How to install a LA3 computer into a Merkur XR4Ti
by Aaron Loynes

Well, today I decided to install an LA3 with a "big" VAM in my XR4Ti, and seeing as I had a digital camera - I decided to take pictures of each step along the way.
The first thing I did was go to and read how to do the LA3 swap, then I went to, did a search on LA3 installations and read all I could on there. I then gathered up everything I would need, which is coincidentally everything that you will need if you plan to do this installation: Procedure:
Okay on with the "how to".

First thing you will want to do is get pins out of your EEC wiring harness. To do this first pry off the black cover from over the wires, then turn the harness so you are looking at the end the computer plugs into - you will see a red piece that is holding the pins in place - that needs to come out. Using your small screwdriver pry that piece out. (refer to photo below)

(click for full size image)

Red piece removed...

Once you have the red piece out all you have to do is grab onto the wires you want to remove and pull - they will pop out. Some stick harder than others, but the will not break if you pull on them. I have been told that turning the pins as you pull them will help get them out if they are not coming out easily (tip from Tony Vissoc). You will only need 3 pins, and I chose to use the pins that "belong" in the spots we are going to add pins... so I pulled out pin 2, pin 24, and pin 25 from my TC wiring harness.

You will then want to cut a length of wire long enough to get from where you plan to place your ACT sensor to where your EEC mounts - and solder one end to the colored wire of the ACT harness, and the other end of the wire to the pin you plan to use. The other wire on the ACT harness may need a foot or so of wire added to it so you can reach a suitable ground or signal return wire with it.
Next I soldered about a foot and a half of wire to the pin I was going to use for Premium Mode grounding (red wire in the picture below). Then I soldered enough wire to get from the EEC to the brake pedal for the "Brake on/off" pin (green wire in picture below).
Now gather up your stuff... and you are ready to go out and install it!

The first place I went was under the hood. Now I wasn't able to pull my lower intake manifold to tap it like you are supposed to... (on the firewall side of the lower intake there is a fitting where the ACT sensor is meant to go on cars that run it... you just have to remove your lower intake and drill the hole out with your 9/16" bit, then tap it with your 3/8" NPT tap, screw in the ACT sensor, and you are good to go.) I read all the posts I could find about ACT placement, and I learned that you can really put it anywhere in the intake after your intercooler - you can even just stick it under the hood in a "cool" spot if you want. That is the option I chose for the time being - so we are skipping the ACT sensor into the lower intake part of this "how to".
Anyway... so under the hood the first thing I did was poke a hole in the rubber grommet that is on the drivers side near the brake booster, and feed my ACT sensor wire through it. (Green striped wire with electrical tape on it)

Then I routed the ground from the ACT plug along some other wires to the TPS signal return wire, which is black with a whire stripe and used a "tap in" connector to splice it in. Any ground or signal return wire will work for this, or you can just ground it to the intake manifold.

(its in the middle of the picture)

Then, since I didn't tap my intake for the ACT sensor yet - I zip tied the sensor to the drivers side fender until I can get a place to do the lower intake R & R.

Next step is pretty easy if you are already using a cone filter... just remove the hose clamps on the VAM, unplug the wiring harness, and take it out. Attach the big VAM where the stock one was... plug it in. The end. If you still have the factory airbox, I understand that you can "wedge" the big VAM into it using the stock hoses - but I haven't tried this method (from Bill Williams). Another option that Tony Vissoc suggested was using the factory air filter, but ditching the frame. Again I haven't tried that, but apparently it works.

Now its time to move on to the inside of the car for one more easy step - tapping into the brake on/off signal. To do this the easy way - remove the lower dash on the driver side, to do it the way I did it - just look up under the dash and you will see a pressure switch attached to your brake pedal. Remove the wiring harness from the switch and use your volt meter to check the current. One side of the harness will read 12V, the other side will read nothing. Tap your "brake on/off" pin that you prepared earlier into the side that read nothing. No picture of this, was too tight of a squeeze under there.

Now on to the "scary" part of the swap... pulling pins out of the harness!
First thing to do is remove the sound deadener from under the dash in the passenger footwell. Mine was already removed a long time ago, so I don't have exact details on how to get it out. Next you want to get your computer out - this is best accomplished by lying on your back in the footwell and looking up under the dash. You will see your computer staring right at you. On the "passenger" side of the computer there are 2 tabs holding it in place that you can push toward the passenger side to get one end of it free - then just pull it down and to the passenger side and it will come down.

Use your 10mm socket to loosen the bolt on the wiring harness, and remove the computer.

Just like earlier, you will want to pry off the black cover from the harness, and remove the red pin retainer. (Pictures for that are above).

Now comes the fun part - moving pins... Be sure you push them in all the way - until you feel a click.
I have pictures of each step so you can see the color of the wire involoved... please note these steps are from an email by Don Sweigart, so he gets all the credit for this information.
Step 1: Pull out pin 43 and push it into pin 27. This connects the VAF signal.

Step 2: Pull out pin 25 and push it into pin 43. This connects the VAT signal.

Step 3: Pull out pin 32 and push it into pin 31. This connects the boost controller signal.

Step 4: Pull out pin 35 and insert it into pin 33. This connects the EGR signal.

Step 5: Put the pin you chose to use for your Premium Mode into pin 24 and attach the other end to ground. I chose to use the ground to the passenger side footwell light. (pictures are blurry, low batteries so couldn't use flash)

Step 6: Plug your ACT sensor wire into pin 25.
Step 7: Finally plug the pin coming from the brake pedal into number 2.

Here is a blurry picture of my harness after the pin install:

Now you ready to re-assemble your harness - so put the red retainer back in first, put the black piece back on, then plug in your LA3 and tighten the 10mm bolt down.
Before you put the computer back up - start the car and make sure everything works!
You will find that putting the computer back into its clips is a real pain - but it does go back!!

Okay... well hopefully that was helpful to someone!