Ceylon

iTrader: (3)

Yeah I did think it may have been done for ease of assembly. Although is having two speed sensors easier than having two clusters? Hard to say . That wouldn't explain how the aftermarket 300kmh gauge faces work accurately, though. If the odo did change with calibration and you adjusted the calibration pot right down to scale the gauge from 180kmh to 300kmh to utilise the new face scale, how would the odo count right? It wouldn't, it'd read low as it would be unaware of the face change. Unless the odo isn't affected by this calibration and the calibration is just to get the analogue face to read right.

I'll find out when my car is back together so then we will know for sure . Or I may rig up my drill and some rpm sensors to test the theory, that's likely an easier route than trying to map out the speedo board.

You're running the rx-8 crown and pinion right? So if the odo isn't corrected your odo would be out by 8.29%, so only 0.83 miles over a 10 mile journey, not overly noticeable unless you do a longer trip. But if compared to GPS would be quite noticeable over a 100 mile trip or so.

DaleClark

iTrader: (55)

I am running the RX-8 4.44 ring and pinion. I have tested over long drives with my GPS and the mileage is accurate. But, it is quite possible that the numbers lining up on the speedo don't equate to them being correct in the odometer, especially with a good deal of change, like changing R&P and going to a 300 KPH speedo, etc.

Dale

CryptoPatriot

Apologize for the bump on an old thread but for those with JDM cars did this work across a range of speeds? When im thinking about it, it seems like the scale between MPH speeds and KPH speeds wouldnt be the same. So you could calibrate a single speed of the KPH gauge to read MPH but wouldnt it be inacurate the further you get from that speed in either direction?

Markus1981

Yes I had a broken 300 speedo and changed over the face to my working 180 speedo . Calibrated it with the trimpot on the side , works like a charm . Did 2 of my friends JDM speedo too with 300 faces . Everything works as described

CryptoPatriot

Excellent. Im doing it then. Thanks!

DJF(NJ)

iTrader: (6)

Bumping this up with a special thanks to Dale for this excellent write up. I’ve had the 4.3 rear for about 15 years and this was always on my to do list when first seeing this thread many years ago. I finally got the courage to pull the cluster and attempt the work. (Another thanks to RuffRotor’s YouTube video for the excellent step by step instructions on cluster removal). I had the benefit of one of those mobile speed radar things on a backroad near where I live. With 245/40/17 tires, my speedometer was showing 60 mph when the radar indicated 54. Did the adjustment with the screw and no kidding, it is very, very sensitive as Dale pointed out. I gave it more than a slight push clockwise and it was now showing about 43 mph when the radar indicated 60. Took about 4 or 5 more adjustments to finally nail it at 60 on the speedometer and 60 on the radar.

Pete_89T2

iTrader: (4)

Bumping this thread to thank Dale for posting this up, and to provide some additional info for those of you who prefer doing their speedometer calibrations on a bench instead of the on the road. In full disclosure I haven't gotten around to doing this procedure on my FD yet, but I will as soon as I can get my hands on a suitable signal generator and an oscilloscope, which are both needed to do this on a bench.

Basically I did a little bit of reverse engineering of the FD Service Highlights and Body Electrical Manual (BEM) to figure this out...



This is a block diagram of the speedometer circuit from the FD Service Highlights document


What we can glean from this diagram and from elsewhere in the applicable BEM and FSM troubleshooting/test procedures is that the OEM speedo sensor is essentially an AC generator that produces a sine wave signal with 8 positive peaks per full rotation of its shaft. We can also glean that its peak output voltage is around ~5 VAC. I derived that 5VAC figure from this speed sensor test procedure in the BEM:





To bench test and calibrate the speedometer, we need to do the following:
1. Apply +12VDC power to pin 2D, and a ground return to pin 1E.
2. Use an AC signal generator to generate a sine wave at various frequencies (I'll get to that soon) and connect that to pin 3C (output terminal on signal gen) and pin 3A (output return terminal on signal gen).
3. To verify the signal generator output, we'll connect an oscilloscope to monitor the signal to ensure it's outputting the correct frequency, amplitude and sine wave shape.

So now that we know the amplitude (~5VAC) we need to know how to set the frequency. And that info can be easily derived from this table in the Service Highlights, which I annotated to show some math...



So if you go to the yellow highlighted 60MPH data, you just need to convert the "pulses per minute" data into "pulses per second" or Hertz (Hz) to get the frequency needed to result in a 60MPH speedo reading. Keep in mind the figures in this table are based on OEM gearing and tire sizes.

To calibrate your speedo, you'll just set up the signal generator to produce frequencies at a few different speeds on the table, adjusting the calibration pot to get it as close as you can.

DonaldDennis

Very nice info!!! This would allow for someone to do a whole build and calibrate for every change all at once. Ie wheel and tire combos and rear end ratios.