AOG with electrical problem - help please!

[robw]

All,

We have one of our planes stuck at a remote strip in Zambia with an electrical fault. Low voltage warning. Front and rear alternators won't engage. Tried resetting master. Tried standby master. Engaged/disengaged voltage regulators. Jiggled switches. Tried revving engines up to 2500. Tried tapping circuit breakers. Nothing helps. Probably got only one start left.

We are suspicious of a faulty generator field circuit breaker but ANY suggestions for things to try would be greatly appreciated.

[CO_Skymaster]

I have a 337A (1966). When I first bought it, I had all types of electrical problems. The issues I found eventually were:
1. Weak Battery in the circuit.
2. I hadto remove some of the electrical connections to some of those relays on the front firewall, on the pilots’ side, and clean the connectors with a steel wool pad. Corrosion had build up on them.
3. The rear alternator didn't work at all, and the front kept kicking off.

Some issues I found out about, after researching was that the alternators required a small amount of current from the battery to engage. If there is a weak battery in the system, the alternators would not engage. Even when I cranked up the RPM, I still couldn't get a charge to the battery. I could even get the aircraft to started, but once that energy was drained after the initial start, or after I turned on all the avionic, the alternator would shutdown.

I had to replace my battery when I first bought the aircraft. Recently, I have another weak battery, but I tried to desulfinate it by using my battery minder which has a desulfinate setting. I never went out to retry it again (That was last Christmas).

Even after replacing the battery, the front alternator would turn on, but would intermittently shutdown. I solve that problem with the cleaning of the electrical contacts. I also replace the Voltage regulators, but I don't think that was the problem.

This is a lot of information, but I hope you can find some solutions in here.

Karl

[JamesC]

Don't forget to clean the connections going to the diode board and the voltage regulators as these tend to be exposed. Clean every blade connector you can find in the cct. esp. the ones at the battery and alternator switches. They may look clean but if you scrape them you will see they are likely coated with a thin layer of corrosion, especially if somebody combined copper and aluminum connectors (yielding galvanic corrosion). All these slight increases in resistance are enough to keep the alternators offline.

[robw]

Thanks James & Karl. Appreciated.

We got it going today but we are not sure why. Which means that the probability of repeat failure is pretty good. We bridged out the field circuit breaker and proved it was not that. Finger of suspicion now rests on the alternator switch. Much cleaning of terminals in confined spaces in 33dC heat is not fun.

Old aircraft! Sigh!

[JamesC]

After it happened to me (and a problem with the landing gear horn sounding) I cleaned every blade and banana plug type connector I could find in the acft. Lots of fun. Battery switch is prone to internal resistance build up with minor internal contact arcing over the years also. Easy to replace both the batt and alternator switches - replaced mine.

[chrisskymaster]

Do you have an alternator reset button? If so, make sure the 4 C batteries are fresh. When you press the alternator reset button it sends a field to the alternator to get them started. Worth a look!

[kragby]

In my '73 P337, it turned out to be the couplings between the engine and the alternators that were slipping. Replace both with rebuilts - expensive at 600 per but fixed the problem.