[Paul462]

Any ideas on how to fix an inappropriately tripping alt field CB would be appreciated in my 1968 T337C, dual alternator installation. During the last hundred hours or so, the alt field CB has taken to tripping occasionally upon landing (after power reduction during the flare), and once during approach (23 in/2400 RPM). In all cases the pulse & strobe lights were on; in one case the pitot heat was on (on approach), in one case they were all on.

We disconnected the HI/LOW volt warning system (its on the same CB); the problem persisted.

The problem occurs with either regulator.

Yesterday I tried load-testing on landing, as follows:

Yesterday I flew N2576S to Flagler, turning off the Front alt/batty switch prior to landing Flagler (then progressively adding load), and turning off the Rear alt/batty switch landing KEVB (New Smyrna Beach), adding load there too. The results:

REAR ALTERNATOR ONLY: Approaching Flagler, Front alt/batty switch OFF, 22 in/2400 RPM: avionics ON, beacon & strobes ON, pulse lights ON – 27.2 volts.
Pitot heat ON: 28v.
Landing & taxi lights on: 27.4v.
20 in/2400 RPM: same
15 in/2400 RPM: 27.5v.
Upon landing, the EDM 760 alarmed @ 23.8v.
Approx. 2 minutes after landing, while taxiing up to the self-serve pumps, the alt field CB tripped.

It took 27 minutes to be able to re-set the alt field CB.

FRONT ALTERNATOR ONLY: Approaching KEVB, Rear alt/batty switch OFF, 22 in/2400 RPM: avionics ON, beacon & strobes ON, pulse lights ON – 27.2 volts.
Pitot heat ON: 26.5v.
Landing & taxi lights on: 27.2v.
20 in/2400 RPM: 26.8v.
15 in/2400 RPM: 25.9v.
Upon landing, the EDM 760 alarmed @ 23.6v.
Turning off the runway after landing the alt field CB tripped.

At one point around when I added the landing/taxi lights it seemed the FRONT alternator voltage was oscillating up and down within a 1v range.

The two alternators appeared to perform in the same ballpark, although the front one landing at KEVB seemed to be a little weaker than the rear one (judging from the voltage it was able to maintain). Is it normal for the voltage to fall that much on landing (to approx. 23.6v)? I’ve been seeing that EDM 760 alarm for a while now.

Any thoughts, other than weak alternator(s)?