Engine sensor wiring for G3X

[jeremiahd]

Excuse me for lacking specifics, as my paperwork and installation manual are at the hangar, and I'm going from memory...

I'm in the very last stages of finishing my RV-10, and the panel is driving me nuts. The immediate issue I'm trying to solve seems pretty straight-forward, so I sense there's a simple solution that I'm missing. It has to do with the pressure sensors (manifold, fuel, and oil) and the oil temp sensor. (They're all the ones provided by Garmin in its engine sensor kit; the engine is a Lycoming IO-540.)

None of the sensors give any readings. They all read zero. All of the pressure sensors have the same three colored wires: black, green, and red. I've assumed the red wire is power (which seems correct, since I checked and they all were receiveing 5 volts from the 720 plug and the GSU). I assumed green was the signal, and black the ground. But the Garmin installation manual calls for a "hi" and a "lo", which I assumed was the green and black wires, repectively. Is that my problem? Today I reversed them hoping that I guessed incorrectly, but that didn't seem to help.

And the oil temp probe only has two black wires. Again, the Garmin manual says "hi" and "lo", but since the wires are the same, does it matter? I've tried both possibilities, but no response.

So far, the G3X seems like a real pain, and I miss the simplicity of the Vision Microsystems unit I installed in my RV-6A. (That's frustration you're hearing.)

Any advice would be much, much appreciated!

Thanks,

Jerry

[Subwaybob]

Sorry you're having trouble Jerry. Here is what I know:

1. Oil temp has no polarity.

All of this stuff connects to GSU73 J732

Now all Reds goes to pin 75 (5V) in everything that follows.

2. Fuel Pressure sensor:
Green is Hi and pin 26
Black is Low and pin 45

3. Man Pressure
Green is Hi and pin 65
Black is low and pin 6

4. Oil Pressure
Green is Hi and pin 69
Black is low and pin 49

45, 6 and 49 are also wired together it would seem but not to ground. Ground may be an internal function of the GSU.

If you want a picture of this send me an email to bob(at)subwaybob.com and I will reply with a pic.

[Bill.Peyton]

Just a note to add for info. You probably know this already, but just in case. When you get the sensors wired properly and you apply power. The oil pressure will read zero, the MP will read the current baro, the oil temp will read current OAT, fuel pressure will read zero. The CHTs and EGTS will read the OAT. Until you put pressure on the pressure sensors, expect them to read zero.

One additional important item. Pin 74 must be connected to pins 45,6,and 49. Pin 74 is transducer power ground. I would also offer to send you a schematic if you need it.

[Auburntsts]

Also, if you haven't already, you need to go to the engine input configuration page and select the appropriate sensor for each input (ie Kalvico P4055-150G or UMA N1EU150G0CS, ect) from the dropdown list as IIRC the default setting is "none".

[jeremiahd]

I really appreciate your help!

Jerry

[jeremiahd]

Well, I went and looked at my wiring records today when I got back to the hangar, and here's what I found: I didn't ground the transducers at pin 74 (for fuel, manifold, and oil pressure) or pin 18 (for oil temp). What was I thinking?

Now, my problem was that to get back to the J732 plug would take me probably four hours of undoing a lot of stuff. So, I tried the following...

Why not just run the ground wire from the oil temp transducer (which goes to pin 14) to ground on the firewall, then to pin 14? I tried that, and -- bingo! -- it worked!!! A five-minute fix.

Now motivated, I did the same for the three pressure transducers. The manifold pressure gauge now reads atmospheric (or close to it), and I'll have to wait until the weekend to start her up to check the oil and fuel pressures. (It's raining here in San Diego, and no one dares go out in that alien stuff.) But, I bet it works.

Anyone see a down-side to what I did? I was assuming that the GSU 73 ground also grounds to the airframe. Seems that's the case.

Now, if only the FAA didn't ban me from taking-off from my home airport, I'd be doing the first flight next week. But, that's another issue.

Thanks, guys, for solving my engine sensor problem!

Jerry

[BobTurner]

Well, technically you've just created some ground loops. Because the fuselage is not a perfect conductor, the potential at the firewall and at the avionics ground may be very slightly different. So if any of the sensor signals are really small, you may note some inaccuracies (I doubt it). The other consideration is that audio signals seem particularly problematic. If you hear alternator whine in your intercom that wasn't there before, the ground loop(s) are the source.

[SteinAir]

As Bob noted, what you did will likely work OK....but, over the long run it'd really be worth your while to avoid known ground loops. It's fairly possible that in the future you may have some weird gremlins, and ground loops make it very difficult to troubleshoot and also to fine tune. The G3X is a very good system (as are most of the current crop of digital instruments), and the Kavlico sensors are also top notch (so much so that other mfgr's have started to follow Garmin's lead and utilize them), but to get the most out of the system overall it'd be best to avoid shortcuts. I know it'll be a pain to modify it now, but it'll be much better than dinking with it later down the road if you experience "anamolies".

Just my 2 cents as usual (and we have a LOT of these installed by customers and ourselves in the field)!

Cheers,
Stein

[Subwaybob]

Stein is right. Do as you wish but I curse myself EVERYTIME I have to redo something. But I do it. No matter the pain. Then I curse some more, then I work some more, then I curse, then I'm happy. Then the next time I hire Stein to do ALL the wiring...