Troubleshooting wild oil pressure readings

For those of you that followed the saga of Mikey's panel renovation (http://www.vansairforce.com/community/showthread.php?t=24139&highlight=Mikey), here's the most recent panel event.

Flying on Sunday, I noted that my oil pressure on my Dynon 180 was fluctuating a bit soon after take-off. It was in the normal range but bumping up and down about 10 units. It settled down a few minutes later and I managed to forget about it. However, taking off for my commute Monday, I noted that the oil pressure was pegged in the red immediately after take-off, then fluctuating wildly. All other readings (and sounds) were normal. The pressure reading soon dropped into the normal range and quit fluctuating by the time I landed. But, the problem now had my attention.

Paul drove over to our local Auto Zone Aircraft Supply store and purchased a $20 mechanical oil pressure gauge. We hooked it to the engine at the site where the annunciator connector normally sit and fired up the engine. As suspected (and hoped), the mechanical gauge was showing normal readings while the Dynon meter was pegged in the red (fluctuating up to 100 psi) when the engine was run up. At idle, both read about the same pressure. Diagnosis? A faulty oil pressure sending unit (transducer).

Before buttoning Mikey back up, we put the annunciator lead back on and also polished up the connector to the lead going to the Dynon. In discussing my problem with Larry Pardue, he mentioned once having fluctuating low oil pressure readings caused by corrosion at a sensor connector so we figured a little preventive maintenance would be wise.

I'll be calling Dynon once they open today and hope to have this problem fixed ASAP. But, at least I know it's a false reading and can keep him in the air.

Bad Ground?

Sounds like a bad ground for the sensor.

Might need to have a specific ground wire to the sensor. Don't know how your sensor is mounted, but if it is in some kind of vibration isolator it won't receive a good ground and give eratic readings.

Check with an ohm meter from chasis ground to sensor body for good continuity.

Then again I suppose it could just be a bad sensor.

Ted

Sensor Ground

Hey Ted, great minds think alike! I check the ground right away, and it is good. (The sensor case is the ground, through the nipple) After Larry's experience, I suspected corrosion on both the ground and sensor lead, and both are good. The fact that this problem came up all of a sudden makes me suspect the sensor.

At least now I had an excuse to buy a mechanical gauge and hose to have in the tool box. Always nice having an excuse to buy another tool...

Paul

Ironflight said:

The fact that this problem came up all of a sudden makes me suspect the sensor.

At least now I had an excuse to buy a mechanical gauge and hose to have in the tool box. Always nice having an excuse to buy another tool...

Paul

Click to expand...

P&L, My money is on the sensor. The fact it was fluctuating then went full "tilt" supports that theory.
Tools are a good thing! You'll use that test pressure gage set up again. I love diagnosing things like this, I just wish I was better at it!

Make sure you are using the restrictor fitting on the oil line to the sensor. I know we think it's primary purpose is for safety if the line should break, but there is another benefit as well. It dampens the pulses from the oil pump and prevents the wiper arm on the electronic sensors from wearing out a particular spot on the sending unit. The symptoms you are seeing are typical of a sensor going bad in this way.

Vic

Hi Louise (and Ironflight),

So the question is still "is it the sensor or something else", correct?

So, this is how I'd go about it. Supplies: mech gauge (which we already have), volt-ohm meter (you do have a simple analog one in your took kit - don't you?), various tools.

- Hook up your mech gauge along w/ your "suspected" sensor. Sounds like Iron has already done this (or knows how to do this).

- Disconnect the leads from you sensor and stow them away safely.

- Set up your VOM to the resistance setting - not sure what the resistance range on a pressure sensor is but I can't imagine anything more than a couple of Kohms (so 10 K setting for starters).

- Startup

Now, very carefully:

- Note behavior of mech gauge (steady reading?)

- Measure the resistance of sensor from the wire connection to sensor case or between wire connections (if the sensor doesn't use the case as a ground).

If the readings are stable from sensor to case (or 2nd wire connection), it's a grounding issue (or possibly a voltage supply issue). Otherwise, it's a sensor issue. You could even go so far as "extending" some wires to the VOM so that you don't have to be so close to the prop.

On second thought, maybe even remove the suspect sensor and connect it to a regulated air supply. Varying the air pressure should result in a linear change of resistance (within reason of course).

If the sensor checks out, then I'd focus on either the ground or supply voltage at the sensor.

Good luck,

Follow-up - problem fixed!

Just before we left for our wedding, Dynon shipped a new transducer for us - I expect it arrived the day we left. As soon as we got back, we installed it (it took longer to pull and replace the cowl) and ran the engine up. The oil pressure was perfect, so the older sensor is on it's way back to Dynon for them to diagnose.

Thanks again for the quick turnaround Dynon!

Paul