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Circuit Doctor 01-21-2015 01:20 PM

LED headlights + Foglight solution
UPDATE 4-4-2015:
Writeup procedure added below!

Hi Guys!
I just picked up a beautiful 1996 LT4 Collector Edition over the weekend. :woohoo: What an awesome car! That LT4 just pulls hard throughout the entire RPM range.

As C4 owners are certainly aware, the headlights are very lacking. I decided I wanted to purchase the GE Nighthawk LED headlights for the car. I installed them, and I am blown away by their performance. However, like everyone else who bought them, my foglights no longer function properly and cause the high beams to switch on.

Since my background is in electrical engineering, I reviewed the schematic of the headlight system and observed the foglight relay's coil gets its ground through the high beam filament. The purpose of this is to disable the foglights while the high beams are on. Due to the higher impedance nature as well as internal circuitry of the LED headlight assemblies, the foglight relay's coil is acting as a psuedo pull up resistor for the high beam input terminal, thus illuminating the high beams when hitting the foglight switch. In fact, if you were to open your hood and switch on the parking and foglights only, you'd see the high beams flicker lightly as a result of this. The solution is to disable the high beam feed to the relay's coil, and ground the relay's coil directly, or provide a low resistance path to ground for the relay's coil.

Anyway...I'm going to put together a write-up with detailed pictures to outline this when I'm not stuck working 7 days a week. The beauty of my plan is that you will not need to hack any of the factory wiring, and add one jumper wire. Would you guys be interested in this? :thumbs:

________________________________________ ________________________________________ _______________________________

Before performing this procedure, please be aware of the following:
You are performing this procedure at your own risk. I am not in any way liable, either express or implied, for any damages caused by this procedure, either deliberate, accidental, incidental, or circumstantial including, but not limited to; damage to vehicle, injury, fire, death, invasion by aliens, or snide remarks from friends. If performing this procedure, having basic electrical knowledge is a plus, as is good hand-eye coordination. The steps outlined below using the tools and materials below will ensure optimum performance and reliability and has been tested with successful results; by deviating, you may be unknowingly compromising the aforementioned parameters. It's a good idea to disconnect the battery before starting this.

- #2 screwdriver with tip in good condition
- Wire cutters/strippers
- Soldering iron (optional)
- 7mm socket and socket handle (recommend 1/4" drive) or 7mm nutdriver
- Pencil
- Xacto knife

- 168 lamp ("Long life" recommended)
- T-3 1/4 socket (Dorman P/N 85816)
- Scotch super 33+ electrical tape (yields highest thermal and adhesion performance)
- 2 wire ties
- Solder flux (optional)
- Rosin core solder (optional)

Raise the hood and rotate the headlight manual adjustment knob until the light is positioned like this. Remove the two Phillips head screws fastening the shroud. Make sure the screwdriver is like new, these screws will strip out very easily with a worn tip!
(Note: You can install this modification on the driver or passenger side, it doesn't matter. I installed on the driver side)

Remove the remaining two 7mm bolts fastening the shroud, and remove the shroud as well.

With the shroud removed, remove the four Phillips head screws holding the headlight retaining ring ONLY. Do not touch any other screws, or the alignment of the headlight capsule will be altered! Then remove the headlight and unplug the electrical connector.

A) With the headlight now removed from the vehicle (or out of the box if you're installing the new light for the first time), look at the connector attached to the light. Orient the connector so the 3 pins create the letter "U"; it will look like there are pins at the "East", "South", and "West" locations, and there will be no pin at the "North" location. What will needs to be done is expose and splice into the wires which connect to the "East" and "West" pins. This H4 pinout diagram should help clarify (this is shown as if the male pins face the observer):
On my GE light, these wires were red and white.
B) Using the xacto knife, carefully, and GENTLY cut the insulation around the entire diameter of the wire in two locations about 1/4" apart from each other. Then, join the cuts together, creating an "H" shape. Repeat this on the wire facing opposite the one that was just stripped. NOTE: Be very careful doing this, when the blade feels like it has hit metal (you'll know), don't cut any farther. Avoid cutting into any of the wire strands.
C) Once the insulation has been cut off the two wires, peel the insulation off to expose the copper strands. Using the pencil, "part" the strands in half.

A) Prepare the #168 socket by shortening the wires to approximately 2" long each. Any additional length is unnecessary. Next, strip approximately 1/2" of insulation off each wire of the #168 socket.
B) Wrap each wire from the #168 socket through the middle of the strands, so it looks like this.
C) (Optional, but highly recommended) Gently push the strands all back together and solder the #168 socket wires to the headlight wires. I highly recommend the use of solder flux in order to achieve the best solder joint possible; this is recommended because these connections will be subject to frequent shock and vibration. At this point, the splices are finished.
D) Wrap each wire with electrical tape.

