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AF-2500 EMS tachometer intermittent/erratic

N444JN

Member
Any ideas? I purchased a used RV-8, after flying it for about 15 hours (120 airplane TT), tachometer started to become erratic. Continually got worse and now pretty much dead… It is the Advanced Flight Systems AF-2500 Engine Monitoring System with the ACS2002 analog module. All wiring looks very neat and connected. Could the transducer fail? I unscrewed the transducer from the right mag (one without impulse coupling) to inspect and is very clean.
 
The transducer can fail. If it?s has a sensor in the magneto vent port, I have seen hose fail before. You could try putting a blast tube on it, but they are not expensive.
 
I?m experiencing erratic tach reading on the AF-2500 also. I replaced the sending unit and still have the erratic readings. The mags, plugs, plug wire harness and mag switch have 137 hours on them. This engine monitor has 500 hours on it and just started the erratic tach readings. The engine runs perfectly. Any trouble shooting guidance would be appreciated.
 
I?m experiencing erratic tach reading on the AF-2500 also. I replaced the sending unit and still have the erratic readings. The mags, plugs, plug wire harness and mag switch have 137 hours on them. This engine monitor has 500 hours on it and just started the erratic tach readings. The engine runs perfectly. Any trouble shooting guidance would be appreciated.

Most erratic issues with sensors using are grounding related. Make sure you have a solid ground, good crimps, no loose wires, etc. Things tend to vibrate loose over time.
 
I'm with Bob on this one. The 3-wire tach sensor needs all 3 wires to be properly connected to the ACS2002 computer module. AND the shield connection on that 3-wire cable should be grounded in order to prevent EMI-related craziness in your tach needle.

Just as an example, if I quickly switch my seat heaters from high to low, the seat heat control relay produces a little bump of energy that's not snubbed by the diode across the coil. When I do this without the engine running I see a momentary jump in the tach reading. All that to say protection of the tach signal from EMI is an important consideration.
 
If you have anything wrong with your ignition system it will cause jumpy RPM readings.

Bad plug, plug wire, points, .........

We had one customer that finally found a bad plug wire, engine ran perfect.

You are checking the resistance of your plugs and wires during your condition inspection?

When it is jumping try running on the left or right ignition to see if it goes away.

I have a number of customer stories about jumpy RPM:

1. Really upset customer on the phone with a jumpy RPM, during his phone rant telling me how bad our unit was he just happened to mention he was also really upset with his Storm Scope. Apparently his Storm Scope was reporting Lightning strikes near the front of his plane all the time. After fixing his ignition system both problems went away, even though the engine ran "perfect and he was positive nothing was wrong with the engine".

2. Another customer had a Mag failure a few weeks after the RPM started jumping and was mad at us because our tech support person told him to replace the sensor and did not mention he could have a mag problem.



Rob Hickman
Advanced Flight Systems
 
Erratic RPM

Happened twice to me. First time I replaced the sensor AND cleaned/secured the in line connectors and the problem went away! Second time I just cleaned and secured the in line connectors and the problem went away. Try cleaning, tightening the connectors, heat shrink, and bundle/secure to something like a mm tube to reduce vibration!
BTW, I am using the sensor in the mag vent on my IO 360.

Hope this helps!

Cheers,

db
 
Thanks to all of you for tuning in with great support. I will follow your suggestions and report back with the solution... you guys are appreciated very much!
 
Still have erratic tach. I did perform an in flight mag test and the tach functions perfectly on the right mag. The sensor is mounted on the left mag. Anyone have a suggestion?
 
Still have erratic tach. I did perform an in flight mag test and the tach functions perfectly on the right mag. The sensor is mounted on the left mag. Anyone have a suggestion?

I suggest re-reading the recommendations from AFS. Did you test the resistance of the plugs and wires connected to the left mag?

Larry
 
Erratic Tach Solved

Rob worked through some testing with me. All pointed to the Hall effect sending unit. AFS sent me a new one, and all works perfectly! Thank you Rob and Tricia.
The sending unit was a year old. The black wire was loose in the epoxy. Apparently it doesn?t take much for a wire there to become loose. Anyway, I?m a happy camper again.
 
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