Testing Rotax 912/914 Generator and Voltage Regulator/Rectifier
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| Our original installation of the Rotax 912/914 regulator/rectifier. 24 years later, the two brown wires on the left were replaced by yellow wires of a new stator, see photo below. |
Most likely the electrical failure is caused by the voltage regulator/rectifier.
To verify this, we now carry in the plane a small multimeter that measures AC/DC voltage and resistance and use the checklist below.
The checklist assumes a standard installation as described in the Rotax installation manual Section 24-00-00, without additional external generator, and that the voltage is not in the normal range.
Learn key strategies for coping with these dangerous situations. They include two tools called Sully's Rule and Forecast Evaluation.
Testing Electrical System Beyond Regulator/Rectifier
It is possible, though quite unlikely, that the electrical failure is caused by something other than the generator and regulator/rectifier.
There is an almost infinite variety of possible causes. A pragmatic approach is as follows. Do the tests described below and carry out any required repairs. If this does not eliminate the problem, measure the voltage at various points, starting with the input to the master relay or, it that is not used, the master switch. Two readers have reported cases where the master relay or two master switches caused a large voltage drop.
Testing Generator
The regulator/rectifier has two yellow wires attached at the two G terminals. Pull off the two wires.
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| Yellow wires at two G terminals |
- Resistance between the two wires should be 0.1 - 0.8 Ohm.
- Resistance between either wire and ground should be infinite; that is, there is an open circuit.
- 15 - 20 VAC during idle
- 30 - 40 VAC during high rpm
If the generator fails any of the above tests, it is faulty and must be repaired; see the discussion at the end of this post for details.
Otherwise, reconnect the two wires. It does not matter which wire goes to which G terminal. Proceed to the next step.
Testing Capacitor
We assume that the installation uses a capacitor in conjunction with the regulator/rectifier.
If such a capacitor is not present, it should be installed. See the Rotax 912/914 installation manual for details.
A capacitor meeting the Rotax requirements is the SANG DCMX223U025AD2B capacitor (22,000uF, 25V, 85 deg max Temp, Aluminum Electrolytic Capacitor) offered by TEDSS.com at reasonable cost.
The capacitor smoothes out voltage spikes and protects the regulator/rectifier if the battery becomes disconnected from the electrical system while the engine is running and the generator is charging. The capacitor can fail in a number of ways:
- Equivalent series resistance (ESR) may be high.
- Capacitance may be low.
- Capacitor may fail open.
- Capacitor may fail shorted.
But the fourth, most troublesome case where the capacitor fails shorted, is readily detected with the multimeter mentioned above:
If the resistance between the plus and minus poles of the capacitor is not infinite, that is, it is not an open circuit, then the capacitor has failed and must be replaced.
Testing Regulator/Rectifier
Normal regulator/rectifier performance is as follows:
- Output voltage range: 13.9-14.5 VDC
- Max current: 22 A
- Max temperature: 80 deg C (176 deg F)
With Master switch on: Measure the voltage between the wire attached to the R terminal and the ground; it should be the battery voltage, about 12.5 VDC. Do the same test for each of the two wires attached at terminals B+ and C.
With Master switch off: There are two possibilities, depending on the wiring schema used in the installation.
First possibility: The above test shows 0 voltage for all three cases.
Second possibility: The above test shows the battery voltage for B+ and R, and 0 voltage for C.
The above photo shows the wiring producing the first situation in our plane. The white wire on the left comes from the master switch and goes to R, then to B+, C, and the capacitor.
For the "Master switch off'' test you may not know which case applies since you may not have a wiring diagram at hand. But don't fret. If you have passed the "Master switch on" test, just proceed. But if you fail that test, then a wiring diagram may be needed, for example to locate a blown fuse for the wires going to B+ and R.
Test that the housing of the regulator/rectifier is properly grounded: Resistance between regulator housing and battery ground should be essentially 0.
If the wiring fails any of the above tests, the wiring problem needs to be isolated and fixed.
If the wiring passes the above tests, yet output voltage of the regulator/rectifier is not in the normal range, then the regulator/rectifier likely is faulty and should be replaced.
Cooling of Rectifier
It is important that the regulator/rectifier is properly cooled.
In 2014 we added a cooling shroud that achieves a significant reduction of the operating temperature. The cooling shroud has worked well for us since so far it has prevented another failure.
Nevertheless, we have bought a spare regulator/rectifier and a small voltage/resistance tester that we carry on all cross-country flights.
