It seems that the VDO resistor-type oil pressure sensors commonly installed on the Jab 3300 routinely wear out every couple of hundred hours - or less!  I'm at 550+ hrs and noticed my 3rd VDO sensor was starting to show signs of failing - usually evidenced by sudden spikes of high pressure.

Some have tried to avoid this problem by either mounting the sensor on the lower oil gallery port (which gives lower oil pressure readings and has to be taken into account) or remotely mounting it via an oil line with the theory the oil line will absorb some of the oil pump's pulsations and extend the life of the sensor.  I tried the lower oil gallery mount and saw no difference in the sensor's life.

It occurred to me to try one of the newer, solid-state oil pressure sensors.  Aircraft Spruce sells a "Swift" 0-5v/100 psi sensor that they say is "similar to the Falcon" and "with no moving parts ... which does not allow the sender to become worn out mechanically." It costs no more (maybe even a little less!) than a VDO sender.

Since it is supposedly resistant to wear, I installed it in the original location that Jabiru uses and plugged the lower galley port.  I display my pressure on the MGL XTreme EMS with the RDAC XF interface, so on the RDAC, I connected the red power wire to the +5v, the black ground wire to ground, and the signal wire to the OIL P connector.  Someone had told me I would need to install a resistor in the signal wire, so I connected a 1/4w, 1 KOhm in series to the signal wire. I set the XTreme menu to "0-5v" sensor and chose the "UMA TIEU100" since both are 0-5v, 100 psi sensors.

Initially, the sensor did not work correctly!  I double-checked and found the  pull-up resistor dip switch was "ON" on the RDAC, so I switched it off and it worked great after that.  Hmmm - makes me wonder if that in-line resistor was really necessary and could I have deleted it and just left the pull-up resistor switch "ON" on the RDAC board? I suspect that's the case but since it is working fine, I just left it as-is.

I had also been advised I might need a capacitor to filter the signal and prevent erratic or jumpy readings. Again, however, it did not seem to be necessary and the oil pressure readings are quite stable.  The person advising this had used a cheap Ebay Chinese sender, making me suspicious that's why he had to modify the installation - perhaps the "Swift" sender is a little better design/quality?

Anyway, now I just need to put a couple hundred more hours on the STOL 750 and see if durability is there!  ;>)

John

N750A

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Replies to This Discussion

Hi John,
What a timely post!
Yesterday on a flight I experienced erratic high oil pressure readings from my VDO sensor. After reading your post I suspect it is beginning to fail. I have 46 hrs. on it.
Thanks for posting.

Glenn Davis
5362P

Hi John, a pull up resistor should not be connected in series. It should go from the input to the +5V. As there seems to be a switch with said pull up resistor, try to put it on ``'`ON`` and remove the resistor you put in. This might work.

Henri

Your comments made me re-think the installation.  In my testing, I remembered that at one time, the display was not reading "0" oil pressure with the engine off - it read about 6 psi.  However, once I finished and test-ran the engine, the operating pressures looked very similar to those previously obtained with the VDO resistive sender.

I ran over to the hangar this a.m. and turned the master on and sure enough, the EMS was indicating 6 psi oil pressure on a stone-cold engine!  

Of course, you are correct that the signal resistor is not acting as a pull-up resistor.  I do think, however, it is probably the source of the false 6 psi of oil pressure!!!  I agree the resistor should be removed.  I'll try that and it will be simple with the DIP switch to determine if the RDAC's PU resistor needs to be ON or OFF. 

Thanks for your input. (pun intended!)

John

I deleted the resistor and the operational pressures again appear to be stable and similar to the previous resistive sender's output.  When the engine is off, it now indicates 5 psi instead of zero.  Since the operational pressures appear normal, I wonder if this is a slight difference in calibration on the low end of the scale between the "Swift" sender and the UMA sender pre-programmed into the EFIS?

It appeared to make no difference as to whether the pull-up resistor DIP switch was ON or OFF, so I left it OFF. (The RDAC XF installation manual says it is usually left OFF except for resistive senders.)

I suspect I could select the "USER" setting on the EFIS and custom-calibrate the sensor, but I suppose that would require temporarily plumbing an analog gauge in-line and it's not worth the effort to me.  I'm going to contact Matt at MGL and see if he has any other ideas about the 5 psi reading with the engine off.

John

I found out what was going-on with the false 5 psi pressure reading.  The Swift sensor is a 0.5-4.5v sensor, not a 0-5v sensor, so therefore it is not the same as a "0-5v" sensor such as the UMA T1EU100.  The Swift is outputting 0.5v at no pressure and the XTreme set-up at "0-5v" is expecting to see zero volts - so, that's why the 5 psi is indicated.

Since my XTreme set-up does not have a provision for 0.5-4.5v sensors, looks like I'll have to go with the UMA after all.  I'm now trying to determine the exact configuration of the UMA sensor (they have multiple configurations for the 100 psi senosr) to be sure I get the right one.

John

Looks like I may not be out of luck with the Swift 0.5-4.5v/100 psi sensor!  I knew it could be configured as a custom sensor using the XTreme's "USER" menu, but thought it would be too big a hassle to plumb-in an analog oil pressure gauge to determine the ADC values to program the XTreme.

THEN, I remembered someone had given me a small aluminum manifold with multiple 1/8 NPT ports.  I can screw the sensor into one of the ports, screw my leak-down compression tester with regulator into another port, and plug the other ports.  Then, leaving the sensor's harness connected to the RDAC XF and EMS, I can determine the ADC values on the EMS to calibrate the sensor to the EMS.

For those not familiar with MGL, if you use a sender that is not pre-set in the set-up menus, the XTreme EMS can be calibrated to display the results of almost any type of custom sender - pressure, fuel level, etc.  During the set-up, you simply input the signal and an arbitrary number is generated - an "ADC"  - and you can then define what number you want displayed with that value. Once 2 or more ADC values are defined, the EMS will then compute a curve so it can display any intermediate value between the ADC points, too. 

John

Today, I calibrated the Swift sensor.  I selected the MGL XTreme EMS oil pressure menu for "0-5v" (even though the Swift is 0.5-4.5v) and selected the "USER" option that lets you calibrate the sensor.

As described earlier, I removed the sensor from the engine and screwed it into the manifold block which was also connected to my leak-down compression tester:

I plugged the sensor back into the harness and then determined the ADC points at 0 psi and 90 psi. I chose 90 because the leak-down gauge only goes to 100 psi and I figured it is probably more accurate at 90 - besides, I only display pressures on the EMS up to 85 psi, anyway.

Apparently the sensor does have a perfectly linear response - after calibration, I found I could vary the pressure anywhere between 0 and 90 and the EMS displayed the exact same pressure as the compression tester gauge indicated.

I then re-installed the sensor on the engine and did a run-up and verified it was working properly ... and FINALLY, it does display "0" oil pressure when the engine is off! Ha!

I'm glad Matt at MGL suggested setting up the custom sender - saved me probably $150 by not having to purchase the UMA sender!

John

... and now to run-up a couple of hundred more hours to see if it wears out or not!  ;>)

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