Where are you measuring the voltage? What do you measure directly at the alternator / battery? At what RPM are checking? Do you have all electrical systems (lights, pitot heat, avionics, etc.) switched off? IMHO you can takle out and bridge the overvoltage protection for troubleshooting, just make sure to keep the avionics switched off (just in case...). But think that won't change anything.
What does the maintenance manual say about troubleshooting? Why did you replace the regulator, same problem? Have you checked your voltmeter?
The clock has a built in voltmeter (compared with my Fluke, and it is accurate). It reads the voltage from where you see the clock in the schematic.
I have tried turning everything on and everything off. The voltage remains at 13.7V regardless.
There is no maintenance manual regarding troubleshooting.
The initial problem was that the system stopped charging in flight. Evetually the battery ran down low enough that nothing really worked except the transponder (it's a periodic load) (had everything else turned off... radio transmit would reset the radio, etc.). This was around 10.8V or so. Turning Alt portion of master switch off and on had no effect. The mechanic said he found a bad main cable from the alternator. This cable was replaced. Everything seemed to work OK.
I went on another flight. The voltmeter read 14.3V at the start of the flight. About 20 or 30 mins in the system stopped putting out enough voltage again. I saw around 11.7V and dropping... Toggling Alt switch did cause the voltage to jump up to around 12.8V for about 30 sec and then drop again. I did this twice with the same result. I turned off all that I could and landed safely, even with the radio used sparingly (towered airport).
Mechanic said replace the voltage regulator. He charged the battery here. He replaced the regulator.
I go on another flight. Voltage starts at 13.7V, which is lower than my previous 14.3V. I get about ten mins out and voltage starts dropping again. It's kinda holding around 12V. I found toggling Alt switch had little if any effect.
Mechanic says replace alternator. So I pay for it...
With new alternator, but not charging battery I get where I am now. At last flight (only flight post alternator) I see about 11.7V or so at startup. Then she makes 13.7V when running. I flew for an hour. It mainly stayed at 13.7V. I saw it dip to 13.6V for a few minutes, and even to 13.5V for less than a minute. I tried turning everything I could ON (including pitot heat, but my lights are LED; so lower draw). Through turning things on and off, or transmitting on the radio I was not able to get the voltage to change. So maybe everything is fine??
Two things make me question this. Other mechanics in a Cessna 172 forum insist it should be between 13.8V-14.4V. OK, take free info with a grain of salt, but they have been doing this a while, and actually seem to understand electrical systems, at least more than my mechanics. Also my Garmin G5 (mini primary flight display) indicated a charging icon for its internal battery, however I watched the charge percentage go from 76% to 74% in about 20-30 mins. This could be due to it getting hotter as I flew longer, since the ground temp was about 92F and I was under 3000Ft AGL.
So that is where I am now, and the whole story as I remember it.