JR's Place - JoeCool's 1987 Ford Thunderbird Turbo Coupe - Motor Trend's 1987 Car of the Year

Build Your Own
Custom Instrument Cluster

Speedometer Pre-Calibration

The Ford Electronic speedometer uses pulses produced by an electronic sender to measure speed and distance; the sensor is located in the transmission. I figured by using an electronic signal source that it should be relatively easy to pre-calibrate the Autometer electronic speedometer before it was actually installed. This procedure would have two benefits. First, the process will make the speedo read at least close to the vehicle speed before performing the actual rolling two-mile calibration described by Autometer. Second, the new odometer could be made to indicate the vehicle's real mileage instead of starting over again at zero mileage. I really liked that idea since I didn't want to have to constantly try to calculate the actual vehicle mileage; I just wanted it to be there on the odometer like it's supposed to be!

It was actually pretty easy to do both. The calibration took just minutes, but running the electronic odometer up to indicate the proper mileage took quite a while. Only three connections were needed to the Autometer speedo; ground, power (+12 volts), and the "sensor" signal. I used an old Circuitmate function generator to simulate the sensor signal. The generator was set to output a sine wave, with the output level set between about 0.5 and 1.0 volts. You can use a digital multimeter set to measure AC volts to obtain the proper level out of the generator.

Precalibration Connection Diagram

Speedo Calibration Schematic

If you don't have access to a 12 volt DC power supply, you could connect to your vehicle's 12 volt battery to obtain the needed voltage. However, the speedo doesn't require much current to run, so I suggest obtaining an inexpensive, low current (1 - 2 amp) 12 volt power supply from Radio Shack or somewhere similar. They can be quite useful for other projects, and are pretty handy to have around the shop.

The first step in preparing the new electronic speedometer for use in the new cluster is to run the new odometer up to the existing mileage as indicated by the old odometer. Begin this process before installation in the new cluster. This best accomplished in two stages. First, connect the new speedo to the calibration setup and apply power and the speed sensor signal (from the generator). Next adjust the signal frequency (Hz) until the speedo indicates 160 MPH. Then prepare to wait... a looong time; at 160 MPH indicated speed, a day (24 hours) of real-time running will put about 3,840 miles on the new odometer. If you have 100,000 miles on your vehicle, be prepared to wait about 26 days to get there. Be sure to check the mileage often - trust me, you do not want to overshoot your mileage target! Stop the process about 200 miles shy of your actual mileage; the final synchronization will be completed just prior to installation of the new cluster in the vehicle.

While the odometer is being run up to the actual mileage, build your new cluster. When the new odometer mileage has been run up to about 200 miles less than the actual mileage, and the new cluster is done and ready to install with the exception of the speedometer, you want to perform the actual speed calibration as described below. You'll need to remove the original instrument cluster from your vehicle before proceeding with the calibration; it's used to set the generator at the exact frequency to indicate 60 MPH.

The actual procedure is pretty straightforward. First connect the old speedometer (which has already been removed from the vehicle) to the calibration setup. After energizing the power supply, apply the sine wave signal and adjust the signal generator frequency so that the old speedo reads 60 MPH. This is going to occur at a signal frequency around 133 Hz. Next remove the signal (without changing the setting) and power. Finally connect the new speedo to the calibration circuit and follow the Autometer calibration procedure, with the following difference: Instead of driving two miles to perform the calibration, apply the generator signal for exactly two (2) minutes. The more accurate you are here, the closer the calibration will be when the cluster is installed in the car.

Now install the new electronic speedometer into the new instrument cluster, and complete the odometer synchronization (new to old) exactly as described above. When the odometers both indicate the same mileage, it's time to install the new cluster into the vehicle. Presto, all done; you've got a new, accurate, custom-built instrument cluster!

Some Notes: (1) Assuming 8000 pulses per mile for the vehicle speed sensor, mathematically calculating the frequency needed to make the speedometer indicate 60 MPH yields a value of 133.33 Hz (cycles per second). Thanks to Mike Walsted, NATO member for this calculation. Empirical measurement at the time of calibration indicated a value of 139 Hz for 60 MPH. The discrepancy between these two values is slightly over 4%, which, given the age of the equipment involved, is probably within the margin of measurement error; 133.33 Hz is most likely the correct value. (2) After getting the new instrument cluster installed, you can calibrate the speedometer to the proverbial gnat's eyeball using a GPS unit. Find a low-traffic two mile stretch of highway and perform the Autometer calibration procedure using the GPS unit to measure the required two miles. Then go for a ride and see how the GPS indicated speed matches the speedometer's speed. If it's too far off for your liking, do it again; it took three tries before the author was satisfied with the calibration.

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This article will be modified and added to as I have the inclination, so stay tuned!
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Copyright © 2003-2017 by D. Joseph Frazier
- All Rights Reserved -

Please note that while I've been as careful as possible with this article,
it may contain mistakes. I assume no liability whatsoever for any
detrimental effects using this information may cause to you or your vehicle.

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