Jdrogers- Is that info specific to the JK?

Redrock- Thanks for the images.

-T

 

Ummmm.... yes, it's from the FSM.

There is also some more information on this thread:

Speed Pulse Wire

(I found it doing a quick search for "speed signal".)

 

:confused: why oh why cant i just do an old fashioned gear swap?

...i have all the numbers worked out on my abacus.

(Sorry but my slide rule has a RAM problem, i stuck it up my physics professer's * )

 

Well for $388 I think I have a solution to speedo recalibration. I am not willing to try it yet, but here are my thoughts. Each wheel has a 2-wire analog speed sensor. This is a very basic item. It gets its signal from a tone ring on the wheel hub. There is a simple signal converter available from Australia that can be used to recalibrate the speedo signal from this type of sensor. It is widely used on sport bikes worldwide. The problem is that you would need 4 of them to calibrate each wheel sensor individually. I am also not sure if the vehicle computer gets a speed signal from any other sources. These would also need to be recalibrated. The signal converters are infinitely adjustable so gearing and tire size are not an issue. You can buy the "Yellow Box" from http://www.blackrobotics.com/. They are about $100/each. Please chime in if you know of other sources that the Jeep is getting speed data from.

I am sure that one of the aftermarket programmer companies will come up with a programmer that will let you modify the tire size in the ECM eventually, but that will also probably cost around $400.

Later-

 

At that price you can have unichip do you a custom tune recalibrated for larger tires/gears. At least they claim they can.

 

From the limited info on unichip's site, you would actually need to purchase the optional display unit to see a recalibrated speed. I'm just going to sit on the sidelines on this one until someone comes out with a definitive way to calibrate the speedometer that's in my dash, plus the other performance gains.

 

Sonic-

The dealer won't program for anything larger than the standard Rubicon tires. I don't know if that is a limitation of the computer, or the legal department at Jeep. Most auto computers have a setting for revolutions per mile, if this is the case, then the dealer could easily fix the calibration. Can we get a dealer tech to chime in?

Tom

 

I don't think you would need 4 of them. I believe the vehicle speed is derived from the transmission output speed sensor. This sensor is connected to the PCM. And from the description of the speedometer operation I posted:

The speedometer gives an indication to the vehicle operator of the vehicle road speed. This gauge is controlled by the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster from the Powertrain Control Module (PCM) over the Controller Area Network (CAN) data bus.

The speedometer gets its information from the PCM.

The wheel speed sensors on each wheel are connected to the ABS control module. The only thing the ABS module would care about is the relative speed of each of the wheels in relation to the other wheels. I doubt the vehicle speed reported to the speedometer is based on the various wheel speeds.

Just my 2 cents...

Ps. An easy test would be to disconnect the output speed sensor on the transmission and see if the speedometer stops working...

 

Jdrogers-

I like your way of thinking. What you described is the typical scenario for most speed sensor/speedo combinations. The speed sensor is usually located on the tranny output, or the rear axle.

On the diagram Redrock provided there are two sensors, 1-input and 1-output. I do not think these are speed related. I believe that those are the sensors for the tranny computer that tell it the relative shaft speed from input to output. This info is used to determine converter lockup, line pressures and slippage.

Is there a sensor on the T-case? That is where the speed sensors have been on most Jeeps 'til now. I am going to spend some time under my Jeep this weekend.

I would think that the JK has a similar system to the Commander and Liberty. Both of these have been out longer. Does anyone have speed correction for these?

Later-

 

The only sensor on the transfer case is the position sensor.

I'm pretty sure the output sensor from the transmission is the one used to determine vehicle speed. It is the output someone used as the vehicle speed pulse for their navigation radio. He tapped into this signal where it enters the PCM. The PCM is located on the driver's side inner fender.

Let's us know what you figure out.