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GEN V LT1/LT4 VSS and tach signal generation

I’ve had quite a few inquires lately in regards to how to power and drive your conventional mechanical or electronic speedometer from the new GM 8L90E 8-speed transmission.  After talking at length with GM Performance and Powertrain they have provided a few solutions that should work.

The 8L90E transmission is a second-generation model that uses an electronic controller built within the transmission itself.  If you’re using the GM “Connect and Cruise” system it includes a separate controller (black rectangle with blue connector above right) that communicates back and forth between the engine ECU and the transmission itself.  This system is part of the CANBUS or GMLAN electronic control system and is for all practical purposes standalone, they don’t want you piggybacking off the system because you can potentially disrupt the network transmission protocol.  For the 8 speed automatic transmissions, the vehicle speed information is fed to the transmission controller over the CAN lines. The Vehicle Speed Sensor connector (on the wiring harness) is not used because there isn’t one on the 8L90E to plug into. The ECM is programmed and looking for 40 pulses per revolution.

I have successfully piggybacked off the VSS signal from a T56 manual transmission and generated the correct speed using the Dakota Digital VHX gauge package with their BIM 01-2 module.  Using the VSS signal from an ECM this way requires a single wire input into the Dakota Digital bus.  Since the signal is generated outside the GMLAN, a simple pulse generated inductive signal from a magnet, I believe it doesn’t disrupt the signal going to the ECM (see my LT1/LT4 installation guide for more guidance).  This is necessary because both the LT1 and LT4 Gen V engines require a VSS signal, without one they go into ‘limp mode’ and run at 1/3 throttle. Set up this way my engine and transmission appear to function correctly because I do not get any check engine codes of MIL lights.

GM Powertrain has brought to my attention that pin 14 (grey/blue wire) out the transmission controller (as referenced above) will provide a non-CANBUS raw signal (pulse generated) that should be able to drive a conventional electronic speedometer. This is an inductive type signal so voltage output is dependent on speed.

One simple solution available is a conventional shaft mounted VSS signal generator like this:

Summit Racing sells this part: https://www.summitracing.com/parts/dak-sen4165/overview/. This is a VSS magnet kit with pick up coil. Normal rear wheel drive domestic installation takes four magneta on the drive shaft with a pickup coil mounted approx 5/8″ away.  It seems odd that you would use a old school mechanical magnets spinning around on the outside of your driveshaft to get a signal that your speedometer can read, but it’s a least a solution.
A little bit more elegant solution is this split-collar speed-sensor that is used on data-acquisition systems:

Another option is a GPS driven signal like this:

The best solution seems to be the recently released STA-100 ODBII interface that not only handles the speedometer issue, but also provides a tachometer signal.  There is also pin C (white wire) output from the bulkhead connector that provides engine speed, but it’s a CANBUS signal and will not drive an aftermarket electronic tachometer unless it’s OEM.

The STA-1000 plugs directly into the OBDII diagnostic connector, providing a user-adjustable traditional Speedometer, Tachometer and Check-Engine output for your choice of aftermarket instruments.

Features:

  • Fully compatible with common aftermarket electronic speedometers 
  • User selectable output of: 4k, 8k, 16k PPM signals, Sine or Square wave options 
  • +/- 35 percent speed signal correction, compensating for tire size or gearing changes
  • Adjustable 4, 6 or 8 cylinder tachometer signal output
  • Check Engine light output provides a grounded (-) output to activate a check engine light of your choice
  • Automatically calibrated output for an aftermarket cruise control can be activated by making connection to the higher-resolution fixed VSS sensor
  • Small footprint measures 4-3/4″x 3″x 1″ making it easy to hide
  • Compatible with J1850 and CAN OBDII protocols

Stay tuned for more….

 

Gen V LT1/8 speed “Connect and Cruise” install on a ’55 Chevy 2 door post

I have made a wee bit of progress, the two steps forward part.  Motor is in, small amount of cutting and grinding and welding…well, ok, a lot of that.  Note I cut off a bit of the motor mount to have access to the oil port for my oil pressure gauge.  Put on the Drive Junky belt system today.  Found out that I can use a rack and pinion steering which takes care of the header clearance problem. 

The fuel tank is ready to be TIG welded (Rick’s Tanks).  However the 500 # gorilla is still in the room, that’s the 8 speed, and yes, you are right, 1st gear is like 4.56,  I found out that the rear end is 2.20:1.  Not sure if that is within the parameters of the ECM, might have to swap gears later.  I have to get the steering in before I can move the car back over to the lift side and slide the tranny under the car.  Then I can see how much of the 60 year old floor I have to cut away.  Anyway it is some progress. – Harry Abbott, Oak Harbor, Wa.



 

New Serpentine drive system for Gen V LT4 with power steering

Drivejunky has just come out with their new system that features the supercharger belt that only runs one thing, the supercharger. The belt length is short, and runs a gates heavy duty automatic tensioner, similar to their limited production Magnuson and Whipple blower builds. The tensioner can be seen under the custom machined cover on the front of the engine. This drive system simplifies and shortens the belt run that the factory engines run in Corvette and Cadillac CTSV configurations. The blower drive, and accessory drive belts feature Gates’ new RPM Racing belt made from a High-modulus, low-stretch polyester tensile cord with the strength to transmit load more accurately with less vibration. All accessories run on a separate belt behind the blower drive. This is the first system that solves the potential bearing failure problem by running the GM type II pump with a 6 rib belt (normal) rather than the 8 rib blower belt.

 

Gen V LT4 PCV solution

If you have a problem with the stock PCV sump and connections, Mighty Mouse have a number of solutions for both the LS and the LT1/LT4 series of crate engines. This kit replaces the factory PCV valve stopping airflow (and oil) into the floor of the supercharger on Gen V LT4 DI engines (2015+ Z06 and CTS-V) LT4 PCV ‘ported’ fitting re-routes the crankcase flow to the outside of the engine, so that it can be used for external ventilation.  The PCV can be set up with an -6AN inlet from valley cover using included adapter.


http://www.mightymousesolutions.com/product-page/cts-v-3-mild