This tech article was written and provided by Chris Darby.
This Tech article shows you how to correct your speedometer,
cruise control, and ABS systems by modifying the DRAC or VSSB unit on 1990-1995 General
Motors trucks, vans, and SUVs with the electronic vehicle speed sensor.
Vehicle Speed Sensor (VSS) is the electronic sensor found on the
tailshaft of the transmission on 2 wheel drives, or the transfercase on 4 wheel
drives. This pulses several times for each revolution of the tailshaft. Most
GM vehicle speed sensors pulse 40 times per revolution of the tailshaft. The output
signal of the VSS is an AC signal.
Digital Ration 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.....
If all of this looks a bit too overwhelming for
you to do, you can get a reprogrammed DRAC/VSSB
from me for $106 shipped but note that it will not have dip switches
installed. I just move the metal jumper wires to the configuration
needed for your application.
Sharp straight pin or paperclip
DIP switch (pick one - or search google for online dip switch retailers)
*only get the 8 position switch if you don't want to order online, and just
want to drop by your neighborhood Radio Shack. The 7 position switch will fit better
(for the 90-91 DRAC).
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)
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
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
Locate & Remove DRAC/VSSB unit
It should be located behind/under the dashboard somewhere, usually
near the ECU. It is in a white plastic box with 2 wiring harnesses connected to
it. Simply, remove the wiring harnesses, and pull the white box out. Open the
white box, and remove the DRAC/VSSB unit.
IMPORTANT - you are dealing with a very sensitive piece of
electronic circuitry! It is VERY sensitive to static electricity, and other forms of
electronic shock. You can destroy the board if it comes in contact with some type of
electronic shock, grease, oil, or other liquids. If you do not feel comfortable
dealing with this, please bring the board to an experienced electronic engineer, or
computer technician to do the soldering on the board. I am not responsible for any
Determine your new Input Ratio
= 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.
(see pictures below)
Note - If you do not feel comfortable working with sensitive
electronic equipment, this is where you need to take your board to an experienced
electronic engineer, or computer technician.
Use a soldering iron to heat up the ends of the jumpers that are
currently on the board. They are soldered on both sides of the board. Once you
heat up one end, pull it out of the board using some pliers. After the jumpers are
out, heat up each hole one by one and use a sharp, narrow straight pin or small paperclip
to poke through the holes to clean all the left-over solder out. The holes need to
be open to fit the DIP switch on. DO NOT remove the #1 jumper if you are installing
2- 6 position DIP switches on the 92-95 VSSB board. Leave both #1 spots alone, as
they do not change.
Be very careful not to spread the solder around too much. it
can cause the board to short out, or produce incorrect signals. Also, be careful not
to damage the board when pulling the jumpers out.
Install DIP switch
If you bought the Radio Shack 8 pin DIP switch, you'll need to
bend the leads on switch 8 out of the way, or cut them off. Be sure they do not lay
across leads when it's on the board. You will also have to slightly move the blue
resistor out of the way, but do not damage it. If you bought a 7 pin DIP switch, it
should fit very nicely.
Carefully, push the DIP switch into the holes. If your using
the 8 pin switch, you may need to trim a small amount of plastic by switch 8.
1992 -1995 VSSB:
The left bank of jumpers should be an exact mirror of the right
bank. For example: if the jumpers of the left bank were numbered as 7654321, and the
jumpers in the right bank were numbered as 1234567, each jumper on the left bank should be
the opposite of the jumper with corresponding number in the right bank. (see picture below for reference) Jumper space #1 does not change
ever. the left bank #1 should stay open, and the right bank #1 should stay closed
(with a jumper).
Carefully, Install the two 6 position dip switches on spots 2
through 7 by pushing the leads through the holes on the board. Leave both #1 spots
as they are. (see pictures below for reference)
Flip the board over, and carefully solder each switch terminal to
the back side of the board. Be careful not to use too much solder as it may short
across other leads.
Use the input ratio you found in Step IV to find the closest value
in the "Jumper Settings" chart. Set the DIP
switches according to the 1's and 0's in the chart next to your input ratio. On the
VSSB, remember we left both #1 jumpers as they were, so the first switch on the DIP box
will be starting with jumper setting #2. Be sure to set the 6 switches on the left
dip switch opposite of the jumpers on the right dip switch in a 'mirror' fashion. (see picture below for reference)
Re-install DRAC/VSSB and Test
Install the DRAC/VSSB board back into it's white plastic
case. The DIP switch should have just enough clearance to fit into the case.
Make sure the case lid isn't hitting the switches.
Install the case into the vehicle and plug both wiring harnesses
Check the accuracy of the speedometer with a GPS unit, or by
pacing another vehicle.