Couple of questions:
Does the alarm come on and stay on? Or just come on momentarily when the load is applied and then go back off?
Do the alarms sound on both engines?
Are you running the standard alternators with standard regulators?
Is there any sort of "smart" regulator
or splitter in the system?
What is the battery
bank arrangement? Start banks, house banks, etc. and how are they connected together?
90% of the VP charge alarm problems that occur with both older and MDI systems are the result of "smart" regulators controlling either Volvo or aftermarket alternators. When these regulators sense that the batteries are fully charged the regulator
goes into float mode and reduces field current
to the alternator
to limit voltage output. This limiting is sensed by the alarm system which then sounds the alarm.
In your case it sounds like you are getting a momentary voltage drop which is deep enough to cause the alarm system to engage. This can sometimes happen so fast that the only way to see it is with an oscilloscope.
The MDI system charge alarm works on both under voltage and over voltage. Early boxes had a somewhat high " low voltage" trip point, and supposedly this was changed in later production to avoid nuisance alarms, though that would require some checking and verification.
Give me some answers to the above questions.