Rev Counters Volvo Penta TMD31 engines

[fishers]

I'm foxed and would appreciate some help my boat has two Volvo tmd31 engines and has volva penta instruments each engine has one rev counter for each helm on initial start up rev counters are ok as revs increase either the port or starboard rev counters cut out. If i stop that engine and restart the rev counters are ok. Increase revs again and they cut out. Any ideas thanks <span style="color:black"> </span>

 

[Travelmate]

If they are driven with pulses from the alternator, the alternator must be charging to get a reading on the tacho.
Check the charging system first.

Tony