Hi all,
We have a slightly peculiar problem with our Cessna 172S. On our way back from Belgium on Saturday, on the last leg when we were about to depart I noticed that the M Batt showed -1.5A and turned yellow. Cycling did not solve it. We departed and monitored it closely, it was VMC so we could easily return, or land enroute if need be.
During the flight, the M Batt fluctuated between -0.5 and -1.5 (it’s first at -1.5A that it turns yellow). Despite the slight discharge, the voltage remained steady between 28.0 and 28.1 on both M and E bus throughout the entire flight of 2h. We also tested increasing or decreasing the electrical load but it did not change anything.
On the ground we switched the ALT off and turned it back on after a few seconds, and the charge quickly went up to 6-8A before decreasing to -0.5/1.5. So it’s obvious that the alternator works and since it was recently changed, it’s unlikely that that would be the cause (in my mind, but this is not my area of expertise).
Any ideas of what could be the issue? The voltage regulator is our main belief at the moment but if anyone has seen similar behavior, or have any other ideas sharing would be much appreciated.
Thanks in advance!
Shitty regulator most probably. We have a lot of problems on our club’s 172R (180hp, 28v). Regulator changed many times by so called genuine but fails… last flight with it a cable broke after too many operations on it…
Quite challenging you departed. Once my alternator light goes red, I closely monitored my voltage from 23.8 to 23v in 15 minutes, with only GNS750 for radio and transponder. It was not a complete failure in your case.
The answer will depend on what that current indication actually is. Is there a circuit diagram somewhere?
Peter wrote:
The answer will depend on what that current indication actually is.
All current to/from the battery except to the starter motor and external power receptacle goes through this meter.
Is there a circuit diagram somewhere?
There is in the POH, but it is rather hard to read. This is a diagram (not showing most ground return paths) that I made for my club. It’s all in Swedish, but you should be able to figure it out anyway.
Could be a voltage indication problem. At 28.0V there should not be current going out of the battery, and especially not at 28.1V. Check the battery with a meter and compare that with what that display (whatever the display is – a G1000?) is saying.
Thanks all, we just had the aircraft at our service org and after cleaning the battery cables and the connection to earth(?j the issue went away. But I will pass this on so any potential problem can be ruled out. (And yes, it’s G1000).
