If your ABS light and brake light come on, in most cases it's probably your Rear Wheel Speed Sensor (RWSS). This sensor is on top of the rear differential.

This sensor is controlled by a "exciter ring" that is interupted by the teeth on the ring sending a variable voltage signal to the computer depending on vehicle speed. small metal shavings can cause it to fail over time. The most common symptoms of the switch's failure are bad speedometer readings and tripping the RWAL computer located under the center of the dash.

Other obvious things can also trip the computer such as:

1. bad pads, shoes, calipers, wheel cylinders.

2. bad fuses.

3. poor connections or broken and/or chaffed wire.


Once you have detemined that none of the above exist, you can try a system reset buy pulling the abs fuse and/or pulling negative battery terminals for 30 sec or so.


If the lights come back, the next thing to try is reading the RWAL fault codes.

To read the codes on 96 and older trucks:

1. turn key to the "on" position.

2. for just a second, ground pin # 13 on the diagnostic plug located on the knee bolster (#13 is a black wire, it's the 4th from right bottom row).

watch your ABS/Brake light for codes.

start counting with the first long flash, end with last short flash.

there are only 13 codes.


1. Not used.
2. open isolation valve wiring or bad control module.
3. open dump valve wiring or bad contol module.
4. closed rwal switch.
5. not used 4wd. over 16 dump pulses in 2wd.
6. erratic speed sensor reading. (most common)
7. electronic control module fuse pellet open, isolation output missing, or valve wiring shorted to ground.
8. dump output missing or valve wiring shorted to ground.
9. speed sensor wire resistance/ high reading.
10. speed sensor wire resistance/ low reading.
11. brake switch always on, RWAL light comes on over 40mph.
12. not used.
13. electronic control module phase lock loop failure.
14. electronic control module program check failure.
15. electronic control module RAM falure.

mine was a code 10. it was a break in the molded section of wire off the RWSS.

*On 97-98.5 12V models, you will end up at the dealer to get the ABS codes.


Hope this helps, and good luck!