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darinh
Joined: 20 Apr 2006
Posts: 327
Location: Utah
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 Posted: Wed May 06, 2009 8:32 am Post subject: Radio Noise Problems - Rotax 914 Alternator Problems |
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I am having a problem I hope someone can help me with. I am not an electronics guy by any means but I did wire my panel and everything has worked well up to this point. Here is a description of the system and the problem:
System:
The engine is Rotax 914 UL and has 40 hours on it since new along with all components. I have a 40 amp alternator (Nippondenso modified by Flightcrafters I will refer to as the ND Alt.) mounted to the back of the gearbox where you would typically mount a vacuum pump. The gear ratio is .43:1 so the alternator does not really come alive until about 3200 engine rpm or so and then will not produce quite its rated capacity due to the gearing. I have set the engine up as a dual alternator system for redundancy so the backup is the Rotax (onboard) generator. Both the ND alternator and the Rotax generator are wired to a relay and a 3 position switch for selection of Main Alt (ND Alt.) - Battery - and Aux. Alt. (Rotax Generator). I have the large capacitor installed on the Rotax generator lead and was under the impression that the ND alternator does not need the capacitor as it is rectified and regulated internally. Again, everything has worked great up until now.
Problem:
Now the problem. At about 38 hours total time my "idiot light" lit up while cruising at 5000 rpm and 31" MAP. The EIS indicated that my oil pressure was too low - 18 psi (55 - 65 psi is normal). Litterally in the course of a couple seconds, the indication went from 18 psi to 75 psi and continued to fluctuate rapidly between the mid teens to mid 90's. I new at this point that is was a faulty indication and not actual loss of oil pressure. Upon landing my suspicions were confirmed as all oil lines were tight and I had lost no oil. A quick check of the system indicated that oil was still circulating. The OP sender was suspect and was replaced and moved from the engine to the firewall to minimize vibration and heat fatigue. The next flight showed OP as normal. All good right? Wrong. The next flight the oil temp sensor started fluctuating rapidly also...210 degrees to over 300 then back to 210 within a second or two. I really didn't think that the temp sensor was going bad but really didn't have any other idea of what it could be. As I was sitting there idling, I switched from the ND alt. to the battery position and the temps immediately stabilized. Next I switched to the Rotax onboard generator and everything stayed normal....then went back to the ND Alt. and the problems came back. With this, I pinned the problem on something to do with the ND alternator.
On the ground I did some run up tests and found that when the ND alternator is providing power my oil temps are either abnormally high or abnormally low depending on rpm setting (high rpm = high temp. low rpm = lower temp). Again, with the Rotax onboard generator providing power, all is normal. Then, during one of my runups, my indicated system voltage plummeted while running on the ND alt. I switched back to the Rotax generator and voltage came right back to 13.8 so I knew I had some type of alternator failure. I removed the ND alt. and found that one of the copper output leads from the coil was completely broken. At this point I thought I had found my problem as obviously a broken lead would cause weird voltage issues and possibly mess with my engine sensors. The alternator was repaired by a reputable shop but because it has a spline gear he could not turn it up to check the rectifier/regulator.
I installed it and am still having the problems with oil temps fluctuating wildly and to add to it, I have a ton of noise in my radio when at cruise power settings. During a test flight today, the tower told me my transmissions were broken and unreadable with a high pitched wine. Isn't this classic alternator interference? By the way, I have traced all grounds on the senors (most ground through the engine) and this has checked out. So I don't think it is a grounding issue. Everything worked fine before this and I haven't changed anything with the electronics with the exception what is mentioned here with the sensors.
Sorry this is so long winded but I wanted everyone to have a complete picture of the issues.
Now the questions:
Could or will broken output leads cause the voltage regulator on the ND alternator to fail? If so, how can I check if it is toast?
From my description above does anyone have any advice or comments that might help?
I am getting pretty frustrated so any help would be greatly appreciated!
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Darin Hawkes
Series 7
914 Turbo
Kaysville, Utah |
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user9253
Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan
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| Posted: Fri May 08, 2009 6:33 am Post subject: Re: Radio Noise Problems - Rotax 914 Alternator Problems |
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Darin,
It sounds like the rectifier/regulator is at fault. Depending on the cost, the easy way to trouble shoot is to replace it and see if that fixes the problem. If you know someone with an oscilloscope, looking at the waveform will tell if the rectifier/regulator is doing its job. A diode or its connections could be open. Instead of outputting steady DC, the output could be pulsing DC. Lacking an oscilloscope, measure the output with a voltmeter set to AC. Compare the voltage readings of each alternator with the meter set to DC and also set to AC. Let us know what you find.
Joe
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Joe Gores |
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darinh
Joined: 20 Apr 2006
Posts: 327
Location: Utah
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| Posted: Fri May 08, 2009 8:08 am Post subject: Re: Radio Noise Problems - Rotax 914 Alternator Problems |
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Joe,
Thanks for the reply. I don't have access to an oscilloscope but can measure voltage using a meter on AC. What in particular would I be looking for when comparing the AC vs. DC? I would expect to see around 14.0vdc but what am I looking for with AC? Maybe a dumb question but I am dumb when it comes to this stuff.
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Darin Hawkes
Series 7
914 Turbo
Kaysville, Utah |
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user9253
Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan
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| Posted: Fri May 08, 2009 2:39 pm Post subject: Re: Radio Noise Problems - Rotax 914 Alternator Problems |
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| Quote: | | What in particular would I be looking for when comparing the AC vs. DC? |
Darin,
I probably gave you bad advice to check the AC output of the alternator because many meters will not give an accurate reading. I used a Fluke 76 meter (which I trust) and measured 0.015 volts AC at the battery on my Ford Expedition with the engine running. However, when I used a couple of other meters, they read anywhere between 12 and 30 volts. Bob Nuckolls book tells how to test an alternator while bypassing the regulator. Other than replacing the rectifier/regulator, the only thing that I can suggest is to test the diodes with an ohmmeter, if they are accessible. Double check all wires and connections. The majority of electrical problems are caused by bad connections.
Joe
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Joe Gores |
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