Tuck the wiring back into the split loom tubing, and fasten the socket to the tubing with two wire ties, like in this photo. Don't forget to install the 168 bulb into the socket!

From here, the reassembly is the reverse of the removal steps.

Once all this is finished, your foglights and highbeams will function as originally intended!

Here's a technical explanation for those interested:

Starting on the top of page "8A-100-1", and ending on page "8A-100-0" (Current path is highlighted in yellow, and the arrows indicate current flow direction). The current into the relay coil originates at the headlight switch, then travels through the foglight switch when desired. The current then travels through the coil of the relay, into node "A" and onto page 8A-100-0. From node "A", the current travels into "S208", into the high beam filament, and finally returns to ground. Since the resistance of the new LED headlights' high beam input is so high, there is not enough current flow through the relay coil to pull it hard to ground, thus rendering it inoperative. Additionally, the new LED highlights' high beam wire can act as a control signal which simply "tells" the high beam to turn on - the coil of the relay provides this current path from the battery since it has a relatively low resistance in contrast to the high resistance of the high beam input.
The solution is to:
A) Provide a low resistance from the relay coil to ground by wiring a #168 lamp in parallel with the high beam wire into the new LED headlight to ground. This, in turn, will restore the original functionality. This also will pull the voltage of node "A" much closer to ground potential, which will no longer cause the high beams to switch on when the foglight switch is turned on.
B) Disconnect the "ground" side of the foglight relay's coil from S208, and ground it directly to chassis.

Clear as mud??

bogus 01-21-2015 03:12 PM

What you say makes sense.

I was watching "Fast and Loud", with their KITT build... and they ran into a similar common ground problem with their LED tail lights and the signals/brakes...

How does this sound... replacing the fog light bulbs with LEDs. Wouldn't that balance the resistance?

V60050 01-21-2015 04:34 PM

Please do "Circuit Doctor"!

Circuit Doctor 01-21-2015 08:30 PM

Originally Posted by bogus (Post 1588772490)
What you say makes sense.

I was watching "Fast and Loud", with their KITT build... and they ran into a similar common ground problem with their LED tail lights and the signals/brakes...

How does this sound... replacing the fog light bulbs with LEDs. Wouldn't that balance the resistance?

It's actually a little more complicated than that. I'll try my best to explain it more clearly. In order to turn the foglights on, a relay must have a voltage applied to its coil, which moves a solenoid, resulting in a switch being turned on or off. In this case, the switch connects the actual foglight bulbs to the battery [or alternator].

A relay is basically a magnetic device which acts as a voltage controlled switch, whose purpose is to allow a low current circuit to control a high current circuit.

In the C4, there is a small button next to the headlight switch which turns the foglights on and off. This switch connects the relay's coil to 12VDC, the current flows through the relay's coil, and passes to ground through the filament of the high beam. Since the relay's coil is in the order of hundreds of ohms, as opposed to the resistance of the headlight filament being less than 1 ohm, the headlight is not visibly lit due to a very small current passing through it. But the plot thickens...see next paragraph!

One of the many benefits of LEDs is they require less current to illuminate than incandescent lamps. Without having a schematic of the LED headlights, my suspicion is that the low beam wire into the LED headlights is the main power source to the light (in other words, provides all power for high and low beams), and the high beam wire is just a signal which "tells" the light to switch on the high beam. By contrast, the incandescent lamps receive separate distinct 12VDC battery voltage from two different circuits.

Does that help? :crazy::willy:

bogus 01-21-2015 09:42 PM

yes. I am not an electrical engineer, but I am pretty savvy when it comes to wiring. I am a bit weak on theory tho, thank's for the excellent explanation!

If I am reading this right, the actual relay is the problem, not the light. The lower resistance of the light, for all intents and purposes, makes the relay redundant. the relay is a remote switch designed to allow the use of smaller wire to control something that requires heavier wire. In short (bad pun), the LED by its very nature removes the high current draw...

In a perfect world, the relay could now be totally removed, if the fog lights had LED bulbs.

IIRC, the entire C4 headlight system is solid state, no relays.

gdl2165 01-21-2015 10:08 PM

Circuit Doctor, I for one would be extremely thankful if you could provide detailed instructions on this. We had a thread going on this very issue about a month ago. I tried a couple of the simpler proposed solutions, including trying the LED fog lights, all with no positive results. This last weekend I spent some time looking over the wiring schematics and came to the same conclusion as you, it's nice to have confirmation that I was on the right track. I'm just not sure where to tie into the wire in order to ground the fog light relay so if you can provide details instruction there I'd greatly appreciate it.

Cliff Harris 01-22-2015 05:10 AM

Originally Posted by gdl2165 (Post 1588775838)
We had a thread going on this very issue about a month ago.

