How-To: "C" Tail Lights on a Base/Touring/Limited (incl. LEDs)

[italianguy63]

Preface: Read this entire post carefully, and beware there is a possible danger in doing this mod. We will take no responsibility if damage occurs. This could possibly void your warranty, etc. etc. etc. This post is for information purposes only. Proceed at your own risk.

Model(s): Base, Touring, and Limited models

Difficulty: (1 to 10, 1=easy) 6. If you are intimidated by cutting or splicing wires, or are "electronically challenged," please seek help . This post assumes you are comfortable using a soldering iron and heat-shrink tubing.

Time Required: Approx. 1.5 hours

Cost (for Mod): $25 or less.

Tools and Materials: Wire cutters, wire strippers, single edge razor blade, soldering iron, solder, heat shrink tubing, black tape, short lengths of wire, LED ballast/resistors (optional).

Theory: The brake lights on the "C" are on their own circuit (different wire from the Touring/Base); the brake lights on the Touring/Base are on a different wire that combines the brake function with the turn signals.

You can use the "C" harness to make the brake lights work alone, but you lose the function of the turn signals. If you use the stock harness without isolating the brake/turn signal... the turn signals come on when you hit the brakes (not acceptable).

The Forward Control Module (FCM) controls the functions of the lights-- and we found no way to change the "software/firmware" involved. Hence, the purpose of this thread-- you need to do a little rewiring at the FCM to fool it to make the lights work right.

In short, this mod makes your car (electonically) have 4 front turn signals and eliminates the rear ones. It isolates the turn signal function from the combined brake/turn functions on the Base/Touring model.

The next page is the background which explains this in more detail. You can skip it if you just don't care!

[italianguy63]

Background:

I am aware that many other folks have tried to figure this problem out before I tried to tackle it. For example I know "Lukeinva" worked alot on it. Since I am relatively new to the forum, however, I am ignorant to most of the history. However, Luke did give me some head's up info. and allowed me to ultimately figure this out. He's the one that told me the FCM was the villan.

Another fantastic gentleman of this forum "Mr. Incredible" helped me extensively on this project. He is extremely educated on the inner workings of the 300, and he also owned a "C" model for reference. Since we are opposite ends of the country-- there were a lot of pictures taken and a lot of e-mails sent. We probably asked every "what if" question imaginable. Ultimately, the solution to the problem was pretty "brute force"-- which I find depressing, because we actually had some pretty clever ideas throughout the process.

Mr. Incredible and myself started out by pouring over the schematics and parts lists. Then we began disecting our cars starting at the tail lights and moving forward. We found the differences in the rear fuse blocks, and I did a bunch of cutting & splicing of wires to try to isolate the turn function from the brakes; and combine the brake function to the 3rd brake light. The brake lights unfortunately are tied to other things back there (such as the ESP relay). The reason I am telling you this is... I tried everything we could think of and about 3 weeks of time to ultimately realize that the brake/turn function is controlled by the Forward Control Module (FCM), and there was no way to circumvent it. Both cars have the same part number for the FCM, so the issue is a "flash" issue. Without having the ability to reprogram the FCM and knowing what this function is tied to in the FCM data... it became a nearly impossible task. If someone has the ability to decode the FCM firmware/software there could possibly be an easier fix. But, it was out of our reach without extensive equipment.

[italianguy63]

Step 1: Location of Components

Under the hood on the passenger side, you will find the IPM (Integrated Power Module) -- this is a really pretentious name for the fuse block. Attached to front of the IPM you will see a silver box called the FCM (Forward Control Module).

Important: Disconnect the battery and wait 10 minutes before working with the FCM. It is static sensitive, and quite expensive if you zap it. Use proper anti-static procedures while doing this!

Release the FCM/IPM assembly from the inner fender by releasing the 3 clips.

Shortly, you will be removing 3 of the wiring clips from the bottom side: C1 (Green, on the IPM), C2 (Black, on the FCM), and C3 (Black, on the IPM). Look at the attached pictures and diagram so you can identify their shape and location.

[italianguy63]

Step 2: Identify Plugs and Wires

Be careful with the plugs—they are somewhat complex to take off. C2 is removed by sliding the red “keeper” towards the base then squeezing the plug near the wires.

C1 and C3 are removed by sliding the keeper, then pulling the “lever” type lock away from the plug (it pivots), then squeeze the plug near the wires. Reattach the plugs later in reverse order.

Use the razor blade to cut off about 6" of the black wrapped insulation from the wiring bundles so you can identify the wires. Be carefull not to nick the isulation on the wires.

The WHITE/YELLOW wire on C1 is easy to locate. It is in the corner of the plug. See picture.

