[Acree]

TLDR: skip to the last paragraph and the attached picture.

From what it sounds like, King Tut might have spliced his gauge illumination wire into the illumination wire going into instrument cluster. Honestly, it doesn't matter what illumination wire you splice in to, it's never going to work.

First and something I'm sure you've noticed, your temperature gauge doesn't have an illumination input on it. That's because it is built exactly like the AEM Uego. They both have an on-board photo-diode that adjusts the brightness automatically based on the ambient lighting amount. So you don't even have to worry about that gauge.

You have to understand how the illumination works through out the car. LEDs are dimmed by pulse width modulation (PWM), where incandescent bulbs can be dimmed with PWM or by varying the supply voltage. Most of the illumination wiring running to the majority of the interior components are PWM controlled. If you understand how PWM controllers work, skip down a couple of paragraphs. Wikipedia has a half decent article on the subject of PWM so I won't go into an in-depth explanation here. But for a super simplified explanation, a PWM controller can only go high or low, nothing in between. If your high voltage is 12v and your low voltage is ground (aka 0v), then the PWM controller can only be 0v or 12v, nothing in between. What happens is the PWM controller essentially toggles between high and low very very quickly. It basically just blinks the LED, but it blinks it so fast that the human eye can't see the blinking. If you think of it like turning a light switch on and off really fast over and over, then the time you leave the light switch on can be longer or shorter than the amount of time you leave the light switch off. The PWM controller does this, and it is referred to as duty cycle.

Let's say you have a 12v incandescent bulb hooked up to a variable voltage supply, and you can select the exact voltage you want going to the bulb. (For this analogy, I'm going to assume an ideal bulb without considering non-linear resistance, in-rush start up currents, etc. In other works, super simplified.) If I consider the bulb at 0 volts to be 0% of it's brightness, and 12 volts to be 100% of it's brightness, and then I set the voltage to 2 volts, then the bulb will be at 16% of it's potential brightness. If I set it to 6 volts, then it would be theoretically 50% bright.

Unfortunately, LEDs don't operate in this linear fashion. Instead, to adjust the brightness level, we use PWM controllers to "blink" the LED. If you want the LED to be 50% of it's total brightness, then you would turn the LED on for the same amount of time as you turn it off, or in other words, 50% duty cycle. But the important thing to remember is no matter what the duty cycle is, the connection (or wire) going to the LED will always be either high, or low, never in between.

On the other hand, your failsafe gauge is looking for an analog voltage like what would be going to an incandescent. Being analog, it can be any voltage level between 0v and 12v, and it never turns on and off rapidly They have a pin on their microcontroller which is configured as an analog to digital converter. It's configurable for either active hi or active low, but it directly correlates the voltage on the pin to a duty cycle for the internal PWM controller. If configured for active high, then basically their pin is looking for 0v (GND) to be completely dimmed, 12v to be fully bright, 6v to be half way dimmed, etc.

So all of that being said, we basically need to find the wire that has the analog voltage, and not the pulse width modulated voltage. See the attached image. I will have to double check tonight when I get home and make sure the G-W wire is running at 12v and not 5v. If it is running at 5v out of the ECU, then I'll have to design a simple resistor network to get the gauge to scale to the full 0-12v range.

-Acree