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AM radio buzz, but only when moving


neutronranger

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I just got a used 2001 Ford Ranger XLT 4x4. I drove it several weekends ago and noted that there was a buzzing noise in the AM radio, but ONLY while the truck was moving. The frequency of the buzz followed the speed of the wheels, not the speed of the engine.

I checked to be sure the frequency of the buzz wasn't engine speed related by shifting into neutral while the truck was rolling and I rev-ed the engine. The frequency of the buzz did not change with engine revs. As I rolled to a stop, the frequency of the buzz got lower and lower until it stopped, when the truck stopped.

Is this a common problem?

Could this be the electronic speedometer signal (PCM speed output signal) I read about in the this tech article?
http://www.therangerstation.com/tech_library/speedometer.shtml

"Starting in 2001, the Ranger gets its raw speed signal from the OSS (Output Shaft Speed) sensor on the transmission/transfer case. The OSS sensor is a magnetic pickup that provides transmission output shaft rotation speed information to the PCM (Powertrain Control Module) where it is corrected for tire diameter and axle ratio.

The PCM uses the OSS sensor signal to help determine EPC (Electronic Pressure Control) pressure, shift scheduling and TCC (Torque Converter Clutch) operation. The PCM outputs the corrected VSS pulse to the speed control and the cluster. In this system the PCM is the correction and distribution point for the VSS signal."

Thanks,
Eric
KAØYWN
 
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ericbphoto

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RF interference in radios used to be real common way, way back. I think modern radios have better filters built in. Interesting that you have that on such a relatively new truck. You might be able to add a filter on your antenna. It would be better if the cable for the speed sensor was properly shielded so it would not send out that interference.

Let us know if you solve it. My solution is to replace AM with XM.

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neutronranger

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sounds like an engineering oversight :(

RF interference in radios used to be real common way, way back. I think modern radios have better filters built in. Interesting that you have that on such a relatively new truck. You might be able to add a filter on your antenna. It would be better if the cable for the speed sensor was properly shielded so it would not send out that interference.

Let us know if you solve it. My solution is to replace AM with XM.
I was hoping this was a common problem with a prescribed fix. If not, I'll likely leave it as-is - especially if it means I'd have to snake a shielded cable through the innards of the truck, just for that signal!

I only listen to AM radio 5 -10 minutes on any given day - to hear complete, concise news updates. This is a seasonal truck, bought to tow my boat, primarily, so it's not worth re-engineering the signal path of that speed signal.

Eric
 

ericbphoto

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I googled filters for am radio in auto and several came up. They actually install in the power wire to the radio. not expensive either.
 

neutronranger

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that's a idea

I googled filters for am radio in auto and several came up. They actually install in the power wire to the radio. not expensive either.
That's an idea. But, a power filter on the radio's DC power will only help if the interference created by that speed signal is being conducted down the DC power wires, into the radio.

If the interference created by that speed signal is being radiated as radio frequency energy (RF) from unshielded wires, then it's being transmitted through the air and being received by the antenna.

I'll take a battery operated AM radio into the truck. That will show whether the interference signal is radiated RF (being received the the antennas) or conducted RF down the DC power wires in the truck.

Eric
 

ericbphoto

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Wellford, SC
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1993
Make / Model
Ford Ranger
Engine Type
3.0 V6
Engine Size
3.0L
Transmission
Manual
2WD / 4WD
4WD
Total Lift
6"
Tire Size
35"
My credo
In theory, theory and practice are the same. In practice, they are different.
That's a good troubleshooting idea.
 


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