A 0-200 ammeter will have an external shunt. The voltage drop (E) is taken across this known, small and stable resistance (R) and is related to current
by Mister Ohm's Law:
I = E/R.
So the ammeter is really a milli-voltmeter with a face painted in amps. Isn't electricity fun?
The first thing to look for is corrosion
at the wiring
connected to the shunt and to the meter. If that is bright and shiny, then check the fuse holder, if installed, to make sure its contacts are bright and shiny.
While at the shunt, check the integrity of the heavy conductors to ensure that those connections are bright and shiny.
After the above remedial action, take your clamp meter and put it around the big cables
going into or leaving the shunt. Start charging
and see what the clamp meter records vice what the ammeter is showing. They should be close with but the clamp meter, if it is of reasonable quality, will be the more accurate.
If the readings are not close, then take your DMM across the two terminals on the ammeter, and record
the milli-volts during a charge. Most shunts are 50mV for full deflection. This means that for your 200A ammeter, you should see about 12.5mV for 50A, 25mV for 100A and so on. The relationship is linear.
If there is a mis-match between actual (clamp meter) and DMM millivolt readings (converted to amps), then the shunt has failed. If there is agreement between actual (clamp meter) and DMM millivolt readings (converted to amps), and you cannot adjust the ammeter with the front adjustment screw, then the ammeter has failed.
Hope this helps.