Disclaimer: I recommend only professionals undertake any electrical work on vehicles. I am not liable for any damage or harm that may come from the use of this writeup.

Backstory: I wanted to mod my interior to have white LED lighting; I thought the yellow tint in the interior looked old. Most of the interior bulbs are 194 and easy LED replacements. However, the rearview mirror uses PCB mounted LED's. No one on the forum really nailed this problem down with technical details so I figured I would give it a shot since I am an EE.

Process: I loaded the rearview mirror circuit with different resistor values to calculate load lines. Load lines are used in non-linear circuits to determine where the operating points will be and how the circuit responds. LED's are non-linear devices. There are 4 different circuits: Driver's side outer, driver's side inner, passenger's side outer, and passenger's side inner. Surprisingly enough the driver's side ended up having roughly 30% more current being drawn as well as a higher overall voltage which I thought was interesting. Also, the voltages seemed to vary a bit based on if the dash lights were on or not. My calculated load lines ended up being too far off from reality (not sure why they were so far off. I assumed the circuit powering these LED's would be a linear circuit but apparently it's not, and it doesnt seem to be regulated either). The load line calculations are not shown since they were not accurate however they provided me a good starting point on resistor values to begin with.

What I ended up doing was wiring up my theoretical values using my loadlines (~60ohm for passenger if I remember correctly) and it ended up being too low of a current. So I basically wired up the LED's, and took measurements as I went. Once I saw the "inner" circuits both draw <24mA w/o a resistor, I wired those up first. Then I tackled the "outer" circuits because they needed a resistor value selected before I soldered it on. Here are some measurements I took (if you happen to want to use one of these different resistor values to change the lighting brightness):

Driver's "Outer Circuit"
40mA w/o a resistor (3 LED's)
25.5mA w/ 35.6ohm
23.2mA w/ 46.5ohm
20.5mA w/ 60.1ohm
17mA w/ 75ohm

Passenger's "Outer Circuit"
30mA w/o a resistor (3 LED's)
18.1mA w/ 35.6ohm
15.2mA w/ 46.5ohm
14.6mA w/ 60.1ohm
12.1mA w/ 75ohm

The reason you cannot always just swap different color LED's is because they have different voltage drops. See my technical spreadsheet listed below for the stock voltage drops vs the new ones if you are interested in seeing the difference it makes.

Results: My solution is to use 3 LED's on each "outer" circuit with a current limiting resistor, and use 2 LED's on each "inner" circuit. It turns out, the inner circuit's run fine by just straight swapping the stock LED's for white LED's. Drivers side ran @ 22mA, passengers side ran @ 16.1mA, which is below the LED's rating of 24mA and keeps the stock config of having the driver's side biased slightly higher current/voltage (slightly more light). However, the outer circuit needs 1 resistor to limit the current. Otherwise, the circuit will draw 40mA+ and shorten the life of the LED bulbs for negligible lighting gain. LED's last a long time so even if you do not use the resistor, you may never have the car long enough to see the effects of running @ a higher current. By having one less LED on the outer circuit you have enough voltage to correctly power the 3 LED's, in a controlled manner. 4 LED's would be too much voltage drop and thus results in issues with inconsistent LED brightness and behavior.

The resistor values were selected to best match the light output of the different LED circuits, and to ensure the current draw is less than the LED rating (to extend LED life). I used LED's I had laying around from various other projects. My values are NOT unique, there are many other combinations of resistor values you can use but this is just what worked for me.

Stock Operation -> My Suggested New Operation
Drivers "Outer Circuit"
4 LED -> 3 LED
0 Resistors -> 1 ~46.5ohm Resistor
50.7mA -> 20.65mA
Drivers "Inner Circuit"
2 LED -> 2 LED
21.1mA -> 22mA
Passenger "Outer Circuit"
4 LED -> 3 LED
0 Resistors -> 1 ~35.6ohm Resistor
37.8mA -> 16.85mA
Passenger "Inner Circuit"
2 LED -> 2 LED
16mA -> 16.1mA

As you can see, the new current values are nicely matched while still providing a slightly higher bias on the driver's side (slightly more light).


Thoughts: Your results may vary based on LED selection and resistor tolerances. I think this mod was worth it in the end however to be honest this is one of the most frustrating mods I have ever done - putting the PCB back in the rearview mirror with the lens is such a PITA . I ended up actually damaging the passenger PCB by putting too much force on the face of the LED's when trying to reinstall back into the rearview mirror. I love the look of the white lighting in the interior and definitely makes it look modernized. Not sure why the pictures are coming up more of a blue but in person it's more of a white.

 

You can use all six 5mm LEDs without the resistors by modifying the circuit boards. (pic supplied by forum member tstar )


I sent my mirror to Radioflyer to have Cree LEDs installed in place of the 5mm ones.
http://forums.corvetteforum.com/c5-g...im-mirror.html