I'm currently planning & procuring for a 5.3L Gen III GM Swap into my '95 YJ, and I hit the GM VSS issue. I've done a lot of lurking around the internet and there are a lot of "Well, this made it work for me" type threads. This is my attempt at a broader overview & deeper understanding of the issue and the various solutions available in one place to help Jeepers decide what's best for their build. If anything needs corrected or there is more info to fill in, by all means comment. So here goes...
What is a VSS?
Background: In the recent past, speedometers were driven by spinning cables attached to gear assemblies in the drive train. At this time, pretty much the only purpose was to drive the speedometer.
Then, computers entered the scene and with them came the electronic VSS. Vehicle speed began being used for a variety of purposes such as automatic transmission control, anti-lock brake systems, and emissions systems like EGR and EVAP. For modern engine swaps, it's no longer just a matter of getting the speedo to read correctly - your computer needs a working VSS it can read.
The output of a VSS depends on how it is driven, what type of signal is produced, and how that signal is conditioned by circuitry on the sensor before it goes to the computer.
A VSS can be driven in two ways: directly with a speedo gear, or passively with a reluctor wheel (aka tone ring).
A VSS generally will produce either an AC Sine Wave signal or a DC Square Wave signal, depending on the type. Some VSSs produce AC Sine Waves naturally, but condition the signal into a DC Square Wave internally.
More Reading on Vehicle Speed Sensors:
The Gear Driven VSS
Early VSSs were just attached to the same gear assemblies that drove cable speedos (many still work this way). These gear driven VSSs come in four main types: AC Generator (AKA Magneto-resistive or MRE), Reed Switch, Hall Effect, & Photo-electronic.
The AC Generator type VSS spins an magnetic wheel past a coil. This generates an AC Sine Wave voltage. In the early 90's this type of sensor was common on GM TPI (tuned port injection) drivetrains in a 4 PPR (pulse per revolution) configuration. As TPI engine swaps are popular, there are still many adapters available to convert cable speedos to this type of output. It should be noted, other manufacturers usually include on-board signal conditioning to convert the AC Sine Wave to a DC Square Wave at the sensor.
A Reed Switch is a literal switch that mechanically changes position when close to a magnet. I believe GM TBI-equipped vehicles used 2 PPR Reed Switch sensors. The output of a Reed Switch VSS is a DC Square Wave.
Hall Effect VSSs are similar to and often confused with AC Generator VSSs. They use a spinning magnetic wheel, but instead of a coil they use a Hall Effect Integrated Circuit to drive a DC Square Wave output.
Photo-Electronic VSSs, as far as I can tell, were never used by GM, AMC, or Chrysler. For the sake of being complete, I'll include them here. They use an LED & Photo-diode to read slots or holes cut in a spinning wheel. Many RC servos, old ball mice for computers, and many other non-automotive applications use this same technology to measure speed and position.
NP231 t-cases from 1992 on came stock with a gear driven Square Wave output VSS. Prior to that, they used a cable-driven speedo. I'm not sure whether they were AC Generator, Reed Switch, or Hall Effect, and really it doesn't matter as all three can be made to output the same Square Wave signal.
The Reluctor Wheel VSS
Electricity tends to flow along the path of least resistance. Likewise, magnetic fields tend to concentrate along paths of low reluctance. Iron has low reluctance, air has high reluctance.
This instructables, while I wouldn't recommend using something cobbled together like this on your rig, shows what's inside the sensor for a reluctor wheel VSS: http://www.instructables.com/id/Magnetic-speed-sensor/
You'll see, the sensor has a permanent magnet and a coil. When a tooth on the reluctor wheel passes the sensor, the low reluctance steel of the wheel changes the shape of the magnetic field. When you move a magnetic field past a wire, you generate electricity. So in essence, this works as an AC Sine Wave Generator.
GM uses a 40 tooth reluctor wheel to drive their VSS in Gen III+ setups. It's located in the tailhousing of the transmission on 2WD setups, or the tailhousing of the t-case in 4WD setups.
The Rubicon 241OR t-case came stock with a tone ring, which I believe has 3 teeth. As the signal from the VSS is compatible with your standard Chrysler computers & speedometer, the sensor probably has on-board signal conditioning to convert to a DC Square Wave output.
A Word on Signal Conditioning
For a signal to be read by a computer, it must be conditioned into a digital signal at some point. GM likes to do the conditioning at the PCM. On the VSS, Chrysler likes to do some of the conditioning at the sensor.
Something about how GM processes the VSS signal at the PCM seems to make it incompatible with Square Wave DC VSS signals. I'm an electrical engineer, so I know a little bit about signal processing (not my field, though). Maybe they are using a thyristor in the setup that requires current to go reverse direction to read a pulse.
SYE Kit Possible Solutions
JB Conversions Super-Shorty SYE with GM 40-tooth reluctor - $340-$425 (depending on yoke selected):
Plug and play solution, which is probably what I will do for my build.
