Speedometer Calibration / DeMetrification

[CTScott]

If you are wondering what the heck DeMetrification is, it is my new term for modifying a device intended to display in Metric units to display in English units.

Last month I finally figured out how to reprogram the odometer on our instrument cluster to allow for easier swapping of clusters (http://www.yarisworld.com/forums/showthread.php?t=41461).

The most common reason for wanting to swap clusters it to gain the OEM tachometer (for those in the US with 2009 or older manual transmission, non-S package Yaris, which did not have the tach). The problem is that S package MT liftbacks were not very common, so few clusters from them are available on eBay or from junkyards. On the other hand, all MT Canadian liftbacks had tachometer equipped clusters, so Kmh clusters are very easy to find for reasonable prices (typically under $100).

The problem, for those south of the border, with running a Canadian Kmh cluster is that (besides the large-number primary speed display being in Kmh) the odometer increments in kilometers.

After driving around for a couple day with an oscilloscope attached to the speed sensor input to the cluster, I was able to develop a device which measures the pulse width of the signal and replicates that signal, with the pulse width proportionally adjusted to allow the cluster to display in MPH as well as increment the odometer in miles. I also added a feature, which allows the user to make a fine adjustment to calibrate the speedometer to a GPS to account for variations in tire diameter, etc.

The device has four wires that connect to the right side connector on the speedometer (power, ground, speed signal in, and speedometer out). Normally the speed signal goes into the cluster and then out of the cluster (unmodified) to the ECM. In my initial testing I found that feeding the modified signal to the ECM on my automatic transmission Yaris caused some really odd shift point changes (since the ECM controls the AT). To eliminate this I simply disconnected the ECM output wire from the cluster and connected it to the input to the cluster (so that it sees the unmodified signal).

Note that once connected and calibrated, this device would not be mounted to the dash, but would simply be tucked under the instrument cluster.

Here is video of it in action. I apologize for the shakiness from my headrest mount.
http://youtu.be/KkUOgEvTYAY