Hi. I have a big issue on my truck. There is Chevy Tahoe 2003 4x2. ABS light is on, speedometer is not working, brake light is on and the transmission not shifting. I replace the code sensor P0502 , and the problem persists. What else what can I do?

Seems the most likely is the control module, but could be affected by bad grounds or possibly a wheel speed sensor.
Read this thread, about 3 pages long, and has info and links for the control module fix and where to check the grounds.
http://www.tahoeyukonforum.com/threads/abs-light-and-brake-light-01-tahoe.39226/

First you need to have EBCM - electronic brake control module check for DTC'S - diagnostic trouble code's . If the light is on there is a code stored . I think most part stores can do this and for free . After you find out what code is stored , looking up that code in service repair information for diagnostic trouble shooting chart would be next step . There are a few codes too.
DTC C0035 or C0040
DTC C0045 or C0050
DTC C0110
DTC C0131
DTC C0161
DTC C0186
DTC C0196
DTC C0201
DTC C0240
DTC C0241 or P0856
DTC C0244 or P1689
DTC C0245
DTC C0283
DTC C0287
DTC C0290 or C0292
DTC C0455
DTC C0550
DTC C0558

Find the code do the diagnostic's , fix the problem , wiring issue's , a sensor , bad module ,what ever it is . The light will go out then . Here is a sample of code discription .
• DTC C0035 Left Front Wheel Speed Sensor Circuit
• DTC C0040 Right Front Wheel Speed Sensor Circuit

images for 2002 Tahoe abs relay location

Chevy ABS and Brake Dash Light fix - YouTube

? 24:15
https://www.youtube.com/watch?v=Jo76ylqgUR8
Dec 22, 2012 - Uploaded by Skizzot7
This is how to fix intermittent ABS and Brake dash lights on all Chevrolet ... That board is rather robust and is installed external to the vehicle in a location that sits .... I took the module out of a2002 Sierra and it is engineered with no T10 .... ABS Sensor and Harness 2000-06 Chevy Suburban Tahoe GMC ...

How I fixed a Chevy with an Abs and Brake light on - YouTube

images for 2002 tahoe wiring

not a good idea to do a voltage test on a computer component, it can result in damage to the ECM.
if only the speedometer is not working and the other gauges do work, the first place to start is the speed sensor, it is the most common issues with inoperative speedometers.

look at that wheel as the metal ring thing that makes the ABS work has come loose on the end of the drive shaft,just tack weld it back into position

I installed and after market stereo and my speedometer quit working 1999 chevy s10 blazer

Well if your brakes are still working fine, there really isn't a big issue. It could be as simple as an ABS sensor. Which then just the ABS part doesn't work, that simply means you brakes will lock up instead of using the ABS technology. You will still be able to stop without issue. This is not a certain though, but 99.9% of the time the ABS light is just the abs itself. If the pedal feels the same when you hit the brakes. Just wait til you can get it looked at. I believe autozone should be able to check the codes for free.
Thank you and good luck,

Lee

the brake light switch. under dash behind brake peddle arm.

driver side rear view mirror turned yellow

I hope you have the problem fixed by now, but, it is the ABS control under the truck. You need to go under the hood and open the fuse box and disconnect the 60 amp fuse for the ABS, this will turn it off before it burns up. This however will also cause the traction assist off light to stay lit all the time. To the fix the problem you can pay tons of money ($700 or so) to have the dealership install a ABS control and the problem will happen again over time. Or, remove it yourself, go on ebay and ship it off to a guy who repairs them for about $150 and you will never have the problem again. Also, if you wrap your cable ends with plastic and protect them good, you can still drive you truck while the part is being worked on. It takes about a week.