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98 Seville STS
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Discussion Starter #1
I have a 98 Seville STS with the Service Charging System, ran the codes and I am getting P1638 Generator F Circuit. I changed the alternator twice and the battery a couple of times. Changed the negative battery terminal. I did see that the positive battery cable has a connector that was broken(the two wires coming off of it), I cut that out and hard wired the wires together with butt connectors still same issue....did anyone else have this issue or can help me out, would be much appreciated.
Thanks
Frank
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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68,413 Posts
The F circuit is the Field excitation circuit. That is what "turns on" the alternator/voltage regulator after engine start and rpm comes up to idle. As field current varies, so does alternator output. Connector(s) are the first suspect.

You'll need a a charging system schematic to trace out the wiring for that circuit. Subscribe the car to www.alldatadiy.com
 

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2002 Eldorado Biarritz
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451 Posts
The F circuit is the Field excitation circuit. That is what "turns on" the alternator/voltage regulator after engine start and rpm comes up to idle. As field current varies, so does alternator output. Connector(s) are the first suspect.

You'll need a a charging system schematicd to trace out the wiring for that circuit. Subscribe the car to www.alldatadiy.com
This is incorrect. On these alternators the turn on signal to the regulator is on a wire called L-terminal. The Alternator then reports back to the PCM on the F-terminal in units of duty cycle as a percent. The code indicates that the F-terminal is reporting under 5%. Typically this code is caused by a break in the wire near the connector to the alternator. But, anything that interrupts the signal from the alternator to the PCM can cause it such as a break in the wire anywhere, the wire being shorted to ground or power or the PCM itself.

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Also, do you have a picture of the repair you made to the positive cable?
 

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2002 Eldorado Biarritz
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451 Posts
The L circuit is (used to be) the on/off trigger for the idiot Light. I should have posted something like "I" or "Ignition" circuit is the excitation.
Even one the pre-98 Sevilles, the L-terminal was the turn on signal. Although it was powered by the IPC at the command of the PCM and not directly by the PCM. The F-terminal was still reported to the PCM. The excitation circuit is part of the primary battery feed and is accomplished internally by the regulator once it receives the turn-on signal on the L-terminal. The regulator may ground the L-terminal if it detects an internal problem and that can trigger a warning message. So in that way it triggers the light. But, the terminals purpose in normal operation is to provide the "OK" to the generator to provide charging.

L-terminal turns it on.

F-terminal is reported by the generator to the PCM for diagnostic purposes.
 

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98 Seville STS
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Discussion Starter #6
could the resistor reading the positive terminal go bad and cause the light to go on? and if so what resistor should i replace it with?
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Some of these models/years have a sensing resistor at the positive battery positive terminal, used to signal the voltage regulator to decrease charge rate into a hot battery. The system was only for a couple of years or so. Like the short-lived water-cooled alternator.
 
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