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- Jun 2, 2012
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- canada
- Vehicle Year
- 1994
- Make / Model
- Ford
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- Manual
What was your Battery Voltage key OFF?
12.8v to 12.3volts is expected but must test it after engine has been off for at least 4 hours, overnight is better
Battery voltage has a "false" higher voltage just after charging, alternator charges the battery
If your battery voltage was under 12.2volts that could explain the 14.9volts at idle
But if you raise RPMs to say 1,500-2,000 the 14.9v should stay the same NOT go up above 15volts
If they do then yes voltage regulator is bad
just a quick "how it works"
The voltage regulator sends 7 to 9 volts to the Rotor inside alternator
The engine spins the rotor and that 7-9 volts generates 13.5v to 14.9volts in the 3 Field coils in the alternators case, that voltage comes out on B+
So the spinning of the alternator multiplies the voltage sent to the Rotor
The Voltage regulator adjusts the 7 to 9volts based on "push back" in the vehicles electrical demands
(extra credit, look up electromotive force)
"Push back" is basically how much "force" it takes to push electricity thru a circuit, once a light bulb is on it becomes harder to "push" more power thru that light bulb, there is resistance that "pushes back" on more voltage/amps coming in
When you start the engine the starter motor uses about 50AMPs of battery power
So "push back" at the battery is low because it was drained a bit, so voltage regulator sends a bit more voltage to the rotor, and voltage is above 14volts, usually 14.5-14.8volts
(and this is where a failing battery comes in, 12.2v or lower means it has very little AMPs left so very little "push back" and voltage regulator sends full 9volts to rotor)
As the battery is re-charged it starts to "push back" so voltage regulator lowers the voltage going to Rotor and voltage drops to under 14volts, 13.5 to 13.8volts is expected
The vehicles electric system also have "push back", say with lights off and heater fan off the engine and cab are using 30amps of power
When you turn on the head lights, "push back" gets less because 10 more amps are needed, so voltage regular sends more volts to Rotor
Same with heater fan on HIGH, need more amps so voltage regulator response
And then there are engine RPMs, "push back" is the same at 0MPH or at 60MPH so the voltage regulator also needs to responded to that
As the RPMs go up the Rotor creates more AMPs/Voltage, but "push back" is the same, so voltage regulator must lower the voltage it is sending to the Rotor to match the "push back"
So another sign of failing alternator is if voltage goes up with RPMs and stays UP, it WILL go up momentarily when you REV the engine but should drop back down under 14volts if you hold RPM at say 2,000rpms
Its a very simple system and easy to diagnose if you don't over think it
12.8v to 12.3volts is expected but must test it after engine has been off for at least 4 hours, overnight is better
Battery voltage has a "false" higher voltage just after charging, alternator charges the battery
If your battery voltage was under 12.2volts that could explain the 14.9volts at idle
But if you raise RPMs to say 1,500-2,000 the 14.9v should stay the same NOT go up above 15volts
If they do then yes voltage regulator is bad
just a quick "how it works"
The voltage regulator sends 7 to 9 volts to the Rotor inside alternator
The engine spins the rotor and that 7-9 volts generates 13.5v to 14.9volts in the 3 Field coils in the alternators case, that voltage comes out on B+
So the spinning of the alternator multiplies the voltage sent to the Rotor
The Voltage regulator adjusts the 7 to 9volts based on "push back" in the vehicles electrical demands
(extra credit, look up electromotive force)
"Push back" is basically how much "force" it takes to push electricity thru a circuit, once a light bulb is on it becomes harder to "push" more power thru that light bulb, there is resistance that "pushes back" on more voltage/amps coming in
When you start the engine the starter motor uses about 50AMPs of battery power
So "push back" at the battery is low because it was drained a bit, so voltage regulator sends a bit more voltage to the rotor, and voltage is above 14volts, usually 14.5-14.8volts
(and this is where a failing battery comes in, 12.2v or lower means it has very little AMPs left so very little "push back" and voltage regulator sends full 9volts to rotor)
As the battery is re-charged it starts to "push back" so voltage regulator lowers the voltage going to Rotor and voltage drops to under 14volts, 13.5 to 13.8volts is expected
The vehicles electric system also have "push back", say with lights off and heater fan off the engine and cab are using 30amps of power
When you turn on the head lights, "push back" gets less because 10 more amps are needed, so voltage regular sends more volts to Rotor
Same with heater fan on HIGH, need more amps so voltage regulator response
And then there are engine RPMs, "push back" is the same at 0MPH or at 60MPH so the voltage regulator also needs to responded to that
As the RPMs go up the Rotor creates more AMPs/Voltage, but "push back" is the same, so voltage regulator must lower the voltage it is sending to the Rotor to match the "push back"
So another sign of failing alternator is if voltage goes up with RPMs and stays UP, it WILL go up momentarily when you REV the engine but should drop back down under 14volts if you hold RPM at say 2,000rpms
Its a very simple system and easy to diagnose if you don't over think it
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