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
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