On a lot of Japanese Imports the speedometer has been converted from kilometers per hour to miles per hour for ease of use on British roads. This is done by essentially altering the signal that is sent to the speedometer by a factor of ~0.6 (there are 0.6 miles in a kilometer).
The alteration is performed by a little black box which sits at some point between the Vehicle Speed Sensor (VSS) in the engine bay and the Speedometer on the dash. With this little box in place and altering the signal, the speedometer will now display measurements in Miles Per Hour and the odometer will also begin to clock in miles rather than kilometers.
If you want to know how the little black box works, read on. If not, then skip the next paragraph :P
The VSS sends a series of pulses to the Speedometer and the frequency of these pulses is used to determine how fast the vehicle is travelling. What the little black box does, is alter the frequency of the pulses to produce a resultant difference of a factor of 0.6, thus reducing the reading that the speedometer gives – i.e. it displays the speed effectively in mph.
Anyway, all is well and good, unless you have a Honda VTEC engine – in which case a little more thought needs to go into the placement of the converter. VTEC gives you a boost of power at a certain RPM and activation is controlled by the ECU. Based on my Honda Prelude, there are three conditions which must be met in order for VTEC to engage:
– The car must be up to normal operating temperature
– The car must be travelling faster than 14kmh (or mph)
– Oil pressure must be high enough
The car is setup as follows:
VSS –> ECU –> Speedo
In theory, you can put the converter inbetween either the VSS and the ECU or the ECU and the Speedo. Either way will produce a reading in mph, but the position used will produce different side effects, especially on Honda VTEC engines.
Fit the converter between the VSS and ECU, in the engine bay.
The VSS is located just on the side of the engine block towards the top of the gearbox. It has three wires coming from it: +ve (yellow/black striped), -ve (black) and the speed signal (orange). The converter needs to fit in-line with these wires.
-Speedometer reads in miles per hour
-112mph limiter removed
-No VTEC in 1st gear.
Fit the converter between the ECU and the Speedo unit, just behind the actual speedo.
The wires required are located on the green, 10-pin connector towards the top left of the speedometer unit (as you look at it). The wires required are +ve (yellow), -ve (black) and the speed signal, which again is orange (sometimes orange with silver dots for some reason).
-Speedometer reads in mph
-VTEC in 1st gear
-112mph limiter still in place.
Why does the position matter so much?
Imagine this scenario:
You are accelerating in 1st gear, and given that all the above conditions are met, VTEC will engage at the correct revs. However consider that now you have the MPH converter in place. This is effectively turning the 14mph condition into a limit that is either out of range of 1st gear or right at the top of the rev range where VTEC engagement would be pointless for such a short time.
In other words, by fitting the converter before the ECU, all the subsequent signals related to the vehicle speed are now in MPH internally, but the ECU expects them to be in KMH. By fitting the converter after the ECU, the internals stay in KMH and only the visual output (i.e. the dials) are changed to MPH.
As for the 112mph limiter…well, thats another story ;)
The main point of this post is to hopefully help anyone with a Honda import vehicle who is unable to get VTEC in 1st gear. For more information check out PreludeUK or other Honda specific forums.