Welcome to our knowledge base of totally random and probably obscure stuff relating to GM 1989-1995 external DRAC modules!

The external VSSB/DRAC (Vehicle Speed Sensor Buffer / Digital Ratio Adapter Controller) module was used in 1989-1995 General Motors vehicles in many applications. Before this point a cable-drive speedometer was used, while afterwards the DRAC was incorporated into the speedometer itself, then later the vehicle computer unit.

The DRAC module is programmed to account for different tire circumferences and gear ratios. At the factory the correct external module could then be quickly installed while using identical guage clusters, speedometers, and engine control modules. It is usually located near the ECM, or behind the glove compartment, or under the dashboard. In the same fashion if gear ratios or tire sizes are changed, the module may be quickly swapped or reprogrammed for the correct ratios.

The unit takes input from the transmission VSS (Vehicle Speed Sensor) to derive pulses per mile, according to the rotation of the transmission tailshaft. It then processes this information to account for the given parameters and outputs the correct adjusted pulse quantity per mile to the speedometer and vehicle computer.

There are two main types of circuit boards used in the DRAC modules covered here.
One: seven jumper slots

Two: fourteen jumper slots

Note that the one set is inverse of the other.
The 'redundant' jumper boards are mostly used in 1994 and later applications.

There are aftermarket boards of both types that have the jumpers replaced with DIP switches. These operate the same as jumpers but may be easily switched for reprogramming.

DRAC module pinout and relevant input/output information:

Programming the DRAC module will be necessary if changing tire diameter or final gear ratios. Otherwise the speedometer will not read correctly and the engine computer will not sense the correct vehicle speed.

After locating and removing the module from the vehicle, carefully open the plastic case and remove the circuit board.

The programming is accomplished by connecting or disconnecting a series of 7 resistor circuits. These 7 circuits can create any combination necessary shown the in the chart here. In order to determine which circuits to connect, first complete the calculation shown and then cross-reference the result in the chart. Then connect or disconnect the proper circuit pins as indicated by soldering (or unsoldering) a lead between pairs of pins.

Cross reference the resulting Divide Ratio on this chart here.
Cross reference the resulting Divisor on this chart here.

Formula above is constructed thus:
Divide Ratio = (63,360 x axle gear ratio x 40) / (tire circumference in inches x 128,000)
Divisor = (63,360 x axle gear ratio) / (tire circumference in inches x 100)
(63360 is the number of inches per mile)
(40 is number of pulses per revolution sent to DRAC, most GM are 40)
(128,000 is the number of pulses per mile DRAC output to RWAL module in most configurations)
(use formula: [Pi x radius in inches x 2] to calculate tire circumference, or look up on a tire chart)
(axle gear ratio is found as an example: 3.73:1 where 3.73 is the number used in this formula)

1 = jumper connected, switch ON, circuit closed
0 = jumper NOT connected, switch OFF, circuit open
(7,6,5,4,3,2,1 = jumper position number)

Jumper position reference numbers for this chart:

Divide Ratio--7--6--5--4--3--2--1
-------------------------------------
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
1.000000--0--1--1--1--1--1--0
1.011719--0--1--1--1--1--0--1
1.022461--0--1--1--1--1--0--0
1.034180--0--1--1--1--0--1--1
1.045898--0--1--1--1--0--1--0
1.057617--0--1--1--1--0--0--1
1.069336--0--1--1--1--0--0--0
1.081055--0--1--1--0--1--1--1
1.093750--0--1--1--0--1--1--0
1.105469--0--1--1--0--1--0--1
1.118164--0--1--1--0--1--0--0
1.130859--0--1--1--0--0--1--1
1.143555--0--1--1--0--0--1--0
1.156250--0--1--1--0--0--0--1
1.168945--0--1--1--0--0--0--0
1.182617--0--1--0--1--1--1--1
1.196289--0--1--0--1--1--1--0
1.208984--0--1--0--1--1--0--1
1.222656--0--1--0--1--1--0--0
1.236328--0--1--0--1--0--1--1
1.250977--0--1--0--1--0--1--0
1.264648--0--1--0--1--0--0--1
1.279297--0--1--0--1--0--0--0
1.292969--0--1--0--0--1--1--1
1.307617--0--1--0--0--1--1--0
1.322266--0--1--0--0--1--0--1
1.336914--0--1--0--0--1--0--0
1.352539--0--1--0--0--0--1--1
1.367188--0--1--0--0--0--1--0
1.382812--0--1--0--0--0--0--1
1.398438--0--1--0--0--0--0--0
1.414062--0--0--1--1--1--1--1
1.429688--0--0--1--1--1--1--0
1.446289--0--0--1--1--1--0--1
1.462981--0--0--1--1--1--0--0
1.478516--0--0--1--1--0--1--1
1.495117--0--0--1--1--0--1--0
1.512695--0--0--1--1--0--0--1
1.529297--0--0--1--1--0--0--0
1.546875--0--0--1--0--1--1--1
1.563477--0--0--1--0--1--1--0
1.581055--0--0--1--0--1--0--1
1.599609--0--0--1--0--1--0--0
1.617188--0--0--1--0--0--1--1
1.635742--0--0--1--0--0--1--0
1.654297--0--0--1--0--0--0--1
1.672852--0--0--1--0--0--0--0
1.691406--0--0--0--1--1--1--1
1.709961--0--0--0--1--1--1--0
1.729492--0--0--0--1--1--0--1
1.749023--0--0--0--1--1--0--0
1.754883--0--1--1--1--1--1--1
1.768556--0--0--0--1--0--1--1
1.788086--0--0--0--1--0--1--0
1.808594--0--0--0--1--0--0--1
1.839102--0--0--0--1--0--0--0
1.849061--0--0--0--0--1--1--1
1.870117--0--0--0--0--1--1--0
1.891602--0--0--0--0--1--0--1
1.912109--0--0--0--0--1--0--0
1.933594--0--0--0--0--0--1--1
1.956055--0--0--0--0--0--1--0
1.977539--0--0--0--0--0--0--1

