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
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.
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:
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
Typical response to my page
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.
Brandon, Mb. Can