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

Fellow electricity enthusiast, I need help. I have looked at the wiring manual and while I have a general idea about it, I really don't understand everything on it.

I will be installing gauges soon and need to tap my gauges to the cluster lights cable in order to have the gauges dim accordingly. Could some help me figure out which cable I should be tapping into? What about the RPM Tach cable?

While we are here, can I can I crimp the power cables for both gauges together and add them to a single "add a fuse"? Each gauge requires 5A Fuse, so I figured I can use a 10 Amp fuse. Those that make sense or I need to add a separate "add a fuse" for each?

Thanks in advance!

Ever think about using the radio dimmer signal wire?

Well, it might work but I'm not sure. I don't know how the circuit within the gauge works. Does it take the signal to turn the cluster lights on to dim the gauge display? And also, how does the radio dimmer work, does it send a signal to dim it or is it actually grading the amount of current down to dim it? I honestly don't know the answer to either but maybe you know more than me and can answer them.

Or maybe you just know using the radio dimmer just works for gauges?

Thanks for the help! Let me know what you think.

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Well, the gauges that I have in my other car just run full brightness, until the dimmer wire gets juice. I have it piped into my radio harness to handle that....

I guess it depends on the gauge, what do the installation instructions say?

Not sure if its exactly what youre looking for, but i wired in some LED's into my air vents using the radio dimmer and they work like a charm. They are off until i turn the lights on.

Mine just illuminate with the illum feed, no dimming. Damn expensive Defi's can't do that unless you go to the newer Advance BF ones.

I pull my feed off the foglight fuse so I can turn off the annoying gauge illumination at night :) The warning light is enough in case there's an issue.

For the fuse, run whatever the load should be. I doubt the gauge really pulls that much. Since mine is control unit based I just needed one feed. In the race car I have them all fed off one power wire with a 5A fuse for 3 autometer gauges. Illumination is pulled off the radio illumination wire.

Look at page 3 here: http://www.aemelectronics.com/Images...%2030-4900.pdf

I guess it does work like you say, they run full brightness until the instrument panel cable gets juice. So then, what is the cable for radio dimmer? Thanks for the help!


I honestly don't know exactly if they Dimm or not, I guess I'll figure it out when I install them. Waiting for the Prime Motoring Cluster. Thanks for the help.

Have any idea on what is the RPM cable in the cluster harness?

Subarus usually have reversed illumination circuits. Personally hate wiring, so I haven't explored this further, but it's on every major Subaru forum for your digestion. Big ole scar from the soldering iron was enough.

I have the dimmer wire hooked up properly to my AEM gauge which allows the dimmer circuit to be either 0 to 12v or 12 to 0v. Unfortunetly, the difference in V between dim and not dim is not enough to change the gauge. I would recommend just leaving that wire unhooked. As for RPM, there are a couple wires that you can use:

Pin 17 on connector D48 Gray Wire
Pin 15 on connector A35 Gray/Red Wire

I haven't gotten around to trying either of these yet though.

That sucks... do you feel they bother much at night? (Thanks for the RPM signal location)

Well my gauge just has the not very bright LED screen in the middle since it only shows black and the white number of my boost or vacuum and the lights on the outside for the air fuel ratio, no backlight, so I think it is fine in full bright mode. I can move the dimmer switch and get the gauge to temporarily change to a dimmer mode, but then it comes right back to full brightness shortly afterwards which is weird. I checked the voltage and it definitely changes on that wire when you move the dimmer switch from full bright to no bright.

I bought the failsafe so this is great info. The other gauge is Oil Temp same series as the failsafe.

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!.

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

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.

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

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.



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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.

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

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

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 :(

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!!

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.

You are correct.

Exactly. But as it sits, you'll only be able to dim it about 1/3 of the way. Tach signal I'll have to look in to.

-Acree

There isn't a good way to get a 0-12v with the newer Subarus which are PWM for most dimming. The older models used a 12v hot side (Ill+) and then a floating ground which was 0-12v (Ill-) which controlled the dimming. With the PWM models the Ill- (the B/W wire in your diagram) is a ground while the Ill+ is still +12v all the time. With the old models you could use a simple circuit to output a 0-12v signal for gauges to use.

