This is a step by step rundown of how to install a Jaycar Speedo Corrector. It was done on my EF Fairmont, but can be applied to other Fords as well or other cars for that matter.
The Jaycar Speedo Corrector is very useful when changing Diff Ratio's or going a wheel/Tyre size different to factory specs. It retails for $50 and can be bought HERE
It is advised to get someone to build it for you as its not for beginners, it cost me an extra $60 for an electronics shop to solder it all together.
First you need to work out what percentage your speedo reading is out as opposed to the real speed you are travelling. Below is a link to work out how much percentage your speedo reading is out. I changed my diff ratio from 3.23 to 3.73 for more acceleration, so in my case it was reading 15.48% too high. Remember the figure you come up with.
(Link is to another Ford site)
Before you start, with a small flat screwdriver, turn the S2 dial to "A", and the S1 dial to "2".
Also place a link ONLY onto LK2. Make sure there are no links on LK1, LK3, LK4, LK5.... you will do this later....
Now the Jaycar Speedo Corrector has 4 long wires coming out of it that need to be attached. Power, Earth, Speedo Input & Speedo Output. First find a source for ignition power and an earth. Best place is the cigarette lighter. The plug going to the cigarette lighter has a blue/white strip wire which you tap into for Power, and a black wire you tap into for Earth.
It is best to solder the joins but im hopeless so i just crimped the connections. Tape up the wires once finished.
Now to connect the Jaycar Speedo Input & Output wires. Find out what the Speedo Input wire going to the cluster is for your car/model. In my EF Fairmont it is the white wire (pin 4) on the right hand side connector which plugs into the cluster. Be aware there was also an identical white wire next to it which was in pin 5..... dont use this one.
Cut the correct wire and join the loom side of it to the Jaycar "Speedo Input" wire. Then join the cluster side to the Jaycar "Speedo Output" wire. I just twisted the wires together because there was no room, probably not recommended though...... Tape up the ends when finished.
Now your ready to "tune" it. Place the Jaycar unit with the box lid off in a temporary visible spot where you can safely monitor and adjust it. You can now refit your speedo cluster making sure the cluster connectors plugged back in.
Take the car for a drive for a minute, check if the Jaycar unit is flashing. (NOTE: The speedo is not supposed to work at this point.)
IF its not flashing, then remove the link to LK2 and place it on LK1 and drive the car again to check for flashes.
Once youve got the thing flashing, Then pull the car over and while its idling, change the S1 and S2 dials both to "0".
Now place another link onto LK3 (leaving the LK1 or LK2 link on) and take the car for a drive again, the speedo should be working as it was before you started.
If the speedo is still not working, remove the link to LK3 and place the link on LK4, drive the car and check if the speedo is working, if not try the link on LK5 and check.
Once the speedo is working, pull the car over and while idling, turn the S1 and S2 dials to the percentage your speedo is out, which is the percentage figure you worked out at the top of the page. S1 is single units and S2 is in lots of ten's. For example, To correct 15.48%..... turn S1 dial to "5" and S2 dial to "1".
You can only correct in 1% increments so i only corrected it by 15% leaving it read .48% high on the safe side.
By default, the Jaycar unit is set to drop the speedo reading down by whatever percent you apply to it, as most people need it to read lower from diff ratio changes etc.
If you find you want it to increase the reading instead, or you find somebody at the factory changed it to read higher (like i found), you must turn the S2 dial to "F", wait 4 seconds and it will flash telling you if its set to increase the speedo higher or lower. One flash means its set to decrease the speedo reading, and two flashes means its set to increase the speedo reading. To change it, turn S2 back to any number for a few seconds and turn it back to "F" waiting for the appropriate number of flashes.
Once its set to read the right way, turn it to the correct number you need for percentage correction
Drive the car and the speedo should now be correctly adjusted. Screw the lid onto the box and place inside the dash somewhere, i found in my EF Fairmont that it fits snugly underneath the Climate control. Tape up any excess wire and tuck away. Finished!!! Its not as hard as it sounds!!!