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Author Topic: Interior Cabin Hum  (Read 3175 times)
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MtLeboAztek
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« on: April 25, 2003, 07:20:23 AM »

My 2003 'tek has approx. 6400 miles and haven't had a problem until now. Has anyone experienced a low, albeit annoying,  hum? This hum just started, too. Maybe a day or two ago. Come to think of it, after reading the "Air Conditioning" thread, maybe it has something to do with the BCM - I have run my A/C recently. But...

Here's what I'm hearing...

After starting the engine, I hear this low(faint) hum coming from the the front of the cabin (this sound does not come from the front speakers or tweeters). When I engage the drivetrain and sit idling, it hums. If the engine temperature reads cold or sufficiently warm, it hums. However, when I accelerate, the hum noticeably disappears. But, if I'm in Park, and rev the engine, the hum's still there. Lastly, and interestingly enough, if I depress the rear defroster, the hum stops all together. Click it off, and the hum returns.

I haven't taken her into the dealer yet, pending any ideas, tips, or suggestions.

Appreciate your time and thanks for reading.
MtLeboAztek

 
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Tom Moog
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« Reply #1 on: April 25, 2003, 08:27:25 AM »

There are 2 things that I see as a pattern here.

The sound goes away when either:

1. There is a load on the engine or
2. There is a load on the electrical system, specifically the rear defroster.

Both of these problems deal with large amounts of current terminating to ground.

These could both be the result of a faulty ground, bad ignition coil pack, faulty ground on the ignition coil packs, or other electrical components.

Electricity is a really funny thing. Sometimes there can be a problem that will go away when you turn on another appliance. All of the appliances in your vehicle use a positive and a negative (Ground) supply from the battery in order to operate. If you take the ground away, or add resistance to it, It can malfunction, but if you turn on another appliance, the first appliance can find a ground path through the other.

These type of problems can be a bear to track down. What makes it even worse is that you have a sound and you don't know where it is coming from. This is where the second half of "Today's lesson" comes in...

Your radio uses a stereo signal, which means that the sound that comes from the Left speaker is not the same as the sound that comes from the Right speaker. They are different signals. This covers the front speakers, but what about the back?

In most car stereos that utilize a front and rear speaker configuration, the front speakers receive a different signal than the rear ones. The front speakers have a right and left, which for the sake of simplicity, the rear speakers may have a minus left and a minus right to give your stereo a "perceived depth" by artificially utilizing a single stereo signal both in-phase for the front speakers, and out-of-phase for the rear speakers.

So here is where it really means something to those of you who are experiencing sounds from electrical problems:

First, The sound you hear might "appear" to be coming from a specific place within the vehicle, For example, from under the right side of the dashboard. This is possible with simple stereo seperation.

Here is the sticky part...

If you suspect that your sound is coming from the stereo, and place your ear next to the speaker but hear nothing, it is still possible and even likely that your sound is still coming out of the stereo. Here's why...

When using a stereo that uses simple stereo, plus reverse phasing for rear speakers, you might listen to your speakers one at a time and hear nothing, but when your ears are oriented in a central postition and receive signal from all 4 speakers, the electronic differences between all of the speakers, summed together at your ears can certainly create sounds.

I know this may seem like a class in rocket science, but here is an experiment you can try... Pull the fuse for your radio and see if the sound is still present. If it goes away, you have electrical interference which is most likely caused by a faulty ground.

If the sound does not go away, it is most likely still the same electrical interference, but you could be hearing it coming from a relay, circuit breaker, or a host of other electrical devices in your car.
« Last Edit: April 25, 2003, 08:28:38 AM by Tom Moog » Logged

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« Reply #2 on: April 26, 2003, 12:26:10 PM »

Shocked WOW you said a mouth full.
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Jack Egan
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Guest_MtLeboAztek
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« Reply #3 on: April 28, 2003, 05:43:50 AM »

Thanks for the detailed advice!!!

My wife and I drove her to Cincy from Pittsburgh this past weekend, and were too busy gabbing to really hear anything (plus the A/C was chillin' the cabin).

Will run through your advice checklist and run the "process of elimination" game. I really appreciate you taking the time to enlighten me as to the nuances of the electrical system.

Will keep you posted on the situation.

Thanks!
MtLeboAztek
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