Probably the easiest way to control dimming is use some kind of potentiometer unfortunately. Or build something to convert the PWM to an analog 0-12v signal...

Those wires you are looking at go to the gauge cluster directly I believe to control lights which are not LEDs? So they use the old method where they are 4.1v hot all the time on one side and reversed 0-3.8v based on the dimmer switch. Just my guess.

That's a very good guess, but incorrect. The Ill- side is switched between ground and high with the PWM controller. The Ill+ is constant high. Ill- definitely never floats. They have pull-down resistors internal with the ECU.

And the gauge cluster is definitely LEDs... trust me on this. Check out my avatar. The picture below is the stock speedometer.

-Acree

As far as the RPM is concerned, the Failsafe gauge is looking for a square wave input. It calculates the RPM based on the programmed pulses per revolution in the AEM Failsafe Configuration software. Unfortunately there is no digital tach signal wire going to the combination meter (aka instrument cluster aka gauge cluster). The current RPM value is shared via CANBUS between the combination meter and the engine control module. The only wire with the necessary square wave signal is going directly from the Crankshaft Position Sensor to the ECM.

I do not know enough about the stock CPS to be able to tell you if this will work or not for the Failsafe Gauge. It depends on two things:
1) Find out what the resolution is on the stock crankshaft sensor wheel. This will depend on what type of sensor it is. It's more than likely a hall effect sensor. I would imagine that it has a pretty high resolution, something along the lines of 50-80 pulses per revolution.
2) Download the Failsafe Configuration software from AEM and plug in your failsafe gauge. Check the RPM Pulses per Revolution option and make sure you can select the same Pulses/Rev as the stock sensor.

If all of this checks out, I can figure out where and which wire you should splice into.

I do have to warn, in general as an EE, I would recommend against messing with any of the stock Crankshaft Position Sensor wiring. If you mess up that sensor, the wire, or the ECM, it's going to be pricey and labor intensive to fix it.

-Acree

I am almost positive I got a PM from someone who hooked up the RPM wire of the failsafe to one of the wires I mentioned and that it worked. I checked my PMs, but I must have deleted it.

Yeah I never said the PWM ground/signal floated, that was the old way Subaru controlled the analog dimming. I guess I didn't finish my sentence the PWM signal does switch between ground and high.

What do you suppose the roughly 4v signals are used for? Are those just regulated voltages for the cluster LEDs?

The voltage level is something that baffles me. In my experience, if a voltage is regulated anywhere between 3v and 5v, it's typically 3.3v. I've never seen 4, much less 4.1v.

What I am fairly certain of is the 4.1v is definitely not providing any power to the LEDs in the cluster. The illumination LEDs are internally regulated and grounded, either through the warning LED driver or the CPU.

King Tut,

I rechecked your post and I have to apologize. I definitely missed your details regarding the two wires. I do believe that the gray wire coming out of D48 would be suitable for the Tach signal. According to the wiring manual, it's coming out of the ECM labeled as "TACH." :thumbup:

-Acree

I can live without dimming, since King Tut says it is livable. I did read King Tut's post about the cables that work, but I wanted one in the cluster :D. But also, I have no idea where are those pins... :( Any time you guys can take a pic or at least some directions and I'll figure it out from there.

I appreciate a LOT all of your help, including @dwx.

Check pages 41 and 42 of the BRZwiring PDF for the locations of the two connectors that should have the tach signal. I don't know that you will find one behind the cluster. The gauge comes with plenty of wire to get to that side of the car though.

@Acree, I wonder if you got any more thoughts/ideas on the gauge dimming source. I still don't have my cluster, but just getting ready for it :)

Hey, as far as hooking up the illumination wire for my Greddy gauge, I used the Ill+ which is just a constant 12v switched on with the headlights. You can use the TAIL or PANEL fuses, they both go to 12v when the headlights are turned on. With that particular gauge, it doesn't use the 12v for powering the LED backlight, it just uses it to trigger the gauge to dim when it senses voltage on the line.

Getting something to dim with the dimmer (if that's your goal) is going to require some custom circuitry just like with the older Subaru models.