Vehicle Speed Sensors

for

GM Conversions in Jeeps

GM Vehicle Speed Sensors for Jeeps are helpful when retaining a Jeep transfer case such as a Dana 18, Dana 20, Dana 300 or NP231, and you need to get a speed signal to your GM PCM for proper operation. See the following modules, below:

If you're running a 4L60E or other automatic transmission, the speed signal is going to be captured by your Novak adapter or the transmission itself. These modules below are primarily for classic, non-electronic automatic and manual transmissions.

VSS for 1993-present SFI and Gen III+

Our #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.

Non-Cable Version

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.

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GM VSS History

Any automotive enthusiast wanting an interesting experience should open up a classic GM truck and car powertrain parts manuals from yesteryear. 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.

Thus, the Digital Ratio Adapter Controller was introduced, to use an electronic solution to simplify mechanical complexities.

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:

• Engine computer and speedometer: 2,000 pulses per mile
• Cruise control module: 4,000 pulses per mile
• Anti-lock Brake System: 120,000 pulses per mile

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.

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.

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