I figured i'd publish this fix for those that have swapped gears and/or tires in their Silverado/Sierra, and are getting ABS DTC C0238, and the dreaded ABS/Brake light and chime.
The issue arises when the gears are changed, or the tire size is changed enough to get outside of the allowed error percentage in the ABS system. These trucks do not have individual rear wheel speed sensors for the ABS system, rather it relies on the VSS signal produced by the PCM on the ABS VSS output pin, which is derived from the transmission output shaft speed sensor. This is why changing gears is usually the primary cause of this light, although going with larger than normal tires, or smaller will produce the same DTC if it's far enough out of the allowed threshold.
To my knowledge, there is only 1 way to properly fix this and do it right, which is the way i'm about to detail and the way that has been successful for me. Changing the ABS module's programmed tire size via a Tech 2 does not seem to be an effective fix in my experience either, as it's not the tire size that's causing the issue per-se, so much as the new gear offset. And, from what I understand, the Tech 2 only allows pre determined tire sizes to be programmed into it, you can't simply put the appropriate tire size you are running in there. This method, will make everything work like stock, with whatever size tire you have on the truck.
And here's what you will see that causes the problem in the ABS module setting the code. The front and rear wheel speeds are not in agreement, even though the tire size is the same:
The subject vehicle:
2004 Silverado 1500
5.3L
34 in tires front/rear (advertised 35", but ALWAYS measure them!!!)
changed from 3.73 (stock) gears to 4.56
So, knowing this first off we calculate the basic's that we need in the tune. I personally, NEVER use the auto calculator for the trans tail rev/mile or vss output pulses/mile, it never seems to come out exact for me and that will cause an issue. However, it do keep it's values for the trans/TCC speed scheduling. Here are the values used on this truck:
Tire Circumference (Pi x Diameter) = 106.81415"
Tire Rev/mile (63360 - number of inches in a mile / Circumference) = 593.179836
Driveshaft rev/mile (Tire rev/mile -593.179836- x Gear Ratio - 4.56 -) =2704.9000
VSS Pulses/Mile (40 teeth on reluctor wheel for 4L60+80E x Driveshaft rev/mile -2704.9000) = 108196.00
Plug All of those numbers into the fields in the tune via HPT:
Now, road test the vehicle and see how far off your rear wheel speed is. Depending on your setup and changes, it will most likely be a little, or a long ways off still. Next, you need to play with this box:
You see, the ABS Module expects 128k pulses/mile to get that rear wheel speed. Changing this value, will change the rear wheel speed reading. It's now as simple as moving the value UP or DOWN (Making the number larger will SLOW DOWN the MPH in the ABS module for the rear wheels, making the number SMALLER will speed it up) and re-logging the speed signal in the ABS, until they match, like this:
No more ABS Light, no more annoying chime. The ONLY stipulation to this will be that you have to run the calculations BEFORE you do your gears/tires, just to make sure the values you need to put in the boxes, don't exceed their maximum value. IF that case does arise, grab a signal conversion module such as the Dakota Digital SGI-5 box, install it in the transmission output shaft speed sensor signal circuit, and do the math on the stepped down pulses.
The value I ended up on this truck was 113,000 on the ABS Pulses/mile to get the rear wheel speed to agree, your results may be different. ALWAYS, ALWAYS, ALWAYS Measure the height, OR perform a roll-out check to get that circumference. Just because a tire is advertised a certain diameter, does NOT mean it measures as such, and that will adversely affect the results of this. Once all this is done, finish out by correcting the speedometer by adjusting the General VSS Pulses/Mile count (remember, ADD MORE to the value to slow down the needle, SUBTRACT to speed it up). Most stock vehicles I have to adjust about 2-4,000 pulses/mile to get the speedo dead on.
That's my solution, and has been working for a few years now.
I appreciate any input or further advice from people who have had this concern and their fix as well. I'm sure there are more than one way to kill this issue, but for me, this seems to have been the most effective, with no dealer level programming of the ABS system.