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post #1 of 7 Old 03-23-2007, 09:47 AM Thread Starter
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JK speed sensor

How does the JK determine vehicle speed?
Engine rpm, driveshaft, wheel rpm?
Something completely different?

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post #2 of 7 Old 03-23-2007, 10:58 AM
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A quick check of the service manual shows that there are two possibilities, but I'm not sure which is actually used. The first is that it uses the transmission output sensor along with the transmission range sensor. The second is that there are four wheel speed sensors as part of the ABS. Not sure which one the PCM uses to determine actual vehicle speed. The section on the speedometer mentions a "Vehicle Speed Sensor" but I didn't see it listed anywhere. Didn't spend much time looking though.

The wiring diagram for the cruise control shows it using the input/output speed sensors, so maybe it is based off the transmission output after all and the ABS sensor use is self contained.
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post #3 of 7 Old 03-23-2007, 12:00 PM Thread Starter
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I am wondering if I get some 35s and regear to 4.56, is the computer stuff to work properly. These tire/gears should put me at factory speed per rpm. But, not factory speed per tire revolution. If the JK is looking at tire rotations as well as rpms it might think something bad is happening.
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post #4 of 7 Old 03-23-2007, 12:12 PM
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In my Mustangs, there was a speed sensor on the side of the transmission. When I regeared, I had to reprogram the computer, same with tire size changes. I am pretty sure the JK is the same set up. It isn't a big deal, if someone would hurry up and come out with a programmer.
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post #5 of 7 Old 03-23-2007, 12:15 PM
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yeah i got 35" tires and im guessing the speeds im at. i have heard jeeps can only calilbrate up to a 33" tire but seen where a few say 32" tire

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post #6 of 7 Old 03-23-2007, 12:22 PM
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32" is what the Rubicon comes with from the factory, so that's the biggest stock size, hence the biggest they probably are set up to flash to now. I wish someone would hurry up and make a programmer for this because it's not just the speedometer, the odometer gets thrown off as well.
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post #7 of 7 Old 03-23-2007, 02:51 PM
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I am certain it get speed from the wheels, it counts the teeth and the computer interprets the info to give us speed.

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