Auburntsts

Auburntsts

Well Known Member
So I'm in the process of setting up my avionics prior to first engine start. So far it's been relatively issue free, but have a few things to iron out. The only major one so far is the static MP reading is between 2.6 and 3.0. Any ideas as to what the problem could be?
 
Do you mean between 26 and 30?

Without the engine running, it should read roughly the local altimeter setting, minus about an inch for every 1000" of field elevation.

26-30 inches would be a normal indication without the engine running.
 
Nope. I mean 2.6 to 3.0 not 26.0 to 30.0. Also the needle on the generated guage is at the exetreme low end of the scale corresponding to the abnormally low indication.
 
Todd,

Check your wiring at the sensor. Sounds like you might have a couple of them reversed. I had a similar indication and upon further checking found that I had a couple of them reversed. Just one possibility.
 
Pat's exactly right. All of the gauges read a voltage, the sensors just turn whatever they're measuring into voltage.

You can verify the wiring at the sensor end with a meter, make sure ground really is ground, and look for voltage (IIRC the manifold pressure should be +5.0v, but it could be 12) on the excitation wire. Those two wires are easy to check, and if they're OK, the most likely culprit is a bad connection for the "sense" wireat the GDU. Go check and make sure the wires are connected to the proper pins, and tug on them to make sure wires are properly crimped.
 
Follow-up...

OK, I finally got back to the hangar to do some trouble shooting based upon your comments. It would appear that nothing was reversed at the sensor (and I did verify that I was working on the right one--Kavlico P4055-30A). One thing I did discover is the problem is apparantly somewhere in the ground (MP Lo) on the GSU 73 J732 side of the circuit. I found if I left the power and MP Hi (output) wires connected to the GSU but grounded the sensor "locally", I'd get a proper MP reading. But if I connected the MP low back up to the GSU I'd get garbage.

I didn't feel like diving under the panel to trouble shoot the J732 connector so the saga to implement a perm fix continues.....
 
Last edited:
It sounds like you have a ground integrity issue. The MP "LO" should connect to Pin 6 of J732. That is the only place it should be grounded.
 
stumped!

OK, I'm officially out of ideas. I've checked continuity of the wiring harness from the applicable J732 pins to the sockets on the plug end of the MP sensor and there's no breaks. Today with everything on and connected the MP guage on the MFD showed between 2.7 (two point seven) and 2.9 with an altimeter setting at the time of 30.25.

If I grounded the sensor locally the reading immediately changed to approximately 30.1.

That leads me to believe that there is indeed a grounding issue but it's internal to the GSU 73. Anyone have any other theories or troubleshooting ideas?
 
Hello Todd,

You may have already encountered this in your troubleshooting, but don't forget that the GSU 73 MANIFOLD PRESS LO pin (J732 pin 6) must be connected to ground also. In the installation manual a splice is shown to the XDCR POWER LO (GROUND) pin (J732 pin 74). This might be easy to miss, and would fit the symptoms you are describing.

If you find that this connection is missing, you can likely get away with providing the ground connection at your firewall ground block, if that is easier for you.

- Matt
 
Matt,
thanks for the input! I didn't build that part of the harness but I did open the connecter up when I was troubleshooting but failed to verify the splice. With the harness in the plane where it is and the bazillion wires and grounds inside the J732, I think the best option is to add an external ground splice somewhere "upstream". If the harness was on the bench I'd fix it proper but in this instance I believe perfection is the enemy of good enough.
 
Auburntsts said:
Matt,
thanks for the input! I didn't build that part of the harness but I did open the connecter up when I was troubleshooting but failed to verify the splice. With the harness in the plane where it is and the bazillion wires and grounds inside the J732, I think the best option is to add an external ground splice somewhere "upstream". If the harness was on the bench I'd fix it proper but in this instance I believe perfection is the enemy of good enough.
Click to expand...

