JDM Speedometer from S13 Silvia or 180sx

Back of the cluster, unplug the odometer and pull out the screws. Four for most, but if you've got the chime that's one more screw.

 Looking in here you'll see a blue potentiometer and a resistor. The picture is terrible, but I swear there's a resistor in there. This particular CA18DET speedometer has a second potentiometer on the right of the picture for the annoying chime.

 Looking at the back of the speedometer, this is approximately what you should see. Like I said, this is from a CA, so it's a little different. A new resistor will need to be soldered in here.

 You can measure the resistance across here, and it should come up around 30k ohms. This will help verify you're at the right spot. I got 29.97k ohms. If you have a frequency generator, now would be the time to get some baseline values for your speedometer. If you're going to try this (more accurate) read on below.

 You can adjust the speedometer with the potentiometer, but you can't adjust it low enough to get to miles per hour because of that base resistance. To lower the base resistance, you put another resistor in parallel which will lower the base resistance. Here's the resistor you'll want.. (12k ohm) There are other resistors you could use, but just use something close to that value.

 Here's what mine looked like after I soldered in the resistor.

 I was actually a little sloppy and you can see those two spots are joined with solder. I didn't even bother to change that, because those two are joined already.

Just for reference:

Measuring across resistor: 8.4k ohms

Measuring across potentiometer: 16.33k ohms

Important one - Tune to this:

Measuring across both: 24.73k ohms

This should be measured to the right of the spot labeled "R15" to the bottom of the resistor.

*Very important note* This does convert the odometer to miles, but will be slightly off. My assumption is that either US or JDM speedo is designed to be a little inflated from the factory, so when you try to match them it doesn't *quite* work out for the odometer. It's less than 10% off, but I don't have hard numbers.

Fine tuning of the potentiometer will allow you to adjust for different gear ratios and tire sizes.

If you need to tweak your existing speedo slightly, then the install of the resistor is not necessary for fine tuning.

Changing the KPH on the face isn't necessary if you want the straight JDM look, but if you get custom overlays, etc... you can put MPH there now.

Wave Generator only - not necessary

Read this section if you have a signal generator to work with. If not, don't worry about it. It should work fine.

I found that if you put in a square wave of 6 volts peak-to-peak with no dc voltage, you can adjust a full sweep of the needle.

Now comes the tuning part. You're going to need to power the gauge, and make sure you hook all grounds together. Just put 12 volts on IGN, hook the grounds, and put the generator on the signal wire. The fourth one is labeled 2X and is an output for the computer, don't need to use it. I put the screws partway in and clip to them.

With a small philips screwdriver tune the speedometer to match a measurement you made with the US speedo. Easy way to do it is tune to 100mph on both. Tune an input (freq. generator) signal to get the USDM gauge to read 100mph, then swap in the JDM gauge and using the same input frequency turn the potentiometer until you see 100. Here's some readings I measured:

Stock Gauge

39 Hz = 10 mph

89 Hz = 20 mph

137 Hz = 30 mph

181 Hz = 40 mph

228 Hz = 50 mph

270 Hz = 60 mph

310 Hz = 70 mph

350 Hz = 80 mph

390 Hz = 90 mph

429 Hz = 100 mph

467 Hz = 110 mph

JDM Gauge Just a few..

167 Hz = 60 km/h

273 Hz = 100 km/h

475 Hz = 180 km/h

This information was contributed to Zeroyon.com by Knate of Fresh Alloy. Thank you man!