Low voltage on main electrical bus

[Geoff]

Any suggestions on how to diagnose a slightly low voltage condition on my main bus?  Mine is making 27 volts when 28.5 are called for in a healthy system. 

[M20TN_Driver]

I think i have a similar issue. Diodes in the alternator are being looked at. Supposedly a bad alternator causes the slightly low voltage. 

[dkkim73]

Or the voltage regulator. I took this as a good opportunity to learn a bit about the system... the "M20TN SERVICE & MAINTENANCE MANUAL " (MAN180.pdf) has a pretty good description of theory and also has troubleshooting and service instructions, including voltage adjust. The voltage regulator ("APAC" in that reference) is actually fairly full-featured. 

That said, if something's changed, you would presumably want to thoroughly check everything before just empirically adjusting the voltage. Perhaps a more experienced person (esp. A&P) can weigh in here on whether these things drift on their own. 

HTH, 
D

[LANCECASPER]

On 5/1/2024 at 10:40 PM, Geoff said:

Any suggestions on how to diagnose a slightly low voltage condition on my main bus?  Mine is making 27 volts when 28.5 are called for in a healthy system. 

First thing I would check would be the logs to see if and when the coupler (or alternator) has ever been changed. You don't want that coming apart.