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i have a question does something other than the alternator control the voltage? Installed new alternator and volts still go up to 18 volts thank you
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Somewhere in the wiring harness the alternator is receiving a low voltage signal instead of actual battery voltage and load, which makes it "think" the battery is still low/undercharged. You need to go on a witch hunt to find the high resistance ground or positive sense problem. Try to work from the alternator to the battery, checking and cleaning connections. Pay attention to the voltage sensing circuit in the manual cirduit diagrams. You're in for a long day................
 

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The Northstar Tuner
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99 STS = Ignition switch and power moding problems caused by Ignition switch.
 

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'98 STS (RIP @ 206,xxx miles)
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i have a question does something other than the alternator control the voltage? Installed new alternator and volts still go up to 18 volts thank you
The PCM is involved in voltage regulation and there is a thermistor for temperature compensation. And of course, your 'new' alternator could be faulty right out of the box.

You could check the PCM connectors for corrosion or obvious damage; located in the engine intake air stream forward of the filter location.

The thermistor is tapped into the battery cable very close to the positive terminal and feeds back to the PCM.
 

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2008 SRX
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You're in for a long day................
The generator charges to the cable running to the front junction block which attaches there to another cable running to the battery, which has a lead running to the rear junction block. If these are loose or corroded, everything on the rear junction block will see low voltage and the generator will charge until this voltage appears normal. Everything on the other half of the car will see high voltage.

You're in for a long day, especially if you don't check the basics and check them thoroughly. By thoroughly, that means putting eyes, hands and wrenches to all related connections.

The case I'm thinking of was your exact symptom due to a poor connection at the rear junction block. It wasn't caught the first time I looked at the cable either.
 
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