[gkey]

Firstly, I am no expert on this, but here goes:

-- QUESTION--
This was my question to the CPA forum many moons ago:

I read the following in my POH (1976 337 G): "An alternator restart system, consisting of a momentary push-button type switch and four alkaline dry cell batteries...". Someone please explain this to me. I know about the push button, but batteries? Are there batteries somewhere that I have to change out for fresh ones? Where are these batteries? How do I know that this restart button will work at all? I cannot find out by reading the POH.

--ANSWER--
For those of you who do not know Tom Carr, he is in my eyes almost like the God of 337's - he is a walking encyclopedia on all technical matters regarding these planes. This was his answer:

"It has been my experience that the owner’s blind faith in the alternator restart system has lead many 337 owners down the primrose path of complete charging system failure and related expenses. I feel the placard on the instrument panel above that push button switch should be reworded to say, “If You Have To Push This Button, Something Is Seriously Wrong And You Had Better Be In VFR Conditions Or Preferably On The Ground”.

Here is my thought process on the restart system. If you look at the electrical system schematic in the owners manuals or POHs for the 337s that use that system you will see the alkaline battery pack will deliver 6 volts to the field terminals of both alternators once the button is pushed. This is assuming of course the small polarity diodes in the circuit just aft of the push button switch are NOT failed in the open position. That is a VERY common fault (reason #1 to REMOVE the battery pack).

I have seen where a new 337 owner listening to a CFI (with no 337 and more importantly no 337 electrical system experiences) gave the advice to just press that button whenever both alternator discharge lights were on. That operation took its toll on his bank account until better instructions were learned about that button.

During a taxi out to the run-up area with both engines pulled back to idle RPMs it is not uncommon for both of those "alt out" lights to come on, due to the low alternator output. The button is pushed and the alternators go into an UNCONTROLLED OUTPUT CONDITION since the regulators are being bypassed. If the button is held longer than it should be, the sound of alkaline batteries exploding can sometimes be heard if the polarity diodes have shorted, since the alternators are trying to charge those D cells. Usually the diodes burn open but not always (back to reason #1).

The uncontrolled output from the alternators can trip off the overvoltage sensor and that shuts down the regulators. Until the master is cycled off and back on, expect no output from the alternators unless you hold the button in again. This will be your second chance to explode the D cells if that has not already occurred.

Let’s say you run the battery down while waiting for a clearance, low engine RPMs, no output from the alternators, battery trying to carry avionics and lighting load and then everything gets quiet, the master contactor opens up. Pressing the button brings the alternators on line and you decide to increase the engine RPMs. The battery now may or may not come back on line depending on how low the charge still in the battery is. If it comes back on then the current surge from the alternators to a very low battery can over heat the cells and short out the battery internally. Short battery life results, forcing you into a very expensive hobby of buying a lot of batteries.

If the battery does not come back on line the alternators will not stay on line very long since the battery serves as an accumulator to the charging system helping to keep the bus voltage even when the regulators are in their off cycle with no output from the alternators. The surging bus voltage also causes the over voltage circuit to shut down the system.

This total electrical shutdown is GUARANTEED when launching off in IFR conditions just after the gear has been selected up, so I have been told. If it is VFR conditions then the total failure may take another ten minutes, again, so I have been told.

I feel that the alternator restart system is nothing but trouble and when it is used can cause some very expensive repairs when you have to buy charging system components.

I would DISCONNECT the battery pack and placard the system as INOP. Just my opinion."

--RESULT--
I have disconnected my system, and labelled it INOP. I only get the alt warning lights come on when taxiing or idling. I just added a little load (like switching on my landing lights), or increased my RPM's to 1200, and the alt warning lights went off.

Someone on the forum then added: "Replacing the regulator with a zeftronics one (which is STC'd) completely removes the need for the battery operated system anyway."