If we ever have a charging problem, we will land at the nearest airport, test the regulator/rectifier, and replace it if needed. The installation requires just a 3/8 in. socket.
Repair of Generator
If the generator fails any of the tests described above, you must first solve this problem before any additional testing of the regulator/rectifier.
Potentially, there are a number of causes. The wiring going to the regulator/rectifier may have become faulty, or one of the coils of the stator has failed, and so on.
We had the case where the wiring had failed. This can happen on older engines like ours, which was manufactured in 1994. On these engines, the wires going from the stator of the generator to the regulator/rectifier and to the ignition module have substandard insulation that cracks and eventually breaks apart.
You can check for this by examining the insulation of the wires where they are connected to the regulator/rectifier. If the insulation seems brittle, or if the insulation has actually broken off and the exposed copper wire is oxidized, you have a sure sign that the wiring has failed.
You may think that you can cut off some part of the wire to the point where the wiring is okay, and attach new wiring. Odds are, no matter how far you cut back, the wire will be defective.
Another wiring check is at the ignition module, where two wires from the generator supply power. Here, too, examine the insulation for cracks and broken-off parts. Here is an example from our ignition module.
The two red wires at the horizontal connector have cracked insulation, indicating that failure of the wires, and thus engine stoppage, can occur at any time.
When any such defect is detected, do not try to repair the wiring, but simply replace the stator with the wiring.
In fact, we recommend that you replace the entire stator with the wiring no matter which part has failed. The stator is not cheap, but you are assured that everything has been properly assembled.
For a summary of the replacement process, go to the post Faulty Rotax 912 Alternator Wiring.
Symptoms of Impending Regulator Failure
Since you are reading this post, you most likely have had a failure of the charging system. The odds are that you believe that this failure occurred suddenly.
Indeed, every time you start the engine and turn on various pieces of equipment, the displayed voltage is low due to the low engine rpm required until the engine oil temperature rises to 120 deg F.
Once the engine oil has reached 120 deg F, you raise the rpm, and lo and behold, once you go beyond 3,000 rpm, the voltage climbs slowly to the desired level of 13.6 V.
Over time, this climbing of the voltage may slow down. That's okay, you think: As long as the voltage rises eventually to the desired value 13.6 V, all is well.
But the slower climbing actually is a symptom of impending failure.
So what should you do?
Upon start of the engine, do not turn on any equipment, but simply watch the indicated voltage. It should readily go beyond 12.4 V and climb a bit, say to 12.8 V or even higher while the engine turns 2,600 rpm for the warm-up period.
If this does not happen, the voltage regulator is on its way out. On the other hand, if you do see this modest rise at low rpm, most likely all is well and the run-up will produce 13.6 V as desired.
Test that the housing of the regulator/rectifier is properly grounded: Resistance between regulator housing and battery ground should be essentially 0.
If the wiring fails any of the above tests, the wiring problem needs to be isolated and fixed.
If the wiring passes the above tests, yet output voltage of the regulator/rectifier is not in the normal range, then the regulator/rectifier likely is faulty and should be replaced.
Cooling of Rectifier
It is important that the regulator/rectifier is properly cooled.
In 2014 we added a cooling shroud that achieves a significant reduction of the operating temperature. The cooling shroud has worked well for us since so far it has prevented another failure.
Nevertheless, we have bought a spare regulator/rectifier and a small voltage/resistance tester that we carry on all cross-country flights.
If we ever have a charging problem, we will land at the nearest airport, test the regulator/rectifier, and replace it if needed. The installation requires just a 3/8 in. socket.
Repair of Generator
If the generator fails any of the tests described above, you must first solve this problem before any additional testing of the regulator/rectifier.
Potentially, there are a number of causes. The wiring going to the regulator/rectifier may have become faulty, or one of the coils of the stator has failed, and so on.
We had the case where the wiring had failed. This can happen on older engines like ours, which was manufactured in 1994. On these engines, the wires going from the stator of the generator to the regulator/rectifier and to the ignition module have substandard insulation that cracks and eventually breaks apart.
You can check for this by examining the insulation of the wires where they are connected to the regulator/rectifier. If the insulation seems brittle, or if the insulation has actually broken off and the exposed copper wire is oxidized, you have a sure sign that the wiring has failed.
You may think that you can cut off some part of the wire to the point where the wiring is okay, and attach new wiring. Odds are, no matter how far you cut back, the wire will be defective.
Another wiring check is at the ignition module, where two wires from the generator supply power. Here, too, examine the insulation for cracks and broken-off parts. Here is an example from our ignition module.
