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I have a 99 Dodge Cummins 2500 4x4. The rear breaks lock up hard when I touch the break peddle. The break light and abs light are on. Also the speed odometer will not read until the truck reaches 30 mph. I am searching for what the problem might be and the correct name of the unit that is bad so that I can replace it. Thanks for the help!
 

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I would put money on it being 1 of 2 things, because the speedo doesn't register, that gives us a big clue. It is either the Tone ring in the rear differential, or the sensor that mounts in the rear diff and reads the tone ring. They work as a pair to read vehicle speed and control the ABS. If you pop your rear differential cover and take a look at the tone ring (sorry I don't have a pic. It is a large ring with small teeth along the outside, bolted to your ring gear in your differential. If the teeth are chipped or damaged on the tone ring, that will result in the problem you are having. The next possibility is that the sensor is bad. This sensor plugs in on top of the diff. and reads the tone ring, I don't know the cost off the top of my head, or if it can be tested, but you might want to call your local auto parts store they can probably help you out with this.
Sean
 

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have you checked the ABS fuse?
 
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