What's a GM DRAC?
Any automotive geek that may want an interesting experience should open up some of the classic GM truck and car powertrain parts manuals from yesteryear and one of the first things they'll notice is that GM had to dedicate many pages to their speedometer gears to cover all the different combinations of transmissions, axle ratios and tire sizes. In fact, the speedometer drive gears, driven gears, and mechanical ratio adapters often required more pages of part listings than the gearboxes they were connecting to.
This sensor (VSS01-4160) will allow you to run a conventional cable drive speedometer and send critical speed data to your GM PCM for proper powertrain operation.
This is a cable-pass-through style speed sensor that can be attached to the Jeep Dana and mechanical New Process transfer case outputs. It generates a usable signal required by modified GM PCM computers from 1993+ trucks to present and 1994+ cars (Vette, Camaro, Caprice), equivalent to 8,000 pulses per mile.
This version is identical to the above, except it does not have provisions for a mechanical cable pass-through. Use this sensor when you are not using a cable-driven speedometer and sourcing your speed signal from the GM ECM to your electronic speedometer.
Note that the above products are not appropriate to rely on for the purpose of shifting an automatic transmission (such as the 4L60E) when a gear reduction transfer case is being used, since when the transfer case is shifted into low range, the vehicle speed will read between 2.0 and 2.72 slower than the transmission output shaft is actually going.
Starting as early as 1982, GM began to offer electronic speed sensing and with it, they connected a buffer unit that converted the AC sine wave signal coming from their Vehicle Speed Sensors (VSS) - which was either an inline sensor on a speedometer cable, or eventually a direct pick-up to a reluctor ring - to a DC signal that their rudimentary engine control modules and cruise modules could utilize. Buffers became more advanced in 1984 when GM introduced a buffer that also acted as a signal splitter, distributing speed signals to the following sub-systems:
An earlier GM speed signal buffer as used on 1982-1984 models such as the Corvette.
Now, instead of using a myriad of speedometer drive & driven gear combinations for the huge variety of vehicle configurations, GM used these buffers which could be easily and inexpensively configured with resistors, jumpers or dip switches at the factory to calibrate to the tire size and axle ratio of the vehicle. These units were officially referred to as a Digital Ratio Adapter Controllers or DRAC's.
This 1984-1993 unit could take in a two wire AC signal and convert it to DC outputs of varying frequencies to each of the systems requiring vehicle speed data.
In 1994 and 1995, GM began to introduce PCM's (Powertrain Control Modules) which had the analog-to-digital (A-to-D) converting circuitry integrated into their circuit boards, and which could be programmed and calibrated via software instead of hardware. As such, DRAC's were phased out during these two years. This was done gradually and installers will need to determine if their donor engine / harness / computer application featured a DRAC, or whether the transmission / transfer case speed sensor was connected directly to the PCM.
Electric Speedometer Drive Kit
Many installers like to retain their Jeep mechanical speedometer, all while running a fully modern powertrain with a VSS. This electronic cable drive can be fully calibrated for a full range of tire sizes and axle ratios, and the unit itself can be mounted under the hood or in the cab of the Jeep. It's GM thread-on cable style is ideal for Jeeps.
This box allows your speed signal to be adjusted for tire size and gear ratio for accurate speedometer reading. It also allows the Jeep 3 wire square wave signal to be simultaneously sent as a 2 wire sine wave for your GM engine PCM on a manual transmission application. The GM PCM will need to be programmed to accept a 8,000 pulse per mile signal.
No known aftermarket inline sensors exist to provide the 128,000 pulse per mile (PPM) signal required by these GM PCM's. However, these PCM's can be programmed by your tuner to scale this input signal for proper operation. If you are having your GM Gen. III+ computer programmed by Novak, let us know if you are using this sensor and we'll perform this change at no extra charge.