Racecarl,
Your alternator might have the capacity to put out 72 amps, but that won't happen unless your old truck has the need for this amount of power.
A analogy would be a huge water tank holding millions of gallons of water. Hook a little garden hose to it with a little flow meter in line, and NO problem. The garden hose will never use the full capacity available to it.
Amperes is a measurement of current flow. If nothing in your truck demands tons of power, little current will flow, even though the alternator has the ability to create it, so your ammeter is safe.
This is very scary territory...I'll quit before I step in a big pile of amperes.
Stuart
