[blackdragon4x4]

Just figured I'd post this in here again, been getting a few requests for it lately.

DRAC/VSSB Calibration




Digital Ratio Adapter Controller (DRAC) and Vehicle Speed Sensor Buffer (VSSB) are both electronic devices that simply convert the input AC signal (from the VSS) to a calibrated digital DC signal. The DRAC sends 2,000 digital pulses per mile to the ECM (Electronic Control Module), 4,000 digital pulses per mile to the cruise control module, and 128,000 digital pulses per mile to the ABS (Anti-Lock Brake) computer. The DRAC and VSSB units are pre-calibrated from the factory according to the vehicle's stock gear ratio, and tire size. But, it can be re-calibrated by simply moving jumpers around. 1990-1991 vehicles should have the DRAC, and 1992-1995 vehicles should have the VSSB unit. The main difference between the DRAC unit and the VSSB unit (from what I can tell) is that the VSSB unit has an extra "bank" of 7 jumpers in addition to the same bank of 7 jumpers that the DRAC unit has. I would assume this extra bank of jumpers is for the electronically controlled transmission that GM started using in 1992.


If you change your tire size and/or gear ratio, your speedometer, cruise control, and ABS systems will not function correctly because they are receiving an incorrect signal from the DRAC or VSSB. Your options to correct this problem are to fork out $300+ for an add-in calibrator, buy a new DRAC/VSSB from the dealer for $$$, send off your DRAC/VSSB to a gear shop for $$$, or spend $2 and some change on a Radio Shack part, and do it yourself. hmmmmm... that's a tough one.....

I. Determine your vehicle's gear ratio



If you're positive that your vehicle still retains it's factory gear ratio, use the RPO code list here to match a code with a code in your glove box.


If you are unsure of your gear ratio, pop the diff cover and see if you can find markings on the ring gear that tell you what ratio they are, or count ring gear and pinion teeth and match the numbers with a ratio here.


Note that your final calculations will be more accurate if you do not round your measurements. Divide your pinion gear tooth count into your ring gear tooth count to get a more accurate value for your gear ratio. (4.56 has 9 and 41.... which comes to 4.55555555)

II. Determine your tire's rolling circumference



First, remove a rear tire/wheel from the vehicle, mark a line on the sidewall with a piece of chalk, and mark a line on the ground. Align the mark on the tire to the mark on the ground, then roll the tire straight one complete revolution, then mark a line on the ground where the mark on the tire is. Measure the distance between the 2 marks on the ground to get your tire's rolling circumference.


You can also look on your tire manufacturer's website to find your specific tire's revolutions per mile at a certain speed. Divide that number into 63,360 (number of inches per mile). The result should be the tire's rolling circumference.


Compare both results. They may vary depending on rim width, tread wear, and the fact that the circumference increases at higher speeds. But, the results should be close. Again, do not round yet. Keep as many digits as possible.

IV. Determine your new Input Ratio



Use your results from Steps I & II in the equation below to calculate the correct input ratio


Retain as many digits as possible to achieve a greater accuracy

Input Ratio = 63360 x Gear Ratio x pulses per revolution of speed sensor [40]
tire rolling circumference in inches x 128000

63360 = inches per mile
*most GM speed sensors produce 40 pulses per revolution
128000 = pulses per mile



Once you have done the calculations, compare your result to the input ratios in the "Jumper Settings" chart below. Find the closest value to your answer.


Go across the chart and look at the correct jumper settings. a 1 means there should be a jumper across the terminals, and a 0 means there should not be a jumper. Compare that to your DRAC/VSSB's current Jumper setup. The jumpers are not numbered on the board itself, but, the way my chart is written, if the black wiring harness plug is at the top, the jumpers are numbered 1-7 from Left to Right.



Check the accuracy of the speedometer with a GPS unit, or by pacing another vehicle.




1 = Jumper connected or switch ON
0 = Jumper NOT connected or switch OFF
1-7 = Jumper/switch #
Jumper Settings
Input Ratio 1 2 3 4 5 6 7
0.500000 1 1 1 1 1 1 0
0.505958 1 1 1 1 1 0 1
0.511230 1 1 1 1 1 0 0
0.517090 1 1 1 1 0 1 1
0.522949 1 1 1 1 0 1 0
0.528809 1 1 1 1 0 0 1
0.534668 1 1 1 1 0 0 0
0.540527 1 1 1 0 1 1 1
0.546875 1 1 1 0 1 1 0
0.552734 1 1 1 0 1 0 1
0.559082 1 1 1 0 1 0 0
0.565430 1 1 1 0 0 1 1
0.571777 1 1 1 0 0 1 0
0.578125 1 1 1 0 0 0 1
0.584473 1 1 1 0 0 0 0
0.591309 1 1 0 1 1 1 1
0.598145 1 1 0 1 1 1 0
0.604492 1 1 0 1 1 0 1
0.611328 1 1 0 1 1 0 0
0.618164 1 1 0 1 0 1 1
0.625448 1 1 0 1 0 1 0
0.632324 1 1 0 1 0 0 1
0.639648 1 1 0 1 0 0 0
0.646484 1 1 0 0 1 1 1
0.653809 1 1 0 0 1 1 0
0.661133 1 1 0 0 1 0 1
0.668457 1 1 0 0 1 0 0
0.676270 1 1 0 0 0 1 1
0.683594 1 1 0 0 0 1 0
0.691406 1 1 0 0 0 0 1
0.699219 1 1 0 0 0 0 0
0.707031 1 0 1 1 1 1 1
0.714844 1 0 1 1 1 1 0
0.723145 1 0 1 1 1 0 1
0.731446 1 0 1 1 1 0 0
0.739258 1 0 1 1 0 1 1
0.747559 1 0 1 1 0 1 0
0.756348 1 0 1 1 0 0 1
0.764548 1 0 1 1 0 0 0
0.773438 1 0 1 0 1 1 1
0.781738 1 0 1 0 1 1 0
0.790527 1 0 1 0 1 0 1
0.799805 1 0 1 0 1 0 0
0.808594 1 0 1 0 0 1 1
0.817671 1 0 1 0 0 1 0
0.827148 1 0 1 0 0 0 1
0.836426 1 0 1 0 0 0 0
0.845703 1 0 0 1 1 1 1
0.854980 1 0 0 1 1 1 0
0.864746 1 0 0 1 1 0 1
0.874512 1 0 0 1 1 0 0
0.877441 1 1 1 1 1 1 1
0.884277 1 0 0 1 0 1 1
0.894043 1 0 0 1 0 1 0
0.904297 1 0 0 1 0 0 1
0.914551 1 0 0 1 0 0 0
0.924805 1 0 0 0 1 1 1
0.935059 1 0 0 0 1 1 0
0.945801 1 0 0 0 1 0 1
0.956055 1 0 0 0 1 0 0
0.966797 1 0 0 0 0 1 1
0.978027 1 0 0 0 0 1 0
0.988770 1 0 0 0 0 0 1