That thread is here:

The problem arises because the fog lights were grounded in different ways in different years. That thread has circuit diagrams that show the variations in the connections.

FRANKENSTEIN4x42000 01-23-2015 01:41 AM

I am very interested as well, thanks

Circuit Doctor 01-23-2015 08:01 PM

Guys, I'm glad to see there's an interest in this. As I had indicated, I have been working 7 days a week for quite some time. Also, I haven't left the office during daylight and fair weather since I bought the car last week, which has inhibited my ability to make this change. But rest assured, I promise I will document this as soon as I can! I'll put together easy to follow instructions. I expect this solution to work for all C4s, but definitely 1991-1996. Please be patient, it'll be worth the wait! :thumbs:

Jagdpanzer 01-28-2015 09:05 PM

Originally Posted by Circuit Doctor (Post 1588791474)
Guys, I'm glad to see there's an interest in this. Please be patient, it'll be worth the wait! :thumbs:

I have pair GE Nighthawks sitting on the shelf waiting for install.
Standing by for your words of expirence so I can do the job correct the first time. Thanks for sharing with everyone and keep us posted.

Circuit Doctor 01-29-2015 08:50 PM

Hey everyone, an update.
We got hit with a pretty bad snowstorm earlier this week, so the 96 got buried pretty good. It's going to be a while until I'm able to get to it to work on the writeup. I do apologize for the delay.

1993C4LT1 01-30-2015 01:40 PM

Cool, look forward to write up:thumbs:

Kenabinkport 02-08-2015 02:46 AM

Looking forward to your guide, I've been trying to figure it out on my 93 for months.

Circuit Doctor 03-14-2015 09:20 PM

I wanted to update everyone on this thread.
It's been a brutal winter with respect to temperature and amount of snowfall here in the greater NYC area. Of course, this all began one week after I bought the 96, so I suppose I'm responsible for it. :hide:
In conjunction, my workload at the office has been nearly unbearable, and 7 day, 60+ hour weeks are the norm for quite some time. :willy:

I ask that everyone please continue to be patient. As the temperature has been rising, snow nearly melted entirely, along with an additional hour of daylight, I expect to test out my solution as early as this week. Once I've done that, and "qualified" the change [as we say in the aviation electronics business], I will post a detailed procedure along with a technical explanation for those individuals interested in theory of operation.


gdl2165 03-14-2015 10:16 PM

Thanks for the update, I completely understand how work can get in the way. Anything you can come up with is greatly appreciated, I think patience on everyone's part is not a problem. We all know to leave the fogs off for now so it's not like we're dead in the water waiting for a solution.

HAD2HAV2 03-14-2015 11:59 PM

I am almost ready to order several size LED's from that include new LED fog lights. Changing to LED driving lights would be better lighting. Make sure you attaché your fix on this Thread.

Circuit Doctor 04-02-2015 04:29 PM

Hey guys, another update!
Following snowstorm after snowstorm, and 7 day weeks at the office, I was finally able to install my solution today! I can confirm that it works exactly as expected - foglights operate as normal and do not affect the highbeams.
I'm going to put a writeup together early next week after I've driven the car a few days and "qualified" the modification.

antfarmer2 04-02-2015 05:48 PM

I am having the same problem with my cibies would this be the same fix Mine is a 94

gdl2165 04-02-2015 06:27 PM

Originally Posted by Circuit Doctor (Post 1589328960)
Hey guys, another update!
Following snowstorm after snowstorm, and 7 day weeks at the office, I was finally able to install my solution today! I can confirm that it works exactly as expected - foglights operate as normal and do not affect the highbeams.
I'm going to put a writeup together early next week after I've driven the car a few days and "qualified" the modification.

Hooray!! I'm looking forward to the writeup.

QZRBLU 04-02-2015 07:21 PM

Fog light circuit 'fix' for LED and HID headlamp conversions>
I modified my fog lamp relay circuit about 5 years ago to make my fogs operate normally when I converted from Halogens to HID's. You need to remove the knee bolster from the passenger side of the dash and locate the fog lamp relay (it should be the far right relay in the relay bracket). . Cut the lt grn wire from pin #5 (relay pin 86) about 2" from the relay base and add a 6" jumper of at least 18 Gauge wire with a ring terminal. Find a convenient screw and ground this new jumper wire to the frame of the dash. I used a small piece of heat-shrink tubing to insulate the end of the .5mm lt grn wire that comes from the dimmer switch.
Now, turn on your parking lights and test your fog lamps, they should light and your high beam indicator should be off (unless you have your high beams on).
Page 8A-100-1 shows the circuit in my 1990 FSM.
To undo this mod, just remove the jumper wire and re-connect the original lt grn wire to the relay pin #5.


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