Toyota Tech: Speedometer Recalibration
http://www.4x4wire.com/toyota/tech/tacoma_speedometer/ Short Cuts

| Toyota Section | Tech | Reviews | Maintenance | Minute Mods |

By: Mark Griese - 3/2004

Many times after adding larger tires, there will be a perceived power loss from the larger tires. Because of the power loss you may end up changing the gears in the axle to regain that power. Either one of these changes could cause your speedometer (and odometer) to read incorrectly. The Dakota Digital Universal Signal Interface (USI) provides a way to recalibrate your Toyota's speedometer. I installed the unit on my Tacoma, but I think it could also be used on 4Runners, Tundras, and Sequoias. Most older Toyotas used a mechanical/cable driven speedometer, so the USI wouldn't help to correct those speedometers. The value in using this unit on newer vehicles is its ability to allow for future changes. The speedometer corrections are accomplished by moving dip switches based on some simple calculations and the values in their table, so the unit will not become obsolete by a new tire size.

Universal Signal Interface

If you don't anticipate any future changes, you might try to determine if one of the factory speedometer drive gears will get your speedometer back in calibration. The USI isn't cheap, so finding the right gear would be a considerable savings. Finding it will involve determining the part number and tooth count of the available gears and is beyond the scope of this article. I'm sure instructions for replacing the drive gear can be found in the Factory Service Manual for your vehicle. I chose the USI because I wanted the higher accuracy, and I hate trying to get info from the dealers.


The two most difficult parts of installing the USI are running the wires from underneath the vehicle and providing weather tight connections. If you can handle those two things, you'll have no problems installing this unit. It only requires four wires; a power, ground, signal input, and signal output. The vehicles listed above use a three wire sending unit that can be found at the rear of the transfer case. On my Tacoma, the green with red stripe wire carries the signal, with blue being ground, and pink being the power. We only care about the signal wire sending info to the vehicle's computer. It must be cut, and have both ends extended to wherever you mount the unit.

Installation

Once under the midsection of the vehicle, look where the driveshaft meets the transfer case and you'll find the signal output electrical connector. The connector locks onto the transfer case, and the latch is on top, so be sure to squeeze that before trying to pull it off. Use a small screwdriver to disengage the clip that mounts the wire loom to the transmission. Once you have the mount off, it's possible to use the small screwdriver to undo the mount's grip around the wire loom and reuse the mount. After the mount is removed, peal back the split loom tubing that covers the wiring. After cutting the signal wire I used a White wire going to the connector, and a Black wire going to the vehicle's computer. Both of these wires will need to be long enough to reach the place where the unit is being mounted. I soldered and shrink wrapped my connections, but I believe it would also be acceptable to crimp and silicone seal them.


Signal output connector Release connector lock Signal wire was green w/red

Release the clips on the mount Undo the mount for reuse Solder or crimp, then seal

After the soldering or crimping is complete, you'll need to find a power wire that is only on when the ignition is on, and a good ground. My unit is mounted under the driver's side dash, near where the large firewall grommet can be found. On my truck there was already a grounding bolt just above the kick panel, and an empty threaded hole I could use for mounting the unit. For power, I pulled an ignition switched wire from my Circuit Boss in through the firewall grommet. Wherever you choose to mount yours, just be sure to provide enough slack wire so it's easy to do the final calibration before mounting out of sight.


Circuit boss under the hood Testing and final calibration Under dash mounting


After it's hooked up and powered up, it's time to get out your GPS receiver (or borrow one) and go for a test drive. Bring the instruction sheet with their table, and a calculator. Drive at a steady speed and check the speed readout on the GPS unit. Then pull over when safe to do so. Divide the actual speed by the reading on the speedometer to come up with a recalibration ratio. Find that ratio in the table and reset the dip switches. Then retest to confirm the settings are accurate. On my initial test, using 'OUT4' and all the dip switches set to 'on', my speedometer had a reading more than twice my actual speed.

Comments

Everything that Dakota Digital publishes warns against using this on any vehicle with anti-lock brakes. I imagine they have to include that statement to protect themselves for any liability problems. I have used this unit for over two weeks on my ABS equipped Tacoma without a single problem or ABS trouble light. I also personally know a few ABS equipped Tacomas and 4Runners who use it without problems. The installation instructions were adequate, and are available on their website if you are interested in learning more about this unique product.


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