on my flight today I did just before landing switched on a lot of electrical users (pitot heat, strobs, nav, lights, etc) and more or less a sec later the “high voltage” and “lo bus” lights came on, also amp meter showed discharge? cessna 182 witn O470
any clues for me where to search? ( alternater belt is ok)
i had to leave a bit in a hurry after landing due to some other commitments so i had no time yet to investigate further
thanks
The first thing to do is to make sure the alternator output rating is indeed sufficient to power everything you switched on. If your aircraft had a lot of avionics upgrades, this issue may have been overlooked. Other than that, the two prime suspects are the alternator and the voltage regulator (if they are separate units).
If the low voltage light came on when the engine RPM went to a low figure, that might indicate
Modern avionics doesn’t draw much current. My phone plane draws 25A (not counting landing gear pump or the flap motor) and modern alternators can do about 70A.
The key on this that you seem to suggest you have seperate high and low voltage lights and that the high voltage light also came on!
Which Cessna type exactly? Do you have a low voltage light to meet UK CAA requirements?
If you have both lights, and high voltage light indicates an high voltage has occured, the overvoltage protection is tripped, also indicating the low voltage.
On some (older) Cessna’s high voltage and low voltage is the same light, as it either indicates a low voltage due to alternator not switched on, or over voltage protection activation (low voltage due to high voltage first) has activated.
On older Cessna’s actual voltage is not measured, it only sees if electrical (bus) power is supplied to the alternator regulator. You might have a UK CAA low voltage warning, which does measure voltage, which would be a good solution.
On newer Cessna’s warnings are generated by measuring voltage, which is again a good solution. To troubleshoot your system it is important to know the difference between this systems, and know which system has been installed.
Efficient troubleshooting is done with multiple analog multimeters.
A high voltage relay will disconnect the field circuit between the voltage regulator and the alternator. Without the alternator operating, you will get both a discharge indication and a low voltage indication. To reset the high voltage relay, on many systems you have to power off the battery master. If you power both the alternator and battery master, and then power them back on, if the system is still operating, the alternator will come back on line. If this resets the alternator, then the over voltage was a false or temporary indication. Sometimes this will occur if you start with the alternator in the On position.
Hello,
thank you all about your help
I will be back in the hangar on friday and i will report you back on the findings.
I hope it will be something like you discribed to reset the system.
I guess the low bus light is as Jesse mention the OK CAA requirement as i dont find it in the POH and the plane is on the G reg It is a 1975 182P
in the meantime i also found a maintance manual and there is also a pragraph on how to reset
(i hope for this , as it would be much cheaper than a replacement alternator, as it is nowadays difficult to find someone that are still repairing alternators as this is no more a market in the car repair world)
With an older Cessna like yours, the high voltage light is actually low voltage (no input to the alternator regulator) it is unlikely your alternator is at fault in this case, something which prevents the regulator from being powered is likely (field switch / field circuitbreaker / overvoltage protection / regulator or associated input wiring)
problem fixed
recycled master switch and all ok again
every day is a school day!!!
thanks to all of u for t help
Activating this old thread seemed better than creating a new one.
NCYankee wrote:
A high voltage relay will disconnect the field circuit between the voltage regulator and the alternator. Without the alternator operating, you will get both a discharge indication and a low voltage indication. To reset the high voltage relay, on many systems you have to power off the battery master. If you power both the alternator and battery master, and then power them back on, if the system is still operating, the alternator will come back on line. If this resets the alternator, then the over voltage was a false or temporary indication. Sometimes this will occur if you start with the alternator in the On position.
I have an intermittent issue at startup with the alternator going off-line, and this is exactly the procedure I use to get it back online. I’m still trying to get to the bottom of the issue. I have a C172 XP, with a 24V system and 60A alternator (direct drive, not belt-driven). While switching the alternator off and back on usually does the trick, it is not recommended by CPA for a direct drive alternator due to the torque load on the coupling. So I’d like to resolve the underlying issue.
As far as I can tell, the problem seems to be related to switching on the avionics bus. I’ve never had it occur during flight, and none of the circuit breakers pop. Speaking with Zeftronics, they suggested checking the battery and avionics bus grounds. If those are both good, checking for a spike above 32V when turning on the avionics bus. The problem with that is the intermittent nature of the issue. If the latter is the case, would it indicate a faulty avionics switch? Any ideas?
A second electrical issue is that the battery is only being charged above 1000-1100 rpm. So the battery is being discharged during taxi unless I keep the rpm up and ride the brakes. I have always thought that the voltage regulator is the culprit here, but the unit is a Zeftronics which has a good reputation. Anyone have any thoughts on that too? Perhaps also a ground issue?
