The FSM has a very good diagnostic procedure for testing alt output but you need a DC current meter to run the test. Also have you considered a bad battery as a possibility? Also, try running a volt meter on the cig lighter to double check your dash meter while underway. What other symptoms do you have other than the gauge dropping? Fan bogging? Lights dimming?

, LMK your thoughts.
, G.
, good job with the fluids. Give us an update on the suspension service (bushes).
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