The VSS provides an indication of road speed to the ECM/PCM. The sender is either a electronic Hall switch or an inductive type, which sends pulses directly or via the dashboard circuit to the ECM/PCM.

If the ECM/PCM receives no pulses from the VSS input, a fault, Code 24 will be set in the ECM/PCM. This will result in increased fuel consumption, poor idle quality or stalling can be experienced. Vehicles fitted with TH700 automatic transmissions will have no torque converter clutch lockup, and vehicles fitted with 4L60E transmissions will not change gear.

The VSS is vital for correct operation of the following functions:

• The ECM will shut off the fuel during deceleration. The VSS must be operating correctly for this to occur.
• The ECM has an in-built memory, it learns the correct amount of fuel to deliver under certain conditions. The VSS plays a vital role in determining when these conditions occur.
• The Commodore automatic transmission has an in-built torque converter clutch, which reduces the efficiency loss of the automatic transmission by manually coupling the input shaft to the engine, under certain load conditions. These conditions are met when the engine coolant temperature is above 42°C, the vehicle speed is above 72 Kph, and the throttle is open. Without a vehicle speed reference this function is not possible.

When adapting the TH700 GM transmission to a four wheel drive transfer case the extension housing is replaced by an adaptor housing.

The original GM VSS is removed with the rear extension housing.

Marks 4WD Adaptors have overcome this problem by designing an in line VSS. The unit fits to the transfer case, where the original speedometer cable is fitted. The cable is then fitted to the top of the new VSS. This allows the speedometer to function normally.

Marks VSS sends pulses to the pre programmed module, which interprets them for the ECM.

The ECM is tricked into believing the engine is still fitted to a Commodore allowing the ECM to function as normal.

In a standard GM application the VSS may be tested for its correct operation by driving the vehicle and noting if the vehicles speedometer is working properly or not. Marks 4WD Adaptors have built into the module, a self test circuit. A green LED light fitted to the module flashes when the module is sending pulses to the ECM.

The unit is very easy to install and only requires three wires to be connected. The unit is supplied with all necessary mounting hardware and full instructions.

Some VSS units will not fit directly to transfer cases such as the Landcruiser 40 Series which is fitted with a transmission brake. To move the VSS unit, to position it where it will clear the transmission brake and or the transfer case an extension piece is required. Marks 4WD have manufactured such a unit which is now a stock item. The unit moves the VSS 85mm away from the speedometer cable drive.