Gauge install, dash cluster light cable, Tach RPM cable help...

[Sportsguy83]

Originally Posted by Acree

I'm an electrical engineer and self-proclaimed wiring guru. I've been doing automotive wiring since I was 16 and for the last 10 years. I'd be glad to help if you still need it.

Can you first provide a link or make and model of the exact gauge(s) you purchased?

-Acree

Of course!! I'm a mechanical engineer so :happy0180:.

Here are the instructions to the gaugues:

AEM 30-4900 Failsafe Instructions:

AEM 30-4402 Digital Water Temp Gauge Instructions:


On the Failsafe, I don't care about the blue, yellow, brown, white cables as those are outputs to a data logger/boost control device and I don't have one. Also, don't care too much about the Tach cable, just figured it would be nice to install it to log AFR, Boost vs. RPM and check it out later on the computer. (Of course ECUTek can do that but for those times I don't want to be dragging the laptop around)

The Oil Temp sensor is pretty straight forward., I don't have questions about it.

My main question at the moment is about the dimming cable and getting it to work right. Power source to the gauges I have pretty much nailed down from the ideas above. See previous posts about how the dimming is kind of partially working for king tut.


Thanks in advance, all help is always appreciated!.

[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

[King Tut]

I can tell you that I saw voltage change with a multimeter when moving the dimmer switch on the wire that I connected to. I don't remember which wire I used, but I think it went from 12v to 8v when I moved the dimmer switch. I did not check the voltage on any of the other wires though. I remember going over that diagram quite a bit until I understood how it worked.

[Acree]

You certainly could have been testing the correct wire, but there are a couple of things to keep in mind. Depending on how your voltmeter was made, a pulse width modulated signal might display as a varying voltage. It depends on the sampling rate of the meter and how they calculate the final display value. They might display just the last voltage amount sampled, or they could use a moving average calculation, or any number of algorithms involving the last 2, or 3, or 4, or whatever number of samples.

The reason I believe you have your gauge tied to the wrong wire is because the brightness quickly returns to full after you stop adjusting the dial. This tells me that the pin is probably seeing the full 12v, which is what would happen on a PWM wire.

-Acree

[Sportsguy83]

Originally Posted by Acree

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

Your explanation is freaking AWESOME!! I wonder why AEM didn't design the failsafe with a light sensor like the oil temp gauge. (Probably next version will have it... Effing cash grab!)

I do understand what you mean with the PWM and the duty cycle. Thanks a lot for your explanation. Very thorough.


I am waiting for my gauge cluster to arrive but as soon as it does, I am eager to try all of this.



Sent from my HTC One X using Tapatalk 2

[King Tut]

Originally Posted by Acree

You certainly could have been testing the correct wire, but there are a couple of things to keep in mind. Depending on how your voltmeter was made, a pulse width modulated signal might display as a varying voltage. It depends on the sampling rate of the meter and how they calculate the final display value. They might display just the last voltage amount sampled, or they could use a moving average calculation, or any number of algorithms involving the last 2, or 3, or 4, or whatever number of samples.

The reason I believe you have your gauge tied to the wrong wire is because the brightness quickly returns to full after you stop adjusting the dial. This tells me that the pin is probably seeing the full 12v, which is what would happen on a PWM wire.

-Acree

Do you have the gauge? Are you planning to check out the wires? If not, I can probably take a look at the wires again this weekend.

[Sportsguy83]

Originally Posted by Acree

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

Thanks for this. Let me know when you check if that cable is running at 12V or 5 V

[Acree]

Originally Posted by King Tut

Do you have the gauge? Are you planning to check out the wires? If not, I can probably take a look at the wires again this weekend.

I do not have that gauge. But I can at the very least check the voltage. I didn't have a chance last night, but I can today.

-Acree

[King Tut]

Thanks man. I realized that since I mailed out my ECU today, I don't plan to hook the battery cable back up till I get my ECU back.

[Sportsguy83]

Originally Posted by King Tut

Thanks man. I realized that since I mailed out my ECU today, I don't plan to hook the battery cable back up till I get my ECU back.

My GOD man.... I feel for you

[King Tut]

Originally Posted by Sportsguy83

My GOD man.... I feel for you

I am hoping the 2 weeks since I finished the install will just make it that much better when I finally get to drive it.

[Sportsguy83]

Originally Posted by King Tut

I am hoping the 2 weeks since I finished the install will just make it that much better when I finally get to drive it.

It will all be good when you get to step on it!!

[Acree]

The G/W wire runs at 4.1v no matter if the car is running or stopped.

The yellow wire goes from 0v at full brightness to ~3.8v at full dim. That's right, it's reversed. I'll have to look at this more closely tomorrow to figure out the simplest way to interface this dimming signal with your gauge.

-Acree

[Sportsguy83]

Originally Posted by Acree

The G/W wire runs at 4.1v no matter if the car is running or stopped.

The yellow wire goes from 0v at full brightness to ~3.8v at full dim. That's right, it's reversed. I'll have to look at this more closely tomorrow to figure out the simplest way to interface this dimming signal with your gauge.

-Acree

Thanks, any thoughts on the RPM cable? Preferably the one going to the Cluster?

Also, saw page 8 of the instructions you can change Dimmer max brightness 12V or ground, my uneducated guess is that that means it can be switched to be 0V full brightness.