Honda VTEC Engagement and KMH to MPH Converters


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.

Welcome back…

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.

Method 1:

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.

End result:
-Speedometer reads in miles per hour
-112mph limiter removed
-No VTEC in 1st gear.

Method 2:

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).

End result:
-Speedometer reads in mph
-VTEC in 1st gear
-112mph limiter still in place.

converter

 

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.


9 comments

  1. i have a Honda Civic Coupe 1.7VTEC and my speedo diplay is not workin. it comeon know and they. i need to buy a new one, where can i get one? is there any common fault with this speedo?

    PLEASE HELP!

  2. If you’re just looking for a replacement, try a local scrappy. You could try following the VSS wire from the sensor on the gearbox to the speedo unit and make sure its connected properly – especially if its a JDM and the converter has come loose :)

  3. Just had a link to this article from
    http://www.ek9.org/forum/general-chat/18782-converting-mph-no-vtec-1st-gear.html

    And just to say: Please READ what I have written. It does not say you WILL lose VTEC in 1st, it says you MAY, depending where you position your converter.

    Further to this, a direct link to the PreludeUK owners club where this information is commonly known:

    http://preludeuk.forumup.com/viewtopic.php?t=25650&highlight=vtec+1st+gear&mforum=preludeuk

    1. Hi Leonard , just wondering if you can help. I have a 2002 Honda step wagon k20a I have purchased an electronic speedo converter that I’m gonna put behind the dashboard . I know your instructions are for a prelude but there is a green left multi plug but I don’t seem to have the colours you said . Any ideas
      Regards Dan

      1. Having had a quick look, there seems to be some information out there but I think it’d be a matter of comparing the information online to what you have in your car. If the K20A ECU is the same across the models (you’d assume so!) then the wiring diagrams would be similar. I’d suggest looking for a pin out of the ECU or Speedo for the VSS Signal wire. The power and ground are the easy bit – use a multi meter to figure out which they are.

        Although it is a different car, it’s still K20A based so may be of some use (no guarantees!): http://www.k20a.org/forum/showthread.php?t=230

        It suggests A18 on the ECU (white and green striped wire) might be the VSS Input and the VSS Output is on 25 (blue and white wire) – if this blue/white exists on the speedo connector that might be the one, the voltage will fluctuate on this wire as speed changes.

  4. from where i can buy the converter.
    My car display digital in km and i need it in order to be able to register the car in uk.

    best regards
    dragos
    07823601279

    1. Hi,
      Plenty of Jap specialists sell the converters and replacement speedo panels etc – just Google it to find a supplier. I couldn’t recommend any in particular as my car came with it fitted and I never had to go through the process myself.

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