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| Defective red generator wires at ignition module |
When any such defect is detected, do not try to repair the wiring, but simply replace the stator with the wiring.
In fact, we recommend that you replace the entire stator with the wiring no matter which part has failed. The stator is not cheap, but you are assured that everything has been properly assembled.
For a summary of the replacement process, go to the post Faulty Rotax 912 Alternator Wiring.
Symptoms of Impending Regulator Failure
Since you are reading this post, you most likely have had a failure of the charging system. The odds are that you believe that this failure occurred suddenly.
Indeed, every time you start the engine and turn on various pieces of equipment, the displayed voltage is low due to the low engine rpm required until the engine oil temperature rises to 120 deg F.
Once the engine oil has reached 120 deg F, you raise the rpm, and lo and behold, once you go beyond 3,000 rpm, the voltage climbs slowly to the desired level of 13.6 V.
Over time, this climbing of the voltage may slow down. That's okay, you think: As long as the voltage rises eventually to the desired value 13.6 V, all is well.
But the slower climbing actually is a symptom of impending failure.
So what should you do?
Upon start of the engine, do not turn on any equipment, but simply watch the indicated voltage. It should readily go beyond 12.4 V and climb a bit, say to 12.8 V or even higher while the engine turns 2,600 rpm for the warm-up period.
If this does not happen, the voltage regulator is on its way out. On the other hand, if you do see this modest rise at low rpm, most likely all is well and the run-up will produce 13.6 V as desired.
Another indication: Carrying a regular amperage load in flight, the voltage declines slightly, say from 13.6 or 13.7 V to 13.4 V. The regulator is sending you a message: It is on its way out. Recommendation: Replace it so that you are not caught short on an extended trip.
Our experience: After 2,500 hrs total time in 2024, we just started on our fourth regulator. The third one lasted just 500 hours.
Have any questions or feedback about testing the Rotax generator and regulator/rectifier? Please share your thoughts in the comments.
Our experience: After 2,500 hrs total time in 2024, we just started on our fourth regulator. The third one lasted just 500 hours.
Have any questions or feedback about testing the Rotax generator and regulator/rectifier? Please share your thoughts in the comments.
Learn key strategies for coping with these dangerous situations. They include two tools called Sully's Rule and Forecast Evaluation.
Thank you Very Much for useful information
ReplyDeleteso clear! tnx!
ReplyDeleteWhat about problems with 'whine' when transmitting on a 8.33KHz radio?
ReplyDeleteThe capacitor would seem to be the main culprit but can the regulator, seemingly working correctly, contribute towards this especially on 8.33KHz radios?
If you are following my advice and always have a spare regulator on hand, simply install the spare regulator and see if the problem goes away. If that doesn't work, advice from an electronics expert is needed. We never encountered a whine problem, even though for many years we did not have a capacitor since it was not specified for the original installation.
DeleteThanks nice sir but We have sudden engine cut during taxi once after that each start found normal probably cause may be generator stator failure??
ReplyDeleteIt is highly unlikely that you had a generator stator failure. The ignition of the engine is powered by two coils and two wires going to the ignition module. How old is the engine? We had the wires go bad after 25 years.
DeleteSuggest that you check the wiring connection at the ignition module and the grounding of that module. Also check the wiring connection at the ignition switch in the cockpit and the functioning of that switch.
On the carburetor side, check that mechanical connections are solid, and maybe drain and clean the carburetor bowls.Check that the idle position of the throttle is against the stop and cannot go inadvertently lower.
If such unexpected stoppage happens again, disassemble both carburetors, remove all jets, and blow out carefully with compressed air. We had an unexpected power reduction during the first 50 hours, and it turned out that there was something left in one of the carburetors from production. Complete disassembly and cleaning solved the problem -- it hasn't happened again in 25 years.
An excellent summary I have copied, printed and keep with my 1997 912UL Maintenance File.
ReplyDeleteI too experienced a sudden engine stoppage while backtracking for take-off and before performing my Mag Check for that flight. (I was fortunate the stoppage occurred when it did and not 5 minutes later.)
The original owner of this engine (1999 - 2004) sold the airplane with no maintenance records and with an estimated 250 hours on the clock. I purchased the engine from the second owner who logged an additional 400 hours and provided very detailed engine maintenance records. There is no mention of compliance with SB 912-026 - ie Stator replacement - or any reference to SB 912-026. The engine serial number is among those whose stator should have been replaced before April, 2000. I will check the insulation onf the charging cables next trip to my hangar.
