EDM 930 showing amp discharge

[CapeCodFlyer]

The JPI EDM 930 on my Skylane 182P is showing a discharge of the amps yet the voltage remains fine at 14+/-V. What's strange is whenever the throttle is pulled to idle, the ammeter needle moves to positive. Same thing happens whenever a load is applied (lights or avionics); the ammeter moves to positive. Whenever I apply power, the needle indicates a discharge of amps. The A&P tried a test today by turning off the alt master; while the voltage dropped as expected, the ammeter indicated positive amps. We removed and cleaned the ground to the battery, cleaned the voltage regulator contacts, and tightened the alternator belt but getting the same results. Since the battery is charging fine and the voltage has been stable, I believe it may be just an indicator problem but I'd still like to get to the bottom of this. Guess I could just fly the plane with every electrical switch turned on and keep power at idle but that doesn't seem too practical.

Any ideas would be much appreciated!

 

[jesse]

Well...it really sounds like your ammeter is hooked up backwards. I believe if they are they will indicate backwards.

Did this happen post MX or just randomly start occurring one day?

The fact that it tanks positive when you turn the alternator off pretty much indicates this has to be the case.

 

[CapeCodFlyer]

Well...it really sounds like your ammeter is hooked up backwards. I believe if they are they will indicate backwards.

Did this happen post MX or just randomly start occurring one day?

The fact that it tanks positive when you turn the alternator off pretty much indicates this has to be the case.

I recently purchased the plane and the broker noticed (and told me) about the issue when he transported it to his hangar. Per the logs, the JPI was installed by a first-rate shop (years ago) and the previous owner was meticulous about the MX on the plane. Given the previous owner's attention to the plane, I can't imagine it would have been installed incorrectly and he would have flown it for 3 years with the EDM giving an incorrect reading.

A new battery was installed just before I purchased it last October, although I doubt that could be related to the problem.

The shop that performed the pre-buy was supposed to take a look at it but just ran out of time.

 

[PaulMillner]

Can't know the history, but the ammeter shunt is hooked up backwards. Either reverse the two big wires (more difficult) or swap the two small wires (easy). Or, continue to wonder about the past and fly with a backwards shunt. Your choice!

Paul

 

[Dan Thomas]

Per the logs, the JPI was installed by a first-rate shop (years ago) and the previous owner was meticulous about the MX on the plane. Given the previous owner's attention to the plane, I can't imagine it would have been installed incorrectly and he would have flown it for 3 years with the EDM giving an incorrect reading.

A new battery was installed just before I purchased it last October, although I doubt that could be related to the problem.

If it was done years ago, some mechanic may have had it disconnected for some reason since then (replace the shunt, maybe) and reconnected it backwards. The symptoms all point to a reversed hookup.

When chasing glitches, go after the most obvious and simplest answers first.

Dan

 

[jesse]

I recently purchased the plane and the broker noticed (and told me) about the issue when he transported it to his hangar. Per the logs, the JPI was installed by a first-rate shop (years ago) and the previous owner was meticulous about the MX on the plane. Given the previous owner's attention to the plane, I can't imagine it would have been installed incorrectly and he would have flown it for 3 years with the EDM giving an incorrect reading.

A new battery was installed just before I purchased it last October, although I doubt that could be related to the problem.

The shop that performed the pre-buy was supposed to take a look at it but just ran out of time.

Hard to say how it happened but given what you're seeing it's what is happening. At the very least it's without a doubt the first thing you need to look into to resolve this.

 

[kontiki]

Without seeing WDM for aircraft and how the EDM Wiring is connected I wouldn't hazard a guess. If i saw a needle deflection I thought I shouldn't have, I might open individual breakers or fuses one at a time to see if the fault is associated with a particular circuit.


Sent from my iPad using Tapatalk

 

[CapeCodFlyer]

UPDATE - Problem Fixed. I was fortunate to receive a reply on the Cessna Pilots Association site from the avionics shop that originally installed the EDM. The owner even sent tons of pics of the panel update. He advised that a small twisted pair of wires had been reversed. My A&P reversed them this morning and problem resolved!

In case anyone is interested, here's my panel with my new Garmin GNC 255a Nav/Com.