We recently replaced our rectifier regulator in CTsw (650 hr) and solved repeated low voltage alarms. We did extensive diagnostics (see below) and found a definite temperature dependence, which pointed directly at the rect-reg as the failing component. Here were our diagnostic steps:
Check all ground wires, clean if corroded, and tighten.
Clean all male (easy) and female (difficult) contacts on the rect-reg plug.
Operate an accessory-plug voltmeter (inexpensive at auto part stores) and watch the D120 indications and independent voltmeter indications in parallel. Take an observer; this is too absorbing for the pilot to do it. The D120 should consistently track about 0.5 volt lower than the accessory plug voltage (we assume that a protective diode at the + power input of the D120 causes the D120 to report its internal supply voltage 0.5 volt lower than the supply voltage--not verified with Dynon). This step verifies that the problem is not internal in the D120.
Make multiple flights from a cold start. What we observed was initial supply voltage of about 12.5 (lower than it should be), gradually dropping as the under-the-cowl temperature increased. In our particular test conditions, 10 minutes into the flight we would get the first alarm.
Sometimes an increased load (landing light) would temporarily restore a higher voltage.
Our voltage bounced around quite a bit, and the largest excursions in the low-voltage direction triggered the alarm.
After replacing the rect-reg, our supply voltage, except at idle RPM, was steady at 13 V (12.5 indicated by D120). Do all the checks, but the rect-reg units are running in a hotter environment than they were designed for, so don't chase your tail forever before replacing them. Ours cost $170; they are available in the U.S. from Lockwood and California Power Systems.
AFTER ANOTHER TEN HOURS OF FLYING...
Well, some of the voltage variation problems returned. Going through the wiring diagram for our plane (unfortunately, wiring seems to have changed constantly at the CT factory, so this may not apply to you) and following suspect wires, we found another problem. There are two wires that go into a single pressure terminal on the back of our ignition switch. Both wires have been "tinned" (multi-strand wires that have had solder applied to the end) and are, therefore, slightly different diameters. One of the wires could be pulled out by hand! We tightened the single screw on this connector, really putting the pressure on the two wires. Since making this adjustment, and with the new regulator mentioned above, we have had no voltage issues with about 100 hr of additional flight time. I guess the lesson here is that wires other than ground wires may be the culprit with CT voltage problems.