Format for part numbers below is: GM part number / letter code

Divisor -- 5 output -- 6 output -- 6 output/velcro
-------------------------------------
18.297 -- 25110156 / AGF -- 16124655 / CAM -- 16158755 / QWR
18.703 -- 25110357 / AFI -- 16124755 / CAO -- 16158765 / QWS
19.563 -- 25110361 / AFM -- 16124765 / CAS -- 16158775 / QWT
19.781 -- 25110121 / AFN -- 16124595 / CAT -- 16158785 / QWU
20.016 -- 25110365 / AFO -- 16124775 / CAU -- 16158795 / QWW
20.234 -- 25110122 / AFP -- 16124605 / CAV -- 16158805 / QWX
20.469 -- 25110157 / AFQ -- 16124665 / CAW -- 16158815 / QWY
20.922 -- 25110123 / AFS -- 16124615 / CAY -- 16158825 / QWZ
21.156 -- 25110369 / AFT -- 16124785 / CAZ -- 16158835 / QXA
21.391 -- 25110124 / AFU -- 16124625 / CBA -- 16158845 / QXB
21.641 -- 25110158 / AFV -- 16124675 / CBB -- 16158855 / QXC
21.875 -- 25110145 / AFW -- 16124635 / CBC -- 16158865 / QXD
22.125 -- 25073955 / AFX -- 16124355 / CBD -- 16158875 / QXF
22.375 -- 25110146 / AFY -- 16124645 / CBE -- 16158885 / QXH
22.635 -- 25110373 / AFZ -- 16124795 / CBF -- 16158895 / QXJ
22.875 -- 25110159 / AGA -- 16124685 / CBG -- 16158905 / QXK
23.141 -- 25110377 / AGB -- 16124805 / CBH -- 16158915 / QXL
23.406 -- 25073956 / AGC -- 16124365 / CBI -- 16158925 / QUW
23.656 -- 25073957 / AGD -- 16124375 / CBJ -- 16158935 / QUX
23.922 -- 25110381 / AGE -- 16124815 / CBK -- 16158945 / QUY
24.203 -- 25073958 / AGF -- 16124385 / CBL -- 16158955 / QUZ
24.269 -- 25073959 / AGG -- 16124395 / CBM -- 16158965 / QWA
24.750 -- 25073960 / AGH -- 16124405 / CBN -- 16158975 / QWB
25.016 -- 25110385 / AGI -- 16124825 / CBO -- 16158985 / QWC
25.297 -- 25073961 / AGJ -- 16124415 / CBP -- 16158995 / QWD
25.594 -- 25110160 / AGK -- 16124695 / CBQ -- 16159005 / QWF
25.875 -- 25110161 / AGL -- 16124705 / CBR -- 16159015 / QWH
26.172 -- 25073962 / AGM -- 16124425 / CBS -- 16159025 / QWJ
26.469 -- 25110162 / AGN -- 16124715 / CBT -- 16159035 / QWK
26.766 -- 25073963 / AGO -- 16124435 / CBU -- 16159045 / QWL
27.063 -- 25073964 / AGP -- 16124445 / CBV -- 16159055 / QWM
27.359 -- 25073965 / AGQ -- 16124455 / CBW -- 16159065 / QWN
27.672 -- 25110163 / AGR -- 16124725 / CBX -- 16159075 / QWP
27.984 -- 25073966 / AGS -- 16124465 / CBY -- 16159085 / QUA
28.078 -- -------- / --- -- -------- / --- -- 16159095 / QUB
28.297 -- 25073967 / AGT -- 16124475 / CBZ -- 16159105 / QUC
28.609 -- 25073968 / AGU -- 16124485 / CCA -- 16159115 / QUD
28.938 -- 25110389 / AGV -- 16124835 / CCB -- 16159125 / QUF
29.266 -- 25073969 / AGW -- 16124495 / CCC -- 16159135 / QUH
29.594 -- 25073970 / AGX -- 16124505 / CCD -- 16159145 / QUJ
29.922 -- 25073971 / AGY -- 16124515 / CCE -- 16159155 / QUK
30.594 -- 25073972 / AHA -- 16124525 / CCG -- 16159165 / QUL
30.938 -- 25110393 / AHB -- 16124845 / CCH -- 16159175 / QUM
31.297 -- 25073973 / AHC -- 16124535 / CCI -- 16159185 / QUN
31.641 -- 25073974 / AHD -- 16124545 / CCJ -- 16159195 / QUP
32.000 -- 25110164 / AHE -- 16124735 / CCK -- 16159205 / QUR
32.375 -- 25073975 / AHF -- 16124555 / CCL -- 16159215 / QUS
32.719 -- 25073976 / AHN -- 16124565 / CCM -- 16159225 / QUT
33.094 -- 25073977 / AHN -- 16124575 / CCN -- 16159235 / QUU
33.469 -- 25073978 / AHI -- 16124585 / CCO -- 16159795 / QSF
33.844 -- 25110165 / AHJ -- 16124745 / CCP -- 16159805 / QSH
34.594 -- -------- / --- -- 16131625 / CCR -- 16159815 / QSJ
35.000 -- -------- / --- -- 16131635 / CCS -- 16159825 / QSK
36.188 -- -------- / --- -- 16451125 / QPW -- 16159835 / QSL
36.594 -- -------- / --- -- 16451135 / QPX -- 16159845 / QSM