Tech Article Title Author Date
Homelink Retrofit TDrain 2007

Overview

I was extremely disappointed to find out that my new 2007 Audi A3 3.2 didn't have a Homelink garage door transmitter installed in it. The 2006 models had it and I can only assume it was removed as a cost cutting measure. Given that most $25k sedans have one, this seems ridiculous to me - the least they could have done was make it an optional extra.

So I set out to figure out how I could retrofit some type of Homelink system. The 2006 system has three buttons and an LED in the visor but, unlike most cars, the visor is not a self contained unit and the actual transmitter is underneath the front bumper. Purchased as parts, the transmitter alone is over $400 which is way too much. The visor is also expensive and I decided that trying to get a wiring harness up to the visor was going to be too much work. So, I needed to come up with Plan B...

I decided that the easiest thing to do was purchase a Homelink transmitter, install it somewhere out of sight, and connect it to a set of buttons and an LED somewhere in the car. Since I'm planning on install a tire pressure monitoring system in the car, I couldn't use the existing button location in the dash (and I needed at least two of

While I was installing my foot well lights, I noticed that there is a nice, flat area on the back of the driver's side lower panel next to the headlight switch that would be perfect for the Homelink buttons. After some searching on the 'net, I found a set of push button switches that have a nice silver bezel and match the A3 silver/black interior perfectly. I also discovered that ebay has tons of Homelink transmitters for sale at very cheap prices (I assume these are from the junkyard but who cares). The only thing to make sure of is that the transmitter you get supports rolling codes. I paid a little bit more for one that the seller explicitely stated had rolling code support. The Homelink visors sold on ebay are completely self contained unit, you can rip apart the visor and extract the unit (I purchased one that was already bare but they're all the same underneath). The ebay search terms I used to filter out unnecessary stuff is:

homelink -mirror -overhead -console

I purchased the switches, LED, and some tiny connectors from Digi-key. Since I wanted to mount the switches on a removable dash component, I wanted to have a nice wire connector so that it can be easily disconnected just like the stock eleltrical systems.

I also picked up some Audi repair wires to use for the power supply. I'm planning or completed several modifications that need switched and unswitched power (foot well lights, Homelink, HUD, TPMS) and I wanted a nice, clean way to deliver fused power so I decided to add two fuses (one switched and one not) to the existing fuse box. I couldn't find the part number for the correct wires but the largest repair wire I new of worked fine.

Parts and Costs

Here is a summary of all the parts that I purchased:

Part Store Part # Image Drawing Price
Homelink Transmitter EBay . . ~$35
Pushbutton Switch (x3) Digi-Key EG1932-ND Blueprint $3.35 x3
8 Position Female Plug Digi-Key WM1780-ND Blueprint $0.84
8 Position Male Plug Digi-Key WM1786-ND . Blueprint $0.84
Female Terminals (x10) Digi-Key WM1837-ND Blueprint $0.179 x10
Male Terminals (x10) Digi-Key WM1841-ND Blueprint $0.179 x10
Red LED w/ Mount Digi-Key 67-1192-ND Blueprint $1.26
Audi Repair Wire 1st VW Parts 000-979-133-A . $2.18
TOTAL COST . . . . ~$55+shipping


Preparing the Transmitter

The first step in the install is to figure out how to connect the switches to the Homelink unit. Here's a shot of the Homelink transmitter that I got from ebay. You can see how it used to be part of a visor (the plastic part sit's in outside the visor and the main unit is under the visor upholstery). The plastic bezel and switches are easily pried off with a screwdriver. Then the black plastic housing can be opened by releaseing several plastic catches on the sides.

Since there are three switches and an LED, I decided to use a stranded CAT-5 network cable that I had laying around to make the connection from the transmitter to the dash board. CAT-5 has 4 twisted pairs of wires which allowed me to use on pair for each switch and one for the LED. In hind sight, it turns out that the power lead to the switches is common so I could have gotten away with using six wires. A close up of the board is show below with the correct locations to solder the wires marked with red arrows. A larger image is available if you click on the picture...



After seeing how closely spaced all the components on the PCB are, I decided to try and solder the wires two the open holes in the board instead of attaching them to the switch bodies directly. Luckily, there are open holes near the switches and LED that are perfect for this. The CAT-5 wires are extremely thin but even so, I had to remove two of the wire strands before they would fit in the holes. I just barely added a tiny amount of solder to the wires to get them to stick in the holes and then flipped the board over to add more solder. My soldering iron is a little large for this type of work so that approach worked better for me. I sized each wire so that the resulting cable would end up directly over the middle switch so I could feed it out the existing hole in the transmitter case. Here's a shot of all the wires soldered on to the board.



Once this is done, test each connection for continuity to make sure everything is working (then go have a beer). Also, be sure you write down which colors go with which switches or you'll have trouble programming the unit when you're done. IMPORTANT: Make sure you note exactly which wires (striped or solid) are attached to the LED anode and cathode terminals since those must match the LED exactly. I carefully ran the cable out the center switch hole in the transmitter case, closed it up, and wrapped the whole thing with several layers of electrical tape to keep it from moving around. You'll end up needing about three feet of cable so make sure there's at least that much available. Here's the result:



Installing the Switches

The next step is to remove the drivers side lower dashboard panel that contains the headlight switch. There are two bolts in the foot well and one in the fuse box that need to be removed. The panel then pulls straight out and you can release the three wiring harnesses that attach to the headlight switch.

