Digital Ration Adapter Controller Calibration

Chris Regan of Houston , Texas has created a dynamic CGI calculator that will automatically produce the Drac code. This is a very special calculator and a great deal of thanks go to Chris for his time and effort. http://www.philter.com/cgi-bin/drac.pl Thank you also to Tom Butcher from the fullsize forums who emailed me additional information.

On General Motors vehicles a Digital Ration Adapter Controller (DRAC) is used to pre-process wheel speed sensor signals before they get to the ECU. The DRAC is a pre-calibrated unit that is manufactured for specific tire size and rear axle combinations. The DRAC receives the AC input from the speed sensor and changes the AC sine-wave signal to a DC square-wave signal. It provides a signal on wheel speed to the anti-lock ECU, the cruise control controller (if equipped), the vehicle electronic control module and the speedometer. The DRAC is located in the instrument panel. Any change in tire size or rear axle ratio will require a recalibration of the DRAC, or on some models, a replacement DRAC. Starting in 1992, most General Motors vehicles began using a Vehicle Speed Sensor (VSS) Buffer. This electronic device performs the same function as the DRAC, changing AC input to DC output.

The DRAC sends out digital pulses at 2,000 per mile to the engine computer and speedometer, 4,000 pulses per mile to the cruise module, and 128,000 pulses per mile to the ABS computer. Most GM vechicle utilize a 40 tooth gear inside the transmission tailshaft. The vechicle speed sensor or VSS sits in a bore just above the gear and has a pick up sensor on the bottom that sends a signal to the drac module by grounding the current leading to the vss. On 40 tooth models the vss pulses 40 times per revolution of the transmission output shaft

Different DRAC modules are used depending upon axle gear ratios and tire size on the vehicle to ensure these digital signals are correct. If you change the gear ratio , your speedometer will read incorrect. NOS Drac modules are very expensive to buy from the dealership , and very expensive to send to be recalibrated at speedometer shops. On the average speedometer shops will charge over $300. Junkyards are a source for cheap modules , but you dont always find the matching module ,and IMO pulling parts and looking up codes on worn out labels is not my way of doing the job right.

The alternative is to recalibrate the unit yourself. You will be able to recalibrate the unit better than using the "correct" module if you are using wheels and tires which have a different circumference than stock or use a gear ratio not offered by the factory.

Modified DRAC
Click to enlarge picture , picture will open in new window!

1992 drac shown
1994 drac has redundant jumpers


The DRAC is a small white plastic box. It is normally bolted to the ecm or directly behind the glove box.
To correct the existing module you must know how to use a soldering iron. You will need a calculator. Determine your driven tires actual circumference. Measure it by removing the tire from the vechicle. Then place a paint mark on the tires outside lip and another paint mark on the garage floor adjacent to the first mark. Roll the tire 1 full rotation and mark the floor again. Measure the distance between the two marks. This is the circumference of the tire.

Follow the mathmatical formula below.
Calculate a number called the “InputRatio” using the following formula:
InputRatio =
63360 x gear ratio x number of pulses per revolution sent to drac via sensor(most gm are 40 )
Then divide that number by
Tire circumference in inches x 128000 .
(63360 is the number of inches per mile) (128,000 pulses per mile is DRAC output to RWAL module in some applications.)


Once you arrive at your Input ratio use the table below to reconfigure the #1-7 jumpers located on the drac circut board. Please note these input ratios correspond to matching GM drac modules that were actually produced. Most aftermarket tire sizes and gear ratios will need to use this calculator http://www.philter.com/cgi-bin/drac.pl

Typical response to my page
"Will, I picked up a DRAC from the wrecker's this morning. I didn't want to change my original one in case I put the original gears back in. I lucked out and the guy charged me $5.00 for one. I looked at a few years. the 92 looked the same as my 93 but the large plug had one less wire. The 94 looked okay but the board had more pins locations. I got a 93 that was identical to my 93, even had the pins in the same locations. I used Chris Regan's calculator and picked the closest one to it from your table. Took it to a buddy who has an electronics repair shop and he soldered it for me. On the way home I noticed the rpms were higher. Followed the wife and my speedo is RIGHT ON! Many thanks to you Will. I always love it when I do it myself and save a bundle. Like many things it's so simple when someone shows you how. Murray Duncalfe Brandon, Mb. Can