It would be accurate to say my engine has been "purring like a kitten" since its 100-hour check in the spring up until the stoppage: there was no warning, no roughness, no slowing of the RPMs - nothing. The engine stopped like I had grounded the mag switches - actually, the first thing I checked.
I have done many tests and checks of the fuel, spark, intake and exhaust systems (I won't go into details, please take my word for it) and have not yet located the problem. I am leaning towards the stator wiring. The stator's two charging coil leads each give "on-spec" multimeter readings of 3.7 ohms when tested to engine ground. (I am away from my hangar now for a few months and did not have the presence of mind to test continuity from one lead to the other. It was clear the leads were not shorted to ground.
So, here's my question:
If the leads were joined as a result of faulty insulation, what reading would you expect to see when testing resistance between he joined leads and engine ground?
More important perhaps, what would happen to the current supply to the modules if such an interconnection occurred while running: a surge of the current?; a reduction of the current?; perhaps no current?
Stay safe; fly safely.
Best regards,
CanadaDan
You pose a number of questions that I cannot answer, not having access to a stator to make the required tests. But if I were you, I would do the following.
ReplyDelete- Remove the upper plugs from the cylinders. Crank the engine with the starter and check for a spark on each cylinder. Suspect you will not see a spark, given the suddenness with which the engine stopped. Caution: Make sure you stay away from the propeller!
- If there is no spark or the sparks are yellowish, you have the answer: It is a faulty stator or ignition module.
- If there are whitish-blue sparks on all four cylinders, install the plugs again and squirt a bit of fuel into the two intakes, of the quantity an engine primer would inject. Try to start the engine.
- If the engine starts and runs for a few seconds, you know that you have a fuel problem.
- If the engine does not start, there is some fundamental mechanical flaw where you check compression with automotive method, that is, with the starter cranking the engine. That way you get a reading on the pistons and the valve train.
- Regardless of all this, definitely replace the stator. When I replaced mine, the insulation of the wiring was literally falling off, it was that bad. When you have the new stator, you can do all the continuity tests and resistance tests you are writing about, and compare values for the old stator and the new one. But regardless of the case, it is mandatory for safe flight that the stator be replaced.
Caveat: This comment is just that: a comment telling what I would do if I were in your position.
Take care, stay safe,
Klaus
HI, The following may be of help for anyone else having this problem
ReplyDeleteIm maintaining a 912 UL and i was alerted to the fuses being melted, not blown but melted.......
After checking the regulator and we replaced it anyway i then started looking at the battery and then the capacitor.
The problem was finding what was loading the system down so the Generator was punching out 10+ amps. i removed the fuse for the capacitor to check the regulator during operation and found it was regulating correctly about 13.2V at idle. The regulator would climb to 13.8v at full power which is normal.
I checked the capacitance of the capacitor with my multimeter and found it was open. I connected the fuse for the capacitor again and tested the system and found the regulator was climbing from 13.7 thru to 14.0 at idle and higher during runup.
I personally have never seen a capacitor load a system down so much. Has anyone else seen this before?
Darren
I too have a melted 30A fuse. Just found it yesterday so Im in the process of troubleshooting. Ive heard that a bad capacitor can cause excessive current draw, but still not sure why the fuse would melt and not blow. Im also planning on changing the inline fuse holder (bayonet style) because Im concerned that anything hot enough to melt the fuse would do some damage to the fuse holder and surrounding wire.
DeleteHello, I'm on my 3rd battery now...they keep self discharging. Seems the "C" is crossed into the "R" and the R along with B+ are directly connected to the bus bar which is always being energized. After all the reading it seems I need to connect the "C" to a switched 12V source (master switch?). I also have the problem with running at idle, one I turn on the Garmin 796 I get "low voltage" warning. However above say 2500-3000 RPMs it doesn't go low voltage. Again, from all the reading it sounds like my voltage regulator is on its way out as well?
ReplyDeleteRotax 914 installed on autogyro Cavalon
Rereading your inquiry and looking at Alex's comment below, I finally understand what is happening. There are two ways to connect the regulator to the electrical system. The photo in the post shows the way our Zenith is wired: R, +B, and C are all connected to the master, and NOT directly to the battery. In the alternate way, only C is connected to the master, and R and +B are connected directly to the battery. There are pros and cons for either choice.
DeleteYou write that at idle you get a low voltage warning. That is normal. The alternator produces sufficient voltage upward from 3,000 rpm depending on the load. So a low-voltage indication at idle is NOT a sign that the regulator is failing.
