ABS Sensor is On..

Everytime I accelerate to about 13mph my ABS Light comes on....any thoughts on why it would do that?

Simple stuff first - take out your tire pressure gage and make sure that your tires are up to pressure according to the placard inside the driver's side door.

Once that's done, and you are experiencing the same trouble again, I would inspect each tire on your truck. Are they the same tread depth? Did you replace one, or a pair of tires and are they the same brand/tread type pattern? have you been rotating your tires properly at about the same time you change your oil? (Every 6 months/6000 miles)

Then it gets technical... But we will not talk about that until you got the basics covered first. ;)

To keep all the confusion from mucking up this thread I am deleting my other posts. I am pretty sure I misunderstood what I read in the Haynes manual, which means I am wrong and there is no reason to keep the other posts.

He can also get a light from changing all 4 tires if they are much different than stock. Then of course there is all the other stuff, that can be checked after the simply stuff.

No, that will not cause an ABS light. It cannot sense tire size.

The ABS light problem is because the wheel speed sensors are probably grimed up and need to be cleaned and reinstalled.

Yes it will. It will turn on the light and set a C0238 code. The system monitors the vehicle speed and compares it to the wheel speed, if there is a difference of 10% the light will come on and the code will be set. A difference in tire size will throw off the comparision.

Tell me how the sensor can sense the size of the tire?

Honestly I don't have a clue how it knows, seems to me that it couldn't know.

IF one is smaller, or not as tall as the others
for that matter. It wont travel the same distance as the
others per rotation. So it will have to spin faster than
the others to go the same distance. Sensor maybe
picking up the difference? and setting a code?
but unless your running different tire sizes would
the difference not matter???

If you are running one different tire then yes I can see that.

But having all four tires different will not trip the ABS code.

Maybe the sensors are calibrated for one wheel size. When you replace the wheels with a size more than 10% larger or smaller the sensor will register the speed and it will be different due to the wheel size. Just a guess.

Well, if it sees a different signal coming from the one smaller tire when compared to the rest of them, I could understand that it could sense that easily.

Did you read the footnote on the trouble code list Blazee posted? The sensor is calibrated for one tire size, say 14 inch wheels, if you put 17's on they will cover more ground per rotation than the 14's. This will cause the sensor to send a signal. it will "think" somethings wrong, it will not match the expected speed. According to the page Blazee posted the EBCM can be reprogramed for different tire sizes.

That is for the odometer, not the ABS system.

If you put bigger tires on, the sensors CANNOT sense the difference seeing it will just think that it is going slower.

During a brake application the EBCM (electronic brake control module) compares vehicle speed (from the vehicle speed sensor) to the program built into it.

The different wheel size throws the system off.

Apparently If it didnt, you would not need to re-program the EBCM for a different size tire/wheel, according to the manufacturer.

My tires are bigger than they should be and I don't have any ABS problems, also never see a problem when I have my 18s on either.

If different wheel size did not cause a trouble code to be generated why would they have it in the DTC?
it may be one of those oddball things that happen occasionally and it hasnt happened to you. I have been filling my gas tank while my engine is running and never had the check engine light activate until last week, I may fill it for a year and not have it happen again. I have an 89 c1500 350 with RWAL and when i drive it the abs light never comes on, when my son drives it it always does, the famous 8 flashs (dump valve if i remember right).
I (and others) think its due to his heavy foot causing the vacuum to drop. With 250,000 miles I'm not worried about it, it still stops when i want it to.

I wonder if Sykopenguin has checked his brake fluid? I think I would do that before looking at the tires.

The ABS system is set up to use 1 or 2 matched set tire size. Going over or under that could but not always set a ABS code. That is right out of the GM ABS Training manual. In 1994-1996 Blazers we had a bunch where the owners would lower the truck and put on very big rims and it was setting wheel speed sensor codes because the speedo and wheel speeds did not match or be with in 3-5% of each other. A prom updat to the pcm fixed most of these problems. I could go into a lot more of why and how this works but I think you get the idea.

I Pulled codes 86 and 13 from the ABS system....13 is under RWAL and 86 is under 4WAL
13 = Faulty ECU.
86 = Check the light and wiring for a short circuit.