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Voltage Problem?

When I take off at accelerated speeds I find that the voltage meter drops to a low reading and the battery light comes on. This stops and goes back to normal once I reach higher gears and the RPM’s lower. What could this be? Alternator problems? car is a 1998 Mustang GT 4.6 V8

Yes, most likely an alternator problem. A charging system test can easily confirm this. Some parts stores, like Autozone, will do this for free.

The brushes in your alternator have worn down and there is no longer enough spring force to hold them against the slip rings when the alternator is spinning at high speeds.

A loose/faulty alternator drive belt will also cause this. High rpm’s will cause the belt to slip on the pully.

It’s a single belt that drives the water pump, alternator, air conditioner, and power steering pump. It’s unlikely to be the problem here. (The OP would be reporting other issues too.) The OP’s 4.6L car is not the first with the alternator to fail this way.

It could be the belt not getting a bite on the pulley. A couple of years ago I replaced the alt. on my Lincoln with a reman unit and within 2 minutes the BATT light was on and no charging at all.

Tests showed the reman alt. was good but the belt was slipping on the pulley (new belt and tensioner, go figure).
This was finally determined to be due to the pulley itself, although visually I could not discern a problem with it.
Changed the pulley out and problem solved.

Yeah, that’s possible. The result is still the same since everybody replaces the alternator and the pulley as a unit anyway. However, I suspect that your replacement alternator may have had the wrong pulley on it instead of a defective pulley.