7 6 5 4 3 2 1 Input ratio
Pin Pin Pin Pin Pin Pin No Pin 0.500000
Pin Pin Pin Pin Pin No Pin Pin 0.505958
Pin Pin Pin Pin Pin No Pin No Pin 0.511230
Pin Pin Pin Pin No Pin Pin Pin 0.517090
Pin Pin Pin Pin No Pin Pin No Pin 0.522949
Pin Pin Pin Pin No Pin No Pin Pin 0.528809
Pin Pin Pin Pin No Pin No Pin No Pin 0.534668
Pin Pin Pin No Pin Pin Pin Pin 0.540527
Pin Pin Pin No Pin Pin Pin No Pin 0.546875
Pin Pin Pin No Pin Pin No Pin Pin 0.552734
Pin Pin Pin No Pin Pin No Pin No Pin 0.559082
Pin Pin Pin No Pin No Pin Pin Pin 0.565430
Pin Pin Pin No Pin No Pin Pin No Pin 0.571777
Pin Pin Pin No Pin No Pin No Pin Pin 0.578125
Pin Pin Pin No Pin No Pin No Pin No Pin 0.584473
Pin Pin No Pin Pin Pin Pin Pin 0.591309
Pin Pin No Pin Pin Pin Pin No Pin 0.598145
Pin Pin No Pin Pin Pin No Pin Pin 0.604492
Pin Pin No Pin Pin Pin No Pin No Pin 0.611328
Pin Pin No Pin Pin No Pin Pin Pin 0.618164
Pin Pin No Pin Pin No Pin Pin No Pin 0.625448
Pin Pin No Pin Pin No Pin No Pin Pin 0.632324
Pin Pin No Pin Pin No Pin No Pin No Pin 0.639648
Pin Pin No Pin No Pin Pin Pin Pin 0.646484
Pin Pin No Pin No Pin Pin Pin No Pin 0.653809
Pin Pin No Pin No Pin Pin No Pin Pin 0.661133
Pin Pin No Pin No Pin Pin No Pin No Pin 0.668457
Pin Pin No Pin No Pin No Pin Pin Pin 0.676270
Pin Pin No Pin No Pin No Pin Pin No Pin 0.683594
Pin Pin No Pin No Pin No Pin No Pin Pin 0.691406
Pin Pin No Pin No Pin No Pin No Pin No Pin 0.699219
Pin Pin Pin Pin Pin No Pin Pin 0.707031
No Pin Pin Pin Pin Pin No Pin Pin 0.714844
Pin No Pin Pin Pin Pin No Pin Pin 0.723145
No Pin No Pin Pin Pin Pin No Pin Pin 0.731446
Pin Pin No Pin Pin Pin No Pin Pin 0.739258
No Pin Pin No Pin Pin Pin No Pin Pin 0.747559
Pin No Pin No Pin Pin Pin No Pin Pin 0.756348
No Pin No Pin No Pin Pin Pin No Pin Pin 0.764548
Pin Pin Pin No Pin Pin No Pin Pin 0.773438
No Pin Pin Pin No Pin Pin No Pin Pin .0781738
Pin No Pin Pin No Pin Pin No Pin Pin 0.790527
No Pin No Pin Pin No Pin Pin No Pin Pin 0.799805
Pin Pin No Pin No Pin Pin No Pin Pin 0.808594
No Pin Pin No Pin No Pin Pin No Pin Pin 0.817671
Pin No Pin No Pin No Pin Pin No Pin Pin 0.827148
No Pin No Pin No Pin No Pin Pin No Pin Pin 0.836426
Pin Pin Pin Pin No Pin No Pin Pin 0.845703
No Pin Pin Pin Pin No Pin No Pin Pin 0.854980
Pin No Pin Pin Pin No Pin No Pin Pin 0.864746
No Pin No Pin Pin Pin No Pin No Pin Pin 0.874512
Pin Pin Pin Pin Pin Pin Pin 0.877441
Pin Pin No Pin Pin No Pin No Pin Pin 0.884277
No Pin Pin No Pin Pin No Pin No Pin Pin 0.894043
Pin No Pin No Pin Pin No Pin No Pin Pin 0.904297
No Pin No Pin No Pin Pin No Pin No Pin Pin 0.914551
Pin Pin Pin No Pin No Pin No Pin Pin 0.924805
No Pin Pin Pin No Pin No Pin No Pin Pin 0.935059
Pin No Pin Pin No Pin No Pin No Pin Pin 0.945801
No Pin No Pin Pin NoPin No Pin No Pin Pin 0.956055
Pin Pin No Pin No Pin No Pin No Pin Pin 0.966797
No Pin Pin No Pin No Pin No Pin No Pin Pin 0.978027
Pin No Pin No Pin No Pin No Pin No Pin Pin 0.988770
No Pin No Pin No Pin No Pin No Pin No Pin No Pin
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