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Overcharging alternator / shifting issues?

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  • Overcharging alternator / shifting issues?

    Would anyone know off the top of their head if an overcharging alternator could cause the transmission to shift hard like bad TSS symptoms? I happened to get a new scan tool the other day, so I checked codes, cleared them (CEL hasn’t come back on yet), but it did say it’s overcharging at 14.8V. From what I understand, the tranny is electronically controlled, so wouldn’t overcharging cause shifting problems the same as undercharging? Tried a little redneck troubleshooting, and turned on the heater full blast when it started acting up and it seemed to stop once the heater was drawing current. I figured maybe it balanced it out. *shrug* Sorry, I may be way off base, but figured I’d ask just in case. It seems fairly logical to me, but I’m not great with electronic stuff, either. LOL

  • #2
    14.8V is just a bit over the threshold for overcharging, but if you think it might be affecting the transmission operation, I'd try removing the alternator output mega-fuse and driving the car for a short distance on just the battery to see if the transmission misbehaving changes or not. Replacing the alternator voltage regulator should fix the overcharging problem, if that's what it is.

    Another way to disable the alternator, assuming it's easily accessed, is to pull the multi-pin connector that's connected to the voltage regulator module on the alternator, that will turn off the regulator and field, might be easier than locating the mega-fuse in the output wire.

    Be aware that 98 Contour and Mystique, both V6 and I4 engines, all had under-hood wire harness insulation failure issues, and the problems you're seeing might be early signs of wire insulation failure.
    2019 Corvette Grand Sport, 1LT, just stupid-fast.
    1985 SVO Mustang (turbocharged track rat)
    1989 Taurus SHO (supercharged track rat)
    1999 SVT Contour (Sedanus-Grocerygetter-Rapidus)
    2008 Mercury Milan Premier (Comfy boat)

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    • #3
      I’ll check that out this weekend. Were there specific areas that the wiring insulation was failing? Or was it an “all over” thing?

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      • #4
        Typically, one of two problems cause overcharging alternators: poor battery condition and failed regulators and a failed regulator — or a failing rotor on the regulator — may trigger an overcharging alternator because the regulator cannot correctly read the electricity level in the battery.

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