You say that the B+ and R pins on the regulator should be zero voltage when the master is off, but that isn't correct I think, definitely wrong for some aircraft. The C42 has these pins wired to the battery positive through a fuse, no switch in the circuit, so these pins remain at battery voltage whether the master is on or off, so long as the fuse is good. I also reviewed the Rotax Owner video where all this information seems to have come from, and they also don't say these pins should be zero when the master is off, just the C pin because it should be wired through the master.
ReplyDeleteHi Alex,
DeleteWe appreciate your comment. It's readers like you that help us with required updates. There are two ways to wire the regulator/rectifier. The post describes the testing when R, +B, and C are connected with the battery by the master switch. Our Zenith 601 HDS is wired that way, as can be seen in the second photo of the post. It shows on the left the white wire coming from the master switch to R and then going to +B, C, and the capacitor. In the alternate case you mention in your comment, R and +B are permanently connected with the battery via a fuse. I have updated the post to include that possibility.
Love this write-up, I’ve used it many times and shared it even more. Thank you.
ReplyDeleteThe one thing I’d add since it had me scratching my head for days, is to test for a voltage drop across the master solenoid relay. I tried three regulator rectifiers before finally probing it directly with the engine running and seeing 16.7VDC, with 12.0V out of the master solenoid.
The premature master solenoid failure was do to horizontal plunger orientation rather than vertical. Everything else was working properly and the regulator rectifiers and stator survived the failing master solenoid, so it ended up being a simple fix but a more difficult diagnosis only because the idea wasn’t mentioned in this article.
Thanks again!!
Thank you for the kind comment. You make a good point: The low voltage may be caused by something other than the alternator or regulator. Have added an appropriate paragraph at the beginning of the post to alert the reader to this possibility. Thank you for pointing out this case.
DeleteGood post
ReplyDeleteHello there, 2006 CTSW 100HP Rotax Dynon panel. Intermittent amperage spike 18 to 25 amps
ReplyDelete5 to 10 times per hr. Voltage rock solid at 13.7 cleaned shunt, changed V/R with used one. Turn gen off amperage stays constant at -1 no fluctuations ??
I suppose that your amperage measurement of -1 with the generator turned off is without any load except for, say, the panel instruments. The generator charging capacity is about 20A, so cannot see how it can produce 25A. Do not believe this has anything to do with the generator or voltage rectifier/regulator. That leaves as only guess that there is a sudden consumption. You can try to isolate this by selectively turning off equipment.
DeleteWith all breakers off still has intermittent amp spike? Only thing I can think is a wire grounding out?
DeleteOr maybe the capacitor?
ReplyDeleteKlaus, thanks for such detailed post. Glad to see it is still alive after 10 years! Anyhow, Rotax problems still alive as well. I am on my 2nd failed VR. First time, battery simply stop charging to the point the electrical system simply shutdown. Replaced the battery (EarthX, which would no longer hold a charge) and replaced rectifier. All was good for a few hours then another low voltage indication. This time I've redone the connectors to the rectified which one the G's came out loose and the other were somewhat loose (no signs of resistance/heat on rectifier connectors). Installed a new rectifier. Charging immediately after engine started. Eventually climbed to 14v while in flight. New day, new flight then.... stuck at 13.2v! no climbing even after run-up. Followed your described procedures above with a multimeter. Everything checks, from the stator G wires with engine on and idle and master on voltage check for all the other wires. I have not tested the capacitor yet. It does not have any apparent signs of failure. I can send you the schematics if you are willing to assist!
ReplyDeleteMy generator/voltage regulator outputs13.4 V max. Your EarthX battery outputs 13.2 V. This means that it cannot be charged with the output of the generator. Suggest that you use a 12 V lead acid battery. Checking the capacitor: It’s main role is prevention of voltage spikes when the master is turned on. We didn’t have one installed for many years since it had not been specified in the original installation. So, to test whether the capacitor is causing problems, disconnect both leads of the capacitor, turn off all electronic equipment and run the engine to check the generator output.
DeleteImportant note: I am NOT telling you what to do. I only state what I would do if faced with the difficulties you have encountered. Hence I do not assume any liability whatsoever.
Lastly: This website will remain operational for many years to come. It obviously is helping a lot of Rotax owners.
Have used EarthX battery since new. It's factory standard. The issue is not the battery for sure. I will install a new 25v circuit breaker and install a new capacitor just in case. Will report back afterwards.
DeleteAre you measuring total current usage? It almost sounds as if you have usage spikes produced somewhere.
DeleteGood
ReplyDelete