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Hagerty Employee

What's the charge? A Mustang's vexing electrical issue | Hagerty Media

Douglas Lemmo writes: A year ago, I damaged the alternator on my 1966 Mustang 289, so I bought a rebuilt alternator from AutoZone. My in-dash ammeter and under-dash voltmeter showed the charging system to be working fine, but both gauge needles would spike from their normal positions, then immediately return to normal.

There's an instrument cluster voltage regulator that could be on its way out as well. They aren't horribly expensive and are relatively easy to replace. The higher output of the new alternator may have hastened the aging of the regulator and it could be an issue.

I've had a lot of experience, mostly bad, with Ford and Lincoln alternators.
One o the failures that happens, and it's not that unusual, is that the diodes fail and allow the alternator to put alternating current into the car' direct current electrical system. Newer cars, with electronics, go wonky. Older cars, like the Mustang, might only show a few quirks, like those jumpy needles.
Measuring the car's electrical system with a DC voltmeter won't show tis problem. Sometimes an AC voltmeter will show AC voltage in the system. Other times you just have to test each diode separately.
In any case, i would start with another alternator. And, preferably, a new, no rebuilt, alternator. There are some Ford cars, like the Lincoln Mark VIII, for which there are no new alternators available, from Ford or anyone else, ad that is nothing but trouble.
Good luck.
Community Manager

Luckily the Lincoln Mark VIII easily accepts a 1996 Cobra alternator, which is readily available, has 10(?) more amps, and only requires the owner to slice open the sleeve over the power wire to get it to fit. Of course, I reckon you already knew that. 😉 But most folks do not, and it's a shame that happened. 


Good analysis Rob;

I suspect a bad body to chassis ground or grounds. I've had similar problems on cars where people had tossed the grounds that go from the firewall to the cylinder heads or valve covers. Might be time, given the cars age, to check those grounds and install a new battery to block ground also.

Your right and to be clear. A large braided steel cable from the engine block to the shock tower and a wire from battery ground to the radiator core support make all the difference in the world for battery, starting and charging on an old Ford or Mercury. The battery ground to the core support will suppress electrolysis damage to aluminum radiators too.