Speed Senser 10.25 axle?

[Michael Fowler]

I am still trying to chase down some AM radio interference.
What is the configuration of the speed sensor in the rear axle? What does the sensor read? Does it read the tips of the ring gear? How many teeth on a 3.25 ratio ring?
Lots of questions, I know. The AM radio noise is a "woosh" at less than 5-7 mph. The "woosh" is followed by "quiet" and then another "woosh". By the time speed reaches 7 -10 mph all the "wooshes" blend into a white noise that competes with the radio signal.
My working hypothesis is that I am hearing the teeth of the ring gear as they generate a signal for the speed sensor. Then I need someone to tell me how to filter out that noise, if that is what it is.
I know some of you are asking " AM Radio--what's that?"

 

[itsacrazyasian]

i doubt thats the noise you are hearing.

The speed sensor does not read off the ring gear but infact off a toothed wheel attached to the carrier. Its simply hammered on with a tooth to keep it from spinning. There are serrated teeth which interrupt the magnetic field of the rear abs sensor which in turns creates current, alternating current which the abs/odo module interprets as vehicle speed based on the current level. If you want to check the sensor, set your multimeter on AC current and attach probes to both pins and spin the wheel fairly quickly (hold one wheel if you have an open diff).

Thats how i usually test the stupid abs sensors integral to a wheel bearing on newer fords.

edit: are you using a factory radio? and what year is your truck?

 

[Diesel_brad]

All AM sounds like crap, thats why everything is on FM these days

 

[Fordsandguns]

Could it be that alternator? I know on some vehicles the alt gives interference with cb's.

 

[RLDSL]

Check and clean your grounds, especially the alternator ground.

 

[Michael Fowler]

Noise is definitely NOT alternator. There is no noise when sitting still and the engine running. As I slowly accelerate, I can hear the individual "wooshes" until they merge into a more or less solid sound.

Radio is a late-model JVC.
Truck is a '94

 

[itsacrazyasian]

Well the radio antenna passes right by the abs/speedo module on the passenger side behind the glovebox. Again this signal is AC current so maybe. but it could also be the output from the abs unit to the speedo which is also waveform.

try driving with the speed sensor unplugged from the rear diff. Who knows it might even be interference as the vehicle among your surroundings.

I don't listen to AM radio, but i haven't had an issue with interference with my CB radio and the antenna wire is run right along side the body harness. granted the cabling is shielded but so should be the antenna wire.