Brief Electrical System Malfunction

[B2C2]

It has been pointed out in several posts but bears repeating that the system voltage will get unstable if you have more than the nominal resistance in the field circuit to the alternators. I have also several times had this issue. The first time all crimps were replaced behind the panel in the circuit feeding the field circuit for the alternators. The second time it was fixed by replacing the regulator selector switch. In my 1969 T337D this switch feeds either Reg 1 or Reg 2. Both regulators were showing jumpy voltages. Occasionally lighting the red overvoltage indicator. Replacing the regulator switch caused the voltage to be rock solid. After this was done I disassembled the switch and sure enough the terminals inside were badly worn. I would guess you had an overvoltage condition causing the alternators to go offline. then on recycling everything you got the circuit back. If you have a voltage indicator in your plane look at this. If its jumpy you probably have a field circuit issue.

[hharney]

Originally Posted by B2C2

In my 1969 T337D this switch feeds either Reg 1 or Reg 2. Both regulators were showing jumpy voltages. Occasionally lighting the red overvoltage indicator. Replacing the regulator switch caused the voltage to be rock solid.

1973 G models and on, DO NOT have the regulator switch. Just so you are not confused the front / rear regulators are automatic in the G and H model Skymasters system. The older system is different and uses a pilot selectable regulator switch.

[edasmus]

Thanks Herb...., you just answered my question!

[B2C2]

I looked at the schematic for this system and it looks to me like both regulators are normally active, with the alternator generating the greater output actually acting as the charging alternator due to the diodes in series with the alternator outputs. At similar output voltages both would be active. This is nice as it would cause no issue with a regulator failure, but would limit you to one alternator in that situation. With the older design one regulator feeds both alternators, so if one regulator fails the regulator switch brings both alternators back on line. The design in the G and H models does make it so that if one of the alternators is spiking it will take down the system, and the regulators will still have the path resistance sensitivity, with the alternator field switch as a possible source of higher path resistance similar to the regulator switch in the older system. If you continue to have problems I would say a look at all the connections along the field circuit path starting at the buss bar is worthwhile to see if you have a source of high resistance.