Todd,

I am not a G3 expert or a wiring expert, but you should be sure that the missing ground is not also to be used for something else. If you KNEW for dang sure that it was only a ground through the 73 harness or external ground with no other use, then external it is. I don't have the crawl under there but, I know very well the influences that keep me from doing something like that. Backtracking and retracing is hard to swallow. The worry and avoidance is usually more than the work.

One last thought - Stein Team did most of my panel wiring, but in the process of wrestling with the harness and making some pin changes, I did find some pins and sockets not fully seated. Only three I think, but it takes only one - obviously. Not faulting them, it would be a fraction of a percent of the work. Maybe a good job for an inspection snake camera?


Congrats on your progress, I hope to fly this year. (too)
 
BillL said:
Todd,

I am not a G3 expert or a wiring expert, but you should be sure that the missing ground is not also to be used for something else. If you KNEW for dang sure that it was only a ground through the 73 harness or external ground with no other use, then external it is. I don't have the crawl under there but, I know very well the influences that keep me from doing something like that. Backtracking and retracing is hard to swallow. The worry and avoidance is usually more than the work.

One last thought - Stein Team did most of my panel wiring, but in the process of wrestling with the harness and making some pin changes, I did find some pins and sockets not fully seated. Only three I think, but it takes only one - obviously. Not faulting them, it would be a fraction of a percent of the work. Maybe a good job for an inspection snake camera?


Congrats on your progress, I hope to fly this year. (too)
Click to expand...

Bill good points. With at least a temp fix for the MP issue, I'm ready to start fuel testing followed immediately there after engine start. According to my wiring diagram that I got from Stein the MP, oil and fuel pressure sensors should all have the same ground splice. So I'll be watching those real close on engine start to see if they exhibit similar behavior the MP did prior to adding the external ground. If they do, then I'll be forced to crack open the J732 again and try to get the splices fixed. Quite frankly the prospect of doing that gives me the shakes because of the work that will be required behind the panel--pain-in-the you know what doesn't even begin to describe it. :D
 
Todd,
I would fix it while it is still fresh. You mentioned that all of the grounds for the sensors tie into the same GSU73 pin through a splice. Without reading all of the posts. I would recommend the following troubleshooting.

1. Verify continuity from the ground pin at the sensor connector to the 732 ground connector pin.

2. Verify that the other grounds you mentioned to the other sensors are truly connected together. Unplug the connectors and test them all for being tied together. If they are not, then you have a bad splice.

3. Now plug the 732 connector into the GSU73 and verify continuity to ground at each one of the sensors. Once again, if two out of three are grounds, then you have a bad splice

4. Lastly, verify that the 732 connector pin is the correct pin for the ground return.

You obviously do not have a proper ground return at the sensor end. If the wire is not cut, then the problem is either the splice, a push out pin, or an incorrect pin.

If you still cant find the problem, have another set of eyes take a look.
 
Bill P.,
I'll definitely do that. After really studying the wiring diagram, the 3 sensors all have their own separate grounds (pins 6, 45, and 49) plus all 3 should be spliced into a single transducer ground (J732 pin 74). I'll check for continuity to that pin tomorrow. I've confirmed pin to pin continuity for the MP sensor (obviously minus the splice to pin 74) so that just leaves the other 2 sensors.
 
Auburntsts said:
With at least a temp fix for the MP issue, I'm ready to start fuel testing followed immediately there after engine start. According to my wiring diagram that I got from Stein the MP, oil and fuel pressure sensors should all have the same ground splice. So I'll be watching those real close on engine start to see if they exhibit similar behavior the MP did prior to adding the external ground.
Click to expand...

Todd - You might think about testing your oil and fuel pressure transducers with shop air. A simple assembly of brass adapters should be enough for you to put in a known pressure and check the resulting pressure display. That may be quicker than trying to run the engine before verifying your wiring is correct. In this photo you can sort of see how I did it on my airplane:

20120812_hose.jpg


- Matt
 
Todd,
You can do what Matt recommends, but the oil temp should read room temp, the MP should read a little below the local baro. The oil pressure and fuel pressure can be tested with shop air with the pressure dialed down very very carefully
 
You must log in or register to reply here.