My ABS and Brake lite have been on for a year but everything still works properly. I checked the fuses and they are good . I even replaced them just for good measure. If anything the rear brakes are not real good but I have adjusted them up quite a bit. Very little wear. I have very good brake pedal feel. Sometimes I think too good.

 

Here is an old post of mine from 9-27-2004 that may help some.


I replaced the rear wheel speed sensor (RWSS) that is located on the top of the rear-end differential and my ABS and BRAKE light have gone off.

Now for a few findings that I came across before replacing it,,,,

*With the rear wheels up on jack-stands, The speedometer did not function at all when in gear at approx 30-35mph. (guesstamated),,,,, however, putting it back down on the ground and driving around,,,,, everything worked fine. (other than ABS and BRAKE light still on)
*When I removed the old RWSS, There was no continuity between the two prongs on the sensor.

I had a few options on what to do,,,
Spend the $60-some bucks for a dealer to put it on their diagnostic gismo (every shop I went to said that only a DC dealership could access the Controller for the Anti-lock brakes. (CAB) and I had already figured out that the CAB does not set any codes in the ECM that are readable by the average DRB scanner.) I figured I would gamble and spend the $33 for the RWSS and the reason for my decision was based on a theory that my front sensors were OK because the speedo and Cruise Control worked fine when all four wheels were on the ground.
This is only my theory and maybe one of our gurus can tell me if I'm correct or not.
On a 4 wheel ABS equipped truck, the CAB must get signals from all 4 of its data gathering sensors in order for it and the speedo and cruise control to function properly.

1. Front Left wheel
2. Front Right wheel
3. RWSS (remember that there is only the one sensor mounted on the pumpkin)
4. And the Transmission Speed Sensor which is mounted in the transmission.

When the RWSS fails on a 4 wheel ABS system, apparently the ABS shuts down and trips the warning lights to let you know it has disabled itself, however the CAB still sends the data from the Front sensors and the tranny sensor to the ECM, which enables the speedo and cruise control to be unaffected.
I also think, this is why the trucks that are equipped with only the Rear Wheel ABS systems have problems with their speedo's and cruise control when the Rear Wheel Speed Sensor goes bad. Evidently this system only uses the two data gathering sensors,,,
The RWSS
And the Transmission Speed Sensor

Does this sound like a fair explanation?

One thing I'm sure of,,,,,,,,,, I don't have those BLASTED idiot lights aggrivating me!

BTW the part number for the RWSS on my 2 wheel drive,,,, 4 wheel ABS equipped 2500 is 56028187AE and this is the same part number for the Rear Wheel ABS only trucks also.
NOTE* However,,, it does appear that the 4 wheel drive trucks have a different part number according to the parts guy I spoke with today.

Keep in mind that this post was from 2004 and concerned my 1999 2wd / 4wheel ABS truck.

 

Great post and explanation of how the ABS systems sense...

 

Funny thing, my ABS/Brake light went off last night. I didn't really do anything (that I know of). I did replace the IAT sensor the other day.....perhaps I wiggled something loose messing around the wiring harnesses which caused the lights to come on. Anyway, I will probably check the sensor connection and harness wiring just in case (PM). At least now I know what to do if the problems comes back

 

Well sure enough the lights came back on so I was out to get a new sensor.
Put the new sensor in yesterday (about $40, just like stated) and worked like a charm. Lights are out and speedometer working fine again. Great thread everyone, took me right to the problem without spending $200 bucks at the dealer.

 

You're right about the PCM and ECM, Bill, but the instrument cluster, being a self contained computer starting in '98, does have the self-diagnosis function built in, and will display the codes and run all diagnostics specific to itself.

Diagnostics routine runs after the odometer RESET button is released, stops after the DTC codes are displayed, and will resume when the RESET button is pressed again.

The diagnostic checks all the indicators, and will cause each gage to display ~1/4, 1/2, 3/4 scale, including the tach and speedometer.

For example, DTC 920, 921 and 999 would indicate no speed or mileage data from the PCM, and a general error - usually related to the speed sensor input from the rear axle.

You may have seen that specific in a recent post, also.


What would code 950 mean?

 

I posted this earlier in common problems section.

This is for 96 and older.

"Oh no, my ABS and Brake Light are on!"

--------------------------------------------------------------------------------

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!

 

Thanks for the informative post, Timmy!

 

Foamdoc what sensor did you replace....I have the same problem as you stated abs light and brake light on and no speedo below 25miles/hr.....which sensor did you replace...I read the posting and was not sure which sensor you actually bought. Thanks.

 

Foamdoc and me both replaced the sensor in the rear axle, it is 90% of the time the cause of this problem. It is the sensor that is pictured above and should be less than or around $50. Simple to replace, take a bolt out, pull the sensor out, put the new one and replace the bolt.

 

Interesting reading, I just might have to try replacing the rear sensor.

Mine is a 2000 CTD 2500 4x4 and the ABS and Brake lights have been on since I bought the truck. What has me wondering is what the speedo uses to get a signal to display the speed. While offroad in some mud it will show the actual vehicle speed and not rear wheel speed (spinning rear tires) and the left front ABS sensor is cut so it must be getting a signal from just the right front.

BTW, brakes work great but will easily lock up the rears if pressed hard enough.