The aim of this document to help the new and old owners of a Nissan Skyline GT-R R32 to come to a decision as whether to mechanically convert the vehicle’s speedometer from KPH to MPH or leave the speedometer in KPH. I found myself in this same decision making position after I purchased a 1994 GT-R in Japan through an agent (a very good one at that) and had to make the same choice myself (I chose not to convert but it is a matter of opinion). I had noted that a number of owners on various forums were claiming that they believed the conversion was causing problems in the normal operating conditions of the vehicle – this document is the result of the research prompted by those owners (thanks to all of you ☺).
To aid in this decision I have highlighted potential problems that this conversion may create. I have researched this project using the R32 Service Manual (highly reliable but scanned in a very fuzzy way!), internet research (not very reliable) and some garages who carry out conversions (fairly reliable but wish to remain anon). Other information has been gained from the sites of manufacturers of vehicle speed sensors, anti-lock brake systems and of course books I currently own (Vehicle and Engine Technology by Heinz Heisler and Internal Combustion Engine Fundamentals by John B. Heywood amongst others).
If I get something slightly incorrect or just plain wrong please let me know!
Speedometer Cable
This is a mechanical rotating cable that is attached to the transmission of the vehicle and rotates at a rate that is in a fixed ratio to the speed of the vehicle i.e. road speed of the vehicle if no tyre slip is occurring.
Vehicle Speed Sensor (VSS)
This is a device that converts the mechanical rotation of the speedometer cable to an electrical one. In most cases this will either be an optical VSS or magnetic impulse VSS like that used in ABS sensors. The VSS works off the speedometer cable and is located inside the speedometer at the back of the instrument panel in the R32. The wiring diagram does not give an indication of the type of VSS as normally the LED type would have two small arrows emitting from it.
The optical types use a photo cell, light-emitting diode (LED), and usually a two blade mirrored reflector to generate an electrical pulsed signal. The LED is powered and emits light whenever the ignition switch is on. When the vehicle is in motion, the speedometer cable spins the two bladed reflector. The reflector rotates through the LED light beam breaking the beam two times for each revolution of the reflector. Each time a mirror passes through the LED beam, the light is reflected to the photo cell. When the mirror isn’t breaking the beam, no light reaches the photo cell. Whenever light hits a photo cell an electric signal is generated.
The signal from the VSS is normally used for the following reasons in most modern vehicles…
• Cruise Control
• Auto Transmission Shifting
• Digital Speedometer
• Idle and low speed fuelling map
• High speed economy mapping modes (highway mode)
• Idle air control valve
• EGR valve (emissions control)
• Canister purge cycle (emissions control)
• ABS and Traction Control operation
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