Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
These symptoms are all related. There is no output from the vehicle speed sensor to the computer. This disables abs system, turning on the abs and brake warning lamps. The computer also does not know vehicle speed so the speedometer does not work and the computer does not shift the transmission. Since you said your truck is a Z71, I will assume it is 4wd. This means the speed sensor is located on the transfer case. Check wiring first to make sure all connectors are in place and wiring is not damaged. If it all looks okay, try replacing the speed sensor on the transfer case to see if the speedo starts working. See attached image for location of the sensors. (#3 in picture)
If you have the NP8 transfer case, you will have two sensors. Swap the two around and see if the speedo starts working. If it does then replace the faulty sensor. Sensors are #5 and #4 in the attached picture.
This may be tied into the ABS and Brake warnings. Once you have repaired the issue with the speed sensor, check to see if the service brake system warning is still coming on. If it is then the problem is typically in the brake assist pump motor. This is located at the brake booster under the hood. See #3 in attached picture.
What have you attempted to do so far?