Quick Mods - Speedometer Corrector

So, After driving the car for a little while with this wheel and tire package, the speedometer has finally driven me nuts.

I knew it would be off, but it's harder to deal with than I thought.

In comes a post I came across over on my350z.com: DIY: Speedometer corrector

The unit I ordered is the SpeedoDRD U1 by 12oclocklabs, the same unit the original poster used.
It's a simple little single board that's actually pretty powerful. It'll let you do +/- spedo percentage corrections all the way up to 100% (double), as well as convert MPH/KPH or vice versa, and even remembers your top speed for later recall.

But installation requires a bit of cutting and soldering. Thankfully, I stashed the entire FSM for an 08 350z in my Google Drive. It makes it accesible anywhere and easily searched. Finding the wiring diagrams and information was just a search away.

Before I started the whole process, I took note last time I drove the car of two different speed measurements, one from the GPS on my phone, and the other from the car's digital speed readout on the multi-gauge. Going 67MPH per the GPS, indicated I was doing 74MPH on the dashboard. Using the handy-dandy calculator 12oclocklabs provides on their website (here) that even tells you how to program the unit.

Digging thought the FSM I found how the gauge cluster works.
Under the stereo, what I thought was just the HVAC control unit actually drives the gauge cluster and triple gauge stack. In the case of the speedometer, it converts the CAN-BUS data being fed from the ABS system into a 0-5V signal that's fed to the gauge cluster, and then from there to the triple stack. It's this signal that we're going to intercept and modify. If you are going to try the same thing, but don't have the resources of an FSM.. the guys over at MightyCarMods did a similar modification with their own kit, and show how to find the signal wire with a multimeter: MCM Speedo Corrector

[caption id="attachment_152" align="aligncenter" width="225"] The AC Module under the stereo. All the connections we need to make are on the middle connector.[/caption]

The white wire is the aforementioned signal wire, so we have to cut it in half.

[caption id="attachment_153" align="aligncenter" width="300"] Snip the white wire, but make sure you give yourself enough distance to make repairs if necessary.[/caption]

With the signal wire split, we need to splice in the SpeedoDRD. The plug side is the source of the signal, and the lead goes up to the cluster. So we connect the white wire from the SpeedoDRD to the plug side of the white wire, and the blue wire which is the output from the DRD to the line that feeds the cluster.

[caption id="attachment_154" align="aligncenter" width="300"] The signal wires connected.[/caption]

I used my soldering iron to do the job, and the provided heat shrink tubing. Next up, the unit needs power. The black wires on this connector are ground, and the Yellow/Green wire is an ignition power wire and will do the job just fine.

[caption id="attachment_151" align="aligncenter" width="300"] Power and Ground Connections.[/caption]

A little electrical tape here and there, and it's good for a test.



A short while later, a test drive comparing to the speedo output of my tablet confirms the car's speedo is now dead-on accurate. Now I know if I'm not speeding enough!