Speedometer and Odometer Technical Details
The following text is a joint discussion by a fellow Supra owner and
myself regarding the operation.
It conflicts slightly with my other details, but that's minor. Just follow the schematic diagram and you'll be O.K.
The other Supra member was named Steve Chohan. The author has had many alias on the UK Supras list but I now know this person to actually be James Ashley who admitted this on the MkIVSupra.net BBS.
This text is replicated from an email posting to the UK Supras web site from James (alias Steve Chohan) with modifications from myself. Information posted on the UK Supras list is public domain and not copyrighted but credit is given to the main contributor of the information on this page.
ALL equipment (cruise, spoiler, speed limiter and AC Amp) is hanging off T7 pin 6 (i.e. the odometer re-broadcast signal) and should receive the ORIGINAL speed signal from speed sensor 1- with one exception in a particular case regarding the Cruise Control.
The anomaly lies in thinking that the speed signal from No.1 speed sensor, on a Jap car is a KPH signal and on a UK car it's an MPH signal.
The signal is THE SAME regardless. Both the
Jap car and the UK spec car has a speed sensor that produces 4
pulses per revolution of the wheel.
In other words, the original signal from No.1 speed sensor is NOT a KPH or an MPH one. It's just "the signal" from No.1 speed sensor. Both signals are the same whether you have a UK or Jap spec car. For example, if you have a UK car the speedo will read MPH and will be calibrated in accordance with the same frequency of speed signal as if you were driving a Japanese car in Japan with a speedo reading KPH. Both speedos would be reading the SAME frequency of signal. The difference would lie in the actual speedometers. Likewise with the odometers, whether you were driving your MKIV Japanese Supra in Japan with your odometer registering KM's or whether you were driving your UK Supra in Britain with your odometer reading MILES:
BOTH odometers would be reading the SAME speed signal. The difference lies in the actual odometer units, NOT in the signal that the odometer is fed with.
You see, that's precisely where the converter guys who modify the actual speed sensor signal go wrong: in thinking if they divide the source signal by five-eigths then all else will follow. No way! You screw up your car that way.
The cruise problem, i.e. stopping at 76mph or thereabouts can be solved by feeding it with an MPH signal. That way you have a cruise control that works from 40mph to 125mph, i.e. just when you need it. And that's the exception. Don't ask me why it stops at 76mph.
So, in converting your Jap spec car, you have the choice of ......