user 21975
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There are a number of mechanical gurus on this forum. I could use some expertise. My A&P has been scratching his head about this for a while. There doesn't seem to be much in the maintenance manual about my specific problem.
It's a 28V Seneca IV. I've owned the airplane for about six months now. It's tough to get a good checkout on the airplane, not many experienced pilots on the late model Seneca's. As part of my run-up, I've been checking the digital ammeter, and testing the left and right buttons for positive amps to ensure the alternators are functioning.
Intermittently, on one out of every 20 flights or so...the ammeter would show 0 amps, left and right at the same time only. I suspected a false intermittent indication as it would happen to both sides simultaneously.
I also recently had a digital clock installed, that also had a function that displays voltage on the avionics bus. The one time I had an intermittent double zero ammeter indication since then, I was able to verify 27.8 volts on the bus, so I knew at least one but probably both alternators were functioning. I figured I'll have the shunts checked out when I got done with the trip (never got to it). Odd that both would show zero at the same time.
Now the plot thickens! My last flight, shortly after takeoff, I got a single red flash from the annunciator panel "Alternator." Then it went dark. Hmmm... I checked the circuit breakers, and sure enough, the right alternator 70amp breaker had popped. I gave it a couple minutes to cool and tried a reset, and it instantly popped out again. Well, that's done. Now I am a bit annoyed that the annunciator panel is dark. It should be showing a steady red "Alternator" light. It isn't. My new clock was still showing 27.8 V, so I knew the left alternator was still cranking out amps. What did the ammeter show? 70.4 Amps left, and 70.4 amps right. That ammeter is apparently useless. I go through the checklist and it gets to the point of "Reduce electrical load as required." Hmmm... I don't have de-ice equipment, the radar was off, and there is no air conditioning. Pitot heat was also off. No big loads. Then I finally realized that on my run up checks, the normal reading of 70 Amps per side isn't giving me any load information. The POH says to use the ammeter "to determine how much the electrical load should be reduced." By the way, the 45 minute flight home on the left alternator only, gradually dropped bus voltage down from 27.8 volts to 27.3 by the time we landed. Again, no real significant loads, just normal day vfr, yet it seemed the left alternator could not keep up with the load?
My mechanic and I got a new alternator, 60Amp alternators, by the way. Got it up and running. Now I am left with an ammeter that shows either a full (more than full) load on both alternators of 70.4 amps, or the rare false intermittently 0 amps indication. Nothing in between. Both sides always the same.
Okay, as part of the test run with the new alternator, with the master switch on, everything else off, the ammeter shows -70.5 amps. Avionics master on, the avionics bus voltage was 23.5V. Good so far, except for the excessive negative indication. I push the test switches in. Got +70.3A on both sides. Odd with a combined -70.5a. Not so good. Then I started the left engine. After start, I still had the Red Annunciator illuminated "Alternator." That is normal and expected as the right engine was not started yet. I push the two test buttons, and got +72.5 on the left, and +70.3A on the right. Probably a very high reading for the left, and a false reading on the right. Then I started the right engine. The "Alternator" annunciator light extinguished. Good. Combined ammeter reading was 71.4A, the left 70.9A, and the right 72.8A. I then pulled the left alternator circuit breaker, and the annunciator panel light FLASHED "Alternator." There is nothing in the POH or the maintenance manual that says there is a function of the annunciator lights that "flashes." While the left Alt CB was pulled, the ammeter showed combined 70.4amps, and each side also showed the same 70.4 amps when the test buttons are pushed. Odd. I reset the breaker, then pulled the right Alt 70amp Circuit breaker. This time no annunciator light for "Alternator" as I would expect. Same as my in flight alternator failure. I then reset the breakers. Next step was to pull the left 5 amp field current circuit breaker. I did get the "Alternator" annunciator light as expected. I reset it, and then pulled the right alternator field current 5 amp circuit breaker. Now I finally get an "Alternator" annunciator light. Of course, the ammeter still falsely showed combined 70.4 amps, and 70.4 amps when testing the left and right switches. When turning off the alternator rocker switches, it gives the same result as pulling the field current circuit breakers.
So, for the gurus out there... what tests should my A&P run, which wires should we check, and finally, which components are suspect? The mech wants to order a new Alternator Control Unit (voltage regulator) to start. I really would rather not buy a new electrical system one component at a time...not only for the expense, but for the significant delays it takes to get a new component.
