This is my '88 GL Wagon. SPFI/EA81, XT6 alt.

 

Tried to catch some of the Tiger's ball game this last Wednesday. FM station was spotty at best as I was at the outer limits of that station range, so I opted to try the AM station as I was closer to that tower at the time.

 

1st time I have tried the AM band on this car, it works, but...

 

There is noise similar to what one would get with bad plug wires, but only when the engine is idling or when you let off the throttle. Under acceleration, the noise goes away and reception is clear.

 

Driving down the road, noise only happens when I let off the throttle to shift gears, or the car is going down a rise or hill, noise is there. Accelerating up thru the gears, or pulling up a hill, noise goes away. And it does match engine rpms, in other words, an audible tach.

 

I've had issues like this on other cars, but they were constant, not depending on accel/decell. Those were traced to either bad alt or bad condenser on the coil.

 

Talked with a few other car buffs, and they are at a loss as I am on it.

 

Any ideas?

Is it an electric hum comming from your speakers? Does the sound increase and decrease with your rpm's? If so, you are sufforing from electro magnetic interferance. this can be caused by a power wire crossing over your speaker wires or antenna cable. Jiggle the stereo wires or fiddle with them, whatever your heart crys out for. Does the noise lessen or stop altogether? The sound maybe comming from your power source. You can solve the problem by re-running your wires or wiring in a filter for such an occasion, your local car stereo hut should carry them.

 

Hope this helps.

Edited June 15, 201312 yr by Godsmulligan

Check and make sure the resistor connected to the coil is operational, it maybe there but be an open circuit letting ignition noise into the wiring.

 

In the late seventies gm used resistors everywhere, on the alt, on the coil, on the distributor, and on any electrical connection under the hood to keep the noises out of the am radio.

 

My opinion is that is why we don't use am that much anymore is because of how sensitive it is to localized electric currents and short of wrapping your engine in a faraday cage it's going to make some amount of noise.

 

I would park somewhere where you get a good signal and listen to the ball game without it cutting in and out from driving to fast lol

 

Oh and bad/old wires can cause this also.

Edited June 15, 201312 yr by mikaleda

Plugs, wires, cap & rotor are fairly new.

 

I'll have to check on the coil condenser. The ones I talked to mentioned it, but didn't think it would be the cause as the noise comes and goes depending on engine load.

 

Definitely rpm related, but is only there when engine is free-running, or being driven by the wheels vs. engine driving the wheels. Happens only at idle, shift points, and going down grade/hills.

 

Noise is more of a pop or crack.

I've been near engines running non-resistor plug wires before, I know what that does to a radio. Very similar sounding.

I get electronic noise thru my am stations in the 95 legacy caused by the HID headlights. I only get the noise on am1130 but not 620 (the stations i listen to) I would imagine you would have to target a specific frequency to filter the noise. My car has a modern sony xplod head unit.

Miles touched on a good point;

Does the noise occur no matter what frequency you're tuned to on the AM band?

What you have is EMI coming from the alternator. Install a ground loop isolator on the switched ignition and ground circuit of the radio. Guarantee this is where it's coming from. Because of the modulation of AM broadcast and the internal amplifier of the radio, the AM frequency is more prone to pick up the EMI. As you throttle down, the need for voltage from the alternator is reduced and it takes some time for the regulator to 'see' this, so you have an increase in line amperage that isn't really going anywhere at the time. You can get the ground loop isolator from Radio Schmuck.

Edited June 16, 201312 yr by skishop69

Thanks skishop69. That does make some sense to me. I'll give it a try.