Speedo correction

BMThiker

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I've been daily driving my FJC since July 2006. I've had larger tires on it since Nov 2006. I put bigger gears in the diffs a little bit later.
But I've been relying on my ScanGauge II for an accurate speedometer reading all this time. I calibrated it between 6-7% depending on what my GPS readings showed as true speed. Not only that, but my odometer is off by the same algebra.

Last week I installed a Dakota Digital speedometer controller. I found out from my friends at ACC Garage that the price of these gizmos has dropped significantly. The bonus is that because of the top mounted dash box, the installation on FJCs make this a super easy mod to do.

I got mine on AMZ: Amazon.com: Dakota Digital SGI-5E Latest Design - Universal Speedometer Signal Interface Controller: Automotive
The only tools you need are: a philips head screwdriver, wire snips & pliers, and small slotted screwdriver.
Supplies you need are: 2 butt splices, 2 add-a-taps, some small gauge (18ga) wire, and a zip tie or two.

Remove the dash box to reveal the top of the gauge cluster. Two phillips head screws secure it. Once the screws are removed, grab the hinged side and pull up. The edge of the box closest to you is secured under the edge of the dash by two tabs, so lifting from the hinge side will pop the clips that hold it down at the back, and it will hinge towards you as you remove it.

Find the dash wiring harness connector on the left side. Unplug it and peel back about 6" of electrical tape wrapped around it.

Locate the red w/blue stripe (power), the white w/black stripe (ground) and the gray w/red stripe (speed signal) wires. These are locations 17, 5 and 28 respectively in the harness plug.

Cut four pieces of your 18ga wire about 6-8" long and strip one end on each. Strip back both ends on two of these. If you have different colored wire available, it may reduce confusion. I just labeled my wires with blue tape.

With the first piece of wire insert your add-a-tap around the red w/blue stripe (17) power wire and crimp in place. Label this lead "power". Hint: you insert the unstripped end into the add-a-tap, leaving the stripped end free.

With the second piece of wire insert add-a-tap around the white w/black stripe (5) ground wire and crimp in place. Label this lead "ground".

The gray w/red stripe signal wire gets snipped in half. Choose a spot about 3-4" from the harness plug and cut it and strip back both tails.

Butt splice each tail of the signal wire with two pieces of 18ga wire that have been stripped on both ends. Label the lead nearest the harness plug "signal out". Label the lead still in the wire bundle "signal in". Strip back the free end of each 18ga lead.

Now the 4 free ends that you have should be labeled 1) power, 2) ground, 3) signal in and 4) signal out. These all get placed into the matching slots on the Dakota Digital converter 1) power, 2) ground, 3) "signal in" and 4) "out 2". The converter has many more slots (and functions) but we're only using these 4.

Using a small slotted screwdriver, turn the screws on top to secure the wires in the slots on the side.

Now you're ready to calibrate. Don't zip tie anything and don't put the glove box back just yet! There are two buttons on the bottom of the converter. Go ahead and turn the key on and bump the calibration up (+) to 1.05 (translate this as 5% above normal). Drive on a long stretch of road and use a GPS or your smart phone to give you real time data. I suspect if you are running 35" tires you'll be in the 6-7% range, but get it dialed in as best you can. If you can use cruise control, then that makes life easier too. Push "Inc" (+) for up and the "Set" (-) for down. Don't worry about any other programming that comes with the converter.

Now that you have it dialed in, you can zip tie the converter box into place so it doesn't flop around. Just make sure it doesn't interfere with the replacement of the dash box when it drops back in.

That's it, you're now calibrated on the dash, and if you're like me, you may even notice that the transmission shifting seems a bit smoother. I honestly thought the transmission was strictly reading the throttle position sensor and wheel speed sensors, but it apparently gets speedo signal from the dash cluster.
1908225
1908226
 

1911

chupacabra
 
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... you may even notice that the transmission shifting seems a bit smoother. I honestly thought the transmission was strictly reading the throttle position sensor and wheel speed sensors, but it apparently gets speedo signal from the dash cluster.
Maybe that is why my cruise control still drives me crazy, even after re-gearing the diffs to compensate for 34" tires.

Good write up, thanks Rick.
 

-MaTTi-

 
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Finland
But I've been relying on my ScanGauge II for an accurate speedometer reading all this time. I calibrated it between 6-7% depending on what my GPS readings showed as true speed. Not only that, but my odometer is off by the same algebra.
...
but it apparently gets speedo signal from the dash cluster.
You’ll notice that the ScanGauge show now the speed right.

I have done the same mod too, but I did the corrector by myself:
1908563

 

BMThiker

I aim to misbehave
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Yep, reset my ScanGauge back to 0% on the speed settings.
 
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