Product Info -> VSS Frequency Divider

Vehicle Speed Signal Frequency Divider Module

This product is under development - (Available soon)


Many replicas use an aftermarket vehicle speed sender (VSS) as an input to the digital speedometer. Popular choices include the Cyberdyne 8901 (8 pulses per rotation) and the Summit Racing TBD (16 pulses per rotation). Early third-generation Firebirds generate a vehicle speed signal from an optical sensor and small buffer module in the stock dash assembly and send this signal to the ECM, which it uses for various control functions. The optical sensor detects the rotation of a reflective metal strip on the back of a cable-driven mechanical speedometer.

If the stock speedometer is removed from vehicles so-equipped, then the ECM will not receive a road speed signal, resulting in the following issues:

  • failure to engage the TCC (Torque Converter Clutch) Lock-Up solenoid on the THM700R4 automatic transmission at road speeds above TBD MPH. This in turn will result in reduced gas mileage and potential transmission damage.
  • failure of the ECM to "tune" the idle speed using the IACVs (Idle Air Control Valves). The procedure to set these requires that the car be driven at 40 MPH for at least one mile. Without a road speed signal, the ECM has no means of detemining when the vehicle speed exceeds 40MPH nor when it has travelled one mile at this speed.
  • "Check Engine" light on the dash and an associated fault code '24'

This page describes a "VSS buffer module", which may be used to take the output from senders such as the Cyberdyne 8901 or the Summit Racing TBD and generate a signal equivalent to the stock VSS signal required by the ECM.


The stock dash on vehicles of model year 1986 and later may be removed without issue because on those vehicles the VSS signal for the ECM is not generated within the dash itself, but is generated by a module located behind the passenger side of the dash. This module feeds a VSS signal to the stock dash to drive its speedometer and a separate signal to the ECM. In summary, on early vehicles, the "VSS" connection to the dash is an OUTPUT from the dash to the ECM, while on later vehicles, the "VSS" connection is an INPUT to the dash from the buffer module located behind the passenger side of the dash.


Module Function

The circuit described in this document takes a road speed signal from a 16000ppm or 8000ppm sender, divides its frequency by 8 or by 4 respectively to produce a 2000ppm signal (equivalent to the signal from the stock sender) and outputs this reduced frequency signal as an open collector drive to the ECM. It is only required on replicas of model year 1985 and earlier from which the stock speedometer (and associated VSS sensor and buffer module) has been removed.


Below is a schematic diagram of the VSS Buffer Module

Theory of Operation

The circuit is based around a CMOS 4520B binary counter (IC1). The signal from an 8-pulse or 16-pulse VSS is fed into the circuit and the output is taken from the 2nd or 3rd stage of the counter respectively to give a 2-pulse signal, which is fed to the ECM. The CMOS IC is powered from a 5V regulator (REG1), while the input is assumed to be a square wave switching between 0V and any value up to battery voltage. If the VSS has an "open-collector" type output, then an additional pull-up resistor should be added (eg 1k) between the input of this circuit ('IN' terminal) and the supply voltage ('IGN' terminal). VSSs encountered to date (including the 16-pulse Summit Racing SUM-G2887-1 and the 8-pulse Cyberdyne 8901) do not have open-collector output, but produce square-wave outputs without any external pull-up and thus no pull-up is included in the default configuration for this module. The input signal is buffered via resistor R1 and transistor Q1, such that a 0V to 5V square wave signal is generated at the CLOCK input of IC1 (pin #1), regardless of the amplitude of the input waveform. Input pulses are monitored by the LED1, which is current-limited via resistor R2. The IC1 produces waveforms at its various outputs which are divided by two at each successive stage. Thus, the frequency of the signal at output Q0 is one half of that of the input waveform, while the frequency of the signal at output Q1 is one quarter of that of the input waveform, etc. A jumper should be fitted in one of the two alternative positions to allow selection of the output from either Q1 ("divide by four" for 8-pulse VSS) or Q2 ("divide by 8" for 16-pulse VSS) in order to give an output equivalent to the 2-pulse signal from the stock sender. Output pulses are buffered by resistor R3 and transistor Q2 and monitored by LED2, which is current-limited via resistor R4.

Prototype Layout

A prototype was built on copper strip board, as shown below.


Position of Cut Traces on Rear of Copper Strip Board

Finished Prototype Build

Production Module

At this time, no production version of this module has been designed. When available, details will be given here. As with all KoE products, anyone is free to build their own from the designs given here, but KoE accept no responsibility for any damage or personal injury resulting from incorrect build or installation of products built using these designs. KoE also offer modules built to the prototype design shown on this page for a nominal cost - please use our General Inquiry Form if interested.