem was finally narrowed down to a bad distibutor cap and rotor. Both were replaced with an Accell cap and rotor. Problem solved!

&CDFiXER% · May 19, 2015

Hmm. I'll be. I sure would have figured ignition source like that would have the AM side sounding horrible, much more so than the FM side. Interesting. Thanks!