JPI EDM 900 Fuel Gauges

[Ctparis]

Has anyone experienced erratic fuel readings with their edm 900? I can fill my tanks and both will show full but after a few minutes or so, usually, the right side will bleed down to empty. The sending unit was replaced at conditional inspection and I have recalibrated the EDM as required. The engine alert light will flash and has to be cleared every few minutes during flight. Super annoying. Anyone got any thoughts?

By the way I’m new to the forum, but am very thankful for the wealth of knowledge here!

 

[JonJay]

It’s possible it’s the EDM but not likely. Is your sender well grounded? You should have a separate ground wire with star washer under the head of one of the sender screws.
You can also test the unit without the sender. The sender is 33-240 ohms I believe and a variable resistor between the input and ground or a decade box can simulate the sender across its range.
If it still shows this issue, you can fit up a dsub and tie directly to the EDM. EDM’s can be easily bench tested if you’re handy at making up dsub connectors. You just need power, ground, and the fuel level inputs. Just a few pins but I believe they are on different dsubs. You could also just pin them directly if you’re careful.
Also, how old is the 900? If it’s old, JPI has a program to swap it out for a brand new unit at a fraction of the cost of new. They don’t like having really old units out in the field.

 

[Ctparis]

A bad ground would make sense. It’s intermittent so that leads me to believe it may just be that.

The 900 is at least 10 years old I do believe. I’ve got the paper work on it at my hangar and I’ll check it out here in a little bit to be sure. I’m very interested in trading for a new one if I can. How do I get more info about this program?

 

[DanBeadle]

JPI recommends a separate ground wire from the tank to the EDM itself instead of relying on the airframe ground.

The EDM does have a filter in the firmware to reduce jitter on the display. As I recall, this filter is a few minutes. If yours is a tail Dragger, the sensor in the tanks will change value when you transition from a low angle of attack to very high nose after touch down. The gage may change a little due to the new high pitch.

Also, though not your complaint, the dihedral “hides” the last 10-15% of the tank. The EDM supports up to 4 data points. I recommend setting the upper calibration point at around 85%. With totally full tanks, you will not see a drop in fuel reported until the top 10-15% is burned off.

The dihedral issue is common to all resistive sensor reporting for all engine managers as it is a geometry issue. JPI offers a Capacitive Fuel sensor. This runs the entire span of the tank, reaching up into the dihedral and reads correctly all the way down. This design is commonly accurate to 1% repeatability.

For best results, take the time to use all 4 calibration points. Something like 80%, 60% and 20%.

 

[Ctparis]

Thanks for the replies. I checked and thought I may have found an issue with a connector on the ground wire that looked bad but replaced it and made no difference.

I did contact JPI about upgrading and they are offering a great deal. So I’m going to take advantage. I’ll update once I get it installed.

 

[elev666]

Thanks for the replies. I checked and thought I may have found an issue with a connector on the ground wire that looked bad but replaced it and made no difference.

I did contact JPI about upgrading and they are offering a great deal. So I’m going to take advantage. I’ll update once I get it installed.

I upgraded to my 930 about 15 years ago for 1K but the 900 unit was still brand new , I never did regret that upgrade the 930 is a beautiful unit !