Once this panel is removed, I attached a ground wire to one of the bolts that goes into the firewall using a ring connector. I used some old 10 gauge wire I had laying around from an amplifier install and then crimped a yellow butt connector on the other end with four 18 gauge wires hooked into it. This will serve as my "ground bus" for all of future electronic gizmos.

I wanted to mount the transmitter as high under the dash as possible to allow for the best range. I really wanted to remove the A-pillar for better access but I ran into trouble doing that. Even with Bentley manual instructions, I couldn't locate the mounting clips holding the trim on and there is an airbag that runs right under the trim so I decided to do the best I could from below. There's a nice spot right behind the bottom of the A-pillar to the left of the drivers air vent where the transmitter fit perfectly. I roughly measured the distance to the dash board and cut the cable from that transmitter at that length.

You can see the mounting location in the image at below. The front of the dash is at the top of the picture (you can see my ground point at the bottom left). The plastic gears and tube at the top of the picture is the driver's side air vent. I decided to mount the transmitter where the arrow is pointing using some heavy duty double stick tape.



Now that the dash board is off, the next step is to mount the switches. I started by making a template to use for the holes. I taped the template to the backside of the dash board and made sure that it was horizontal. As you can see in the image below, there really isn't a lot of flat surface to drill into - there are raised plastic bits everywhere so I felt pretty lucky that the available space was going to be aesthetically pleasing.





After the template was attached, I carefully drilled small pilot holes through the center of the circles from the back to the front. In woodworking, you always drill from the good side to the bad since the wood may tear out a little as the bit comes through so I assumed that I should do the same here. However, when I drilled the LED hole from the front side, the bit grabbled a little bit of the textured dash board material and tore it (you can see it if you look carefully at the close up picture later on). After that experience, I drilled the larger switch holes from the back to the front and they came out very nice after a little bit of sanding to remove any plastic burrs.

Here are the Digi-key parts that are used for the installation. Three switches, an LED, a male and female connector, and the corresponding pins for the connectors.



The next step is to attach wires to the LED and switches. I trimmed the leads on the LED and soldered on short (1.5') lengths of the CAT5 wires. Make sure you match up the correct wires for the LED cathode and anode to the wires you used in the transmitter. After soldering each wire, I covered the leads with heat shrink.



The next step was by far the biggest headache of the whole process: attaching the wire harness pins to the leads. These pins are extremely tiny and are supposed to be crimped onto the small CAT5 wires. I tried crimping them with various pliers (the crimp tool is over $50) with no luck. I'd strongly recommend that you get extra pins since they're very cheap and you'll probably need them. After some frustration trying to crimp them, I just ended up soldering the wires into the pins. This worked much better and faster. If I was doing this project again, I might try to find a larger connector with Audi style spade connectors. I don't really trust these molex style connectors all that much though I doubt it will get removed more than a couple of times in the life time of the car. Once the pins are attached, insert the buttons through the holes and tighten the nuts from the backside. The LED is held in place with friction alone. Here are all the parts mounted in the dash board panel.





Before I did anything else, I connected the pins from the dash components to the transmitter and hooked up a 12V power supply (8 D cell batteries) to the transmitter for a test. Unlike most of my projects, everything worked fine the first time I tried it. The LED lit up correctly and I could program the Homelink! Now that the electrical connections are all working, attached power and ground wires to the transmitter and wrapped them with friction tape part way down the cable. Then insert the male and female pins into the correct connectors (see the Digikey site for help with this) and cover the resulting connection with some heat shrink tubing. After the connectors are attached, re-test everything to make sure the pins are in the correct locations. Then I twisted the dash board component wires together, covered them in electrical tape, and routed them down towards the headlight switch so they wouldn't get crushed when the panel is re-installed. Here are the resulting two components ready to be installed in the car.



Mounting in the Car

The first step in mounting the transmitter is to get the power and ground connections configured. For a ground, there are any number of bolts around the fuse box that can be used. I used one of the large bolts going into the firewall. For power, I wanted a very clean install so I decided to add a switched and unswitched low amperage fuse to the existing fuse box. Using my multi-meter, I determined that the second row of full size fuses from the top is constant 12V and the next row down from that is switched. Both of these rows had unused locations in my car that I could use for my circuits. Two bolts hold the fuse box in place and three plastic clips can be pried open to get access to the back of the fuse box.



The only thing needed is to add a connector to the lower wire location in the correct slots (the top location is the power source). I couldn't find any information on the actual correct part number for the wires in the fuse box so I decided to use the largest repair wire I had (part number 000-979-133-A). I cut one repair wire in half and used one for each fuse. Make sure you insert the wire with the connector in the proper orientation to accept the fuse blade. This repair wire is smaller than the correct connector for this harness - it will click into place but it does wobble around a little bit. I tried inserting fuses several times and they always made a good connection so I'm not too worried about it. I wrapped each wire in friction tape and used a butt connector on the other end to tie them in to my circuits. Here are the two five amp fuses that I added for my circuits.



All of the hard work is now done. Attach the power and ground wires from the transmitter harness into the ground and switched or constant power wires. I used the un-switched power wire to allow me to use the Homelink even if the car is off but that's not required (the Homelink works fine with intermittent power). Since my car is always garaged, I'm not too worried about someone breaking in to my car to get access to the garage. After connecting the wires, I covered everything with friction tape to get a factory look and to protect the wires. Remount the dash board panel and plug in the Homelink connectors just like you would with the head light switch cables.

Here is the final result. All in all, it went pretty smoothly and I think the silver switches look great and really match the interior pretty well (they almost look like miniature air vents!).




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