C402C Voltage Reg / Alternator Mystery Fault
I'm writing this on behalf of a 71 year old engineer who has a total aversion to computers.
We are preparing a C402C for a ferry flight from Indonesian to Australia , the aircraft hadn't flown for 18 months due to a number of electrical problems that the local engineers couldn't figure out.
The Australian engineer has spent 3 days resolving the following problems
- mismatched alternators on either side of the aircraft , 100A and 60A , now both 100A
- faulty wiring including the LH alternator being wired directly into the field of the RH alternator, it took about 2 days to go over all of the wiring and return it to spec in accordance with the wiring diagram. There were a number of incorrect connections which had burnt out the diodes on both of the original alternators (now replaced).
The current fault that has him a bit perplexed is this. When testing the LH alternator and setting the voltage regulator to 28 volts at 1,200rpm and then loading it with 15 amps for 2 mins as per the maintenance manual and then engaging the RH alternator with the engine off everything is fine and both voltage regulators show 28 volts. As soon as engine rpm goes to 1,500rpm the voltage goes to 31 volts and trips the over voltage switch taking the alternators off line , we have tried 4 different voltage regulators including a brand new unit.
Has anyone run across this before ? I don't rule out the possibility of a further wire being hooked up incorrectly (even though he's been over it a number of times).
Any suggestions of what else we can look at would be appreciated.