The WHITE/DARK GREEN wire on C3 is tricky. There are 3 wires with "green" next to each other at the top of the plug. The WHITE/DARK GREEN one is the “middle” one. Be sure to identify it before cutting!

The wires on C2 are on the bottom of the plug WHITE/TAN and WHITE/LIGHT GREEN (and next to each other). Refer to the pictures, and be confident you have the right wires before you strip back the isulation to splice the other wires on.

[italianguy63]

Step 3: Cutting and Splicing

The right rear blinker is controlled from plug C1 of the Integrated Power Module at pin 17. That wire is WHITE/YELLOW and goes back to the rear lamp. Cut the WHITE/YELLOW wire on C1 and splice it into plug C2 of the FCM at pin 8. That wire is WHITE/TAN. This signal controls the right front blinker.

Similarly, the left rear blinker is controlled from plug C3 of the Integrated Power Module at pin 3. That wire is WHITE/DARK GREEN and goes back to the rear lamp. Again, cut the WHITE/DARK GREEN wire on C3 and splice it into the plug C2 of the FCM at pin 10. That wire is WHITE/LIGHT GREEN. This signal controls the left front blinker.

This places the rear blinkers in parallel with the front blinkers and will bypass the "brake light" signal sent to the rear (Base/Touring) lamps via the non-"C" FCM firmware/software.

Note: I did not cut the wires on plug C2. I used a razor blade to remove a bit of the wire isulation, then wrapped the stripped ends of the wires from C1 and C3 on the exposed portions of the (C2) wires. Solder the 2 connections and wrap with black tape. Leave about 2 inches of wire on plugs C1 and C3 to allow you to attach ballasts/resistors. It is also neccesary to add extra length to the wires coming from C1 and C3 to keep the install tidy.

Re-wrap the three wiring bundles with black tape to retain the "factory" look of the install. Take your time and do it right! Remember to leave the 2 (cut off) short leads from C1 and C3 accessible for the next step.

Reconnect the 3 plugs to the IPM and FCM.

Now, the FCM realizes there is an (open) fault with the (now missing) rear turn signals. A "fast flash" scenario occurs, so dummy loads (ballasts or resistors) will need to be added to the two unattached leads and tied to ground to fool the FCM. In the next step you will attach the two hanging leads from C1 and C3 to ballasts or resistors and then the other end to ground. That will fix the "fast flash" issue. If the "fast flash" situation does not bother you, you can just insulate the two leads with heat shrink tubing and/or black tape, and skip Step 4.

[italianguy63]

Step 4: Adding ballasts or Resistors

The FCM now thinks the two rear turn signal bulbs are burned out (because they are disconnected). The FCM requires a load of 3 to 4 ohms resistance so that it does not "fast flash." In short 2 resistors or LED ballasts need to be added to the short leads from C1 and C3 and tied to "ground" (-12V). This can be accomplished in many ways, but this is my suggestion:

There are cheaper ways to do this, but I wanted a very clean, factory looking solution.

I found a motorcycle "dual ballast" solution on the Internet. It is great because it is very small, good looking, and very well made (aluminum case, epoxy filled, etc.).

The product is made by Signal Dynamics Corporation under the name "Dual-Load Equalizer" part number #01008. You can easily find one online from almost any motorcycle parts shop. Mine came with a part number of #340-300 and cost $18.

Turn Signal Module

Simply connect one of the "equalizer" wires to each of the short pieces of wire you left coming out of the ICM plugs (see pic).. again use solder and heatshrink (note the red heatshrink in the pic).

Connect the 3rd wire (black) to ground. The shiny FCM case looked like a great candidate! So I soldered (and crimped) a lug onto the black wire-- and put it under the bottom left screw holding the FCM to the ICM. The wire lengths are perfect as is.

I just double sided taped (supplied) the SDC equalizer to the mount for the FCM/ICM on the left side (see pic). It is almost hidden from sight and barely fits.

If you choose to use resistors or single ballasts instead-- remember they get HOT. Keep them isolated from wiring, etc. Note: The "cheap" fix is (2) 3.5 ohm 5 watt ceramic resistors from Radio Shack.

[italianguy63]

Step 5: Reassembly and Tail Lights

Re-clip the IPM/FCM assembly back onto the inner fender.

In the trunk, remove each of the stock tail lights by removing the 3 large wing nuts. Remove the light from the fender, and unplug the harness by first sliding the keeper and disconnect the fixture from the harness.

Just replace the tail lights with your new "C" type tail lights. The harness for the "C" type light is built into the fixture (don't swap out the wiring!). Just plug the new lights in.

Remember to reconnect the battery, and check to make sure everything works right.

Enjoy!

Here is a before and after picture of my car:

Cheers!
Italianguy63 & Mr. Incredible