Alternator shunt value

[SteveG]

Aircraft - C337A, #337-0452, 1966. Symptom - Electrical system under day-VFR ops manages OK but under higher loads will eventually trip an alternator circuit breaker, the load then shifts to one alternator and if load is not shed it will shortly thereafter trip its breaker.
In trying to troubleshoot this problem it became apparent that the EI volt/ammeter was falsely indicating amperage. In trying to determine why, it appears that the instrument inself is OK, the present working hypothesis is that it may not be calibrated correctly for the original equipment shunt value.
The immediate problem is to correct the ammeter display so the first question is: does anyone know the alternator and battery shunt value for this acft. or will it have to be physically measured? This value is not stamped on the shunts, does not appear in the parts or service manual, is not known by CPA or EI tech staff. Cessna part no. is #813489.
Any further thoughts on other possible causes of the ammeter problem or experience with troubleshooting the overload problem would be appreciated.

Thanks, Steve G.