Ideas?
Am attaching a couple schematics to help...
It's a 28V Seneca IV. I've owned the airplane for about six months now. It's tough to get a good checkout on the airplane, not many experienced pilots on the late model Seneca's. As part of my run-up, I've been checking the digital ammeter, and testing the left and right buttons for positive amps to ensure the alternators are functioning.
Intermittently, on one out of every 20 flights or so...the ammeter would show 0 amps, left and right at the same time only. I suspected a false intermittent indication as it would happen to both sides simultaneously.
I also recently had a digital clock installed, that also had a function that displays voltage on the avionics bus. The one time I had an intermittent double zero ammeter indication since then, I was able to verify 27.8 volts on the bus, so I knew at least one but probably both alternators were functioning. I figured I'll have the shunts checked out when I got done with the trip (never got to it). Odd that both would show zero at the same time.
Now the plot thickens! My last flight, shortly after takeoff, I got a single red flash from the annunciator panel "Alternator." Then it went dark. Hmmm... I checked the circuit breakers, and sure enough, the right alternator 70amp breaker had popped. I gave it a couple minutes to cool and tried a reset, and it instantly popped out again. Well, that's done. Now I am a bit annoyed that the annunciator panel is dark. It should be showing a steady red "Alternator" light. It isn't. My new clock was still showing 27.8 V, so I knew the left alternator was still cranking out amps. What did the ammeter show? 70.4 Amps left, and 70.4 amps right. That ammeter is apparently useless. I go through the checklist and it gets to the point of "Reduce electrical load as required." Hmmm... I don't have de-ice equipment, the radar was off, and there is no air conditioning. Pitot heat was also off. No big loads. Then I finally realized that on my run up checks, the normal reading of 70 Amps per side isn't giving me any load information. The POH says to use the ammeter "to determine how much the electrical load should be reduced." By the way, the 45 minute flight home on the left alternator only, gradually dropped bus voltage down from 27.8 volts to 27.3 by the time we landed. Again, no real significant loads, just normal day vfr, yet it seemed the left alternator could not keep up with the load?
My mechanic and I got a new alternator, 60Amp alternators, by the way. Got it up and running. Now I am left with an ammeter that shows either a full (more than full) load on both alternators of 70.4 amps, or the rare false intermittently 0 amps indication. Nothing in between. Both sides always the same.
Okay, as part of the test run with the new alternator, with the master switch on, everything else off, the ammeter shows -70.5 amps. Avionics master on, the avionics bus voltage was 23.5V. Good so far, except for the excessive negative indication. I push the test switches in. Got +70.3A on both sides. Odd with a combined -70.5a. Not so good. Then I started the left engine. After start, I still had the Red Annunciator illuminated "Alternator." That is normal and expected as the right engine was not started yet. I push the two test buttons, and got +72.5 on the left, and +70.3A on the right. Probably a very high reading for the left, and a false reading on the right. Then I started the right engine. The "Alternator" annunciator light extinguished. Good. Combined ammeter reading was 71.4A, the left 70.9A, and the right 72.8A. I then pulled the left alternator circuit breaker, and the annunciator panel light FLASHED "Alternator." There is nothing in the POH or the maintenance manual that says there is a function of the annunciator lights that "flashes." While the left Alt CB was pulled, the ammeter showed combined 70.4amps, and each side also showed the same 70.4 amps when the test buttons are pushed. Odd. I reset the breaker, then pulled the right Alt 70amp Circuit breaker. This time no annunciator light for "Alternator" as I would expect. Same as my in flight alternator failure. I then reset the breakers. Next step was to pull the left 5 amp field current circuit breaker. I did get the "Alternator" annunciator light as expected. I reset it, and then pulled the right alternator field current 5 amp circuit breaker. Now I finally get an "Alternator" annunciator light. Of course, the ammeter still falsely showed combined 70.4 amps, and 70.4 amps when testing the left and right switches. When turning off the alternator rocker switches, it gives the same result as pulling the field current circuit breakers.
So, for the gurus out there... what tests should my A&P run, which wires should we check, and finally, which components are suspect? The mech wants to order a new Alternator Control Unit (voltage regulator) to start. I really would rather not buy a new electrical system one component at a time...not only for the expense, but for the significant delays it takes to get a new component.
Ideas?
Am attaching a couple schematics to help...