Novak Ultra-Short SYE with 3-tooth reluctor - $269:
I think it could be possible to either fab a 40-tooth reluctor that would fit this kit, or program the GM PCM for 3 PPR, and use a GM VSS instead of the included Chrysler VSS. To do this, the VSS port might have to be modified to fit the GM VSS. I'm not sure how the two VSSs compare in size or thread pitch, but the distance from the end of the VSS to the reluctor is critical, and would need to match whatever GM originally designed. Even then, I'm not sure a 3 tooth wheel would produce enough voltage at low speeds. The PCM is expecting 0.5V minimum.
JB Conversions NP231J HD Super Short SYE with 3-tooth reluctor - $299+:
Probably could be modified as described for the Novak kit. If you read JB's marketing, they are pretty proud of this kit. I'm pretty sure the Brand X kit they compare it against is the one sold by Novak (not sure who manufactures it).
Advance Adapters True Pulse Tailhousing Kit - $243.92 + $199 (Fixed yoke kit required) = $442.92:
This is Advance Adapters' 40-tooth reluctor option for a t-case mounted VSS. Note that this kit comes with a GM VSS ($153 if you buy it alone from AA), while the JB Conversions option does not (they say they can provide one at extra cost, but not how much extra cost). This setup would retain whatever Jeep VSS is on your current rig at the expense of a few inches length over the JB super-short option.
Cable Speedo Adapter Solutions
There are some options for adapting cable outputs to provide the AC signal preferred by GM PCMs. I'm not certain, but it should be possible to adapt later-model NP231 gear driven electronic speedo outputs to work with these adapters.
Novak GM 8-pulse VSS - $62.91:
Speed & Position Sensors for Jeep Conversion Applications
According to Novak, it's just a matter of programming the correct pulse count into a Gen III+ PCM to make this particular adapter work.
Dakota Digital GM 8-pulse VSS - $29.95:
Speedometer 8K Pulse Generator
If Novak's works with a PCM reprogram, I don't see why this one wouldn't.
Dakota Digital GM 40-pulse VSS - $119.95:
128k High Frequency GM Pulse Generator
Dakota Digital says this is a square wave output and a direct replacement for 40-tooth reluctor wheel systems
This goes against what I've read & been told elsewhere, that GM PCMs require AC Sine wave VSS signals.
Jags That Run Diff-mounted VSS - $104 Reluctor + $40 bracket = $144:
I don't really like how exposed this is to flying gravel for a Jeep application, but it could work.
Advance Adapters 4L60E / 4L65E VSS 40 pulse kit - $243.92:
This kit mounts in AA's GM transmission to Jeep t-case adapters. This is by far the most common solution I've seen used around the web on Jeep GM Gen III+ swap threads. It has the added benefit that you don't have to mount a switch on your Jeep t-case to give the PCM a low-range indication. I'm using a manual transmission for my swap, so this is no help to me.
Dakota Digital SGI-5 Universal Speedometer Signal Interface - $84.95:
This is what Novak used to recommend for electronic speedo conversions, and I've seen a few recommend it around the web, but success seems to be mixed. This has the advantage of being tuneable to your setup. The disadvantages: it can only go up to a 4X multiplier of the input signal (12 PPR for your Jeep Reluctor setups). It won't get you to the 40 PPR the PCM wants to see.
Homemade - <$100
I've seen a few threads where people fabbed their own setup similar to the Jags That Run differential mounted solution or the Advance Adapters setup that sits next to the t-case yoke.
Absolute Cheapest Option:
The Dakota Digital cable-to-8-pulse setup would probably come in at the lowest cost for a setup either with or without a SYE kit. SYE kits built for cable speedos tend to be cheaper anyway. However, I haven't seen any builds on the web that went this route.
Most Reliable Options:
Any option that uses the ACDelco VSS and a 40-tooth reluctor that is sealed inside a housing is going to tend to be the most reliable and straight forward. This narrows it down to the Advance Adapters option that goes between the transmission and t-case, and the JB Conversions SYE. Personally, I favor the JB Conversions option since I'm going with a manual transmission in my build (manual transmissions don't need the low-range switch). I would probably still prefer that option even if I were going with the stock auto transmission. It's not a big deal to me to mount a limit switch with a custom bracket to give low range indication.
- PCM Programming for Lower Pulse Counts - We know it can be done, but is it a good idea? Has anyone dropped the PPR from 40 to 3 or 8 in their PCM with success? Are there issues with idle or mpg while cruising due to the low resolution signal?
- Square Wave vs. Sine Wave - As an electrical engineer, this issue bothers me. Most circuits I can think of to process a pulse count wouldn't care if it was Sine or Square - once the voltage passed a threshold it would count that as a pulse. However, If GM is using a thyristor instead of a transistor to condition the VSS output for the PCM, that would require the signal to be AC. Thyristors turn on when you show them a positive voltage, and stay on until you show them a negative voltage.
- Has anyone tried replacing the 3-tooth Rubicon style reluctor with a custom 40-tooth reluctor? Did it work?
- Has anyone tried replacing the Chrysler VSS with a GM VSS on a 3-tooth Rubicon style setup? Did it work when the PCM is programmed for 3 PPR?
- If you have done a Gen III+ swap and retained the computer, what was your solution? What did it cost? Did it have any issues?