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Unread 04-21-2011, 02:21 AM   #1
JeepManZJ
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Question Alternator field terminals both read full voltage when...

One is supposed to read full available voltage, and one is supposed to be 3-4 volts lower.

Without a working alternator, my battery was at 12.2 volts and one is supposed to read 3-4 volts less than that.

Supposedly the dark green wire is supposed to be the ground or negative field terminal on the back of the alternator.

Of course, my alternator is not working. Anyone know what's going on? thanks so much.

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Unread 04-21-2011, 05:18 AM   #2
86cj74.2L
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You can put 12v to the field terminals of the alt and see if it pushes voltage. If it doesn't its dead. If it does it could be the PCM. (hardly ever the PCM)
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Unread 04-21-2011, 06:08 AM   #3
JeepManZJ
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Sorry, I should have added that I bought a new alternator to try to fix this, nothing changed
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Unread 04-21-2011, 01:49 PM   #4
86cj74.2L
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the voltage regulator comes from the PCM. I have never heard (always a first) of a Voltage regulator going bad

You can still full field the ALT and verify your system voltage goes up.
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Unread 04-22-2011, 04:18 AM   #5
JeepManZJ
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the voltage regulator comes from the PCM. I have never heard (always a first) of a Voltage regulator going bad

You can still full field the ALT and verify your system voltage goes up.
I attached a jumper wire from one of the field terminals to the negative terminal of the battery and the Jeep stalled and blew the fuel injector fuse.
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Unread 04-22-2011, 04:34 AM   #6
86cj74.2L
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on most alt's applying 12vdc to the + and - to the field terminals will full field it.

Never done it for long time just long enough to verify if voltage goes up.
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Unread 04-22-2011, 06:02 AM   #7
JeepManZJ
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on most alt's applying 12vdc to the + and - to the field terminals will full field it.

Never done it for long time just long enough to verify if voltage goes up.

so how would i apply the power to the field terminals?
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Unread 04-22-2011, 06:35 AM   #8
86cj74.2L
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If you blew a fuse I wouldn't contune expecially if the wire you grounded was the dark green one you mentioned. You don't want to ground out the 12v supply to the field.

My wife has the jeep so I can't go out and look.

I just run a hot and a ground from the battery with a 10a fuse with everything but the charge wire disconected.

The 12v is always on the field + terminal I believe for the jeep, and the ground gets pulsed to control the charge output based on system voltage.

Have you looked in the junction box under the hood for a blown chargeing fuse? My cummins has a 100a fuse on the charging wire (output).
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Unread 04-22-2011, 06:47 AM   #9
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So I want to run the 2 wires from the back of the alternator to the negative and positive posts on the battery and start it up?

I wish it was a fuse, to my knowledge there are no fuses for the alternator/charging system.

Thanks for your help.
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Unread 04-22-2011, 08:47 AM   #10
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Are you working on a ZJ or a WJ?

On my WJ, the charge wire runs directly to the main fusebox terminal, crimped with the battery cable.

I know on the WJ that the battery temperature sensor under the battery will tell the alternator what to output based on the temperature of the battery.
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Unread 04-22-2011, 10:14 AM   #11
86cj74.2L
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Quote:
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So I want to run the 2 wires from the back of the alternator to the negative and positive posts on the battery and start it up?

I wish it was a fuse, to my knowledge there are no fuses for the alternator/charging system.

Thanks for your help.
Yes that is how you do it but once again if you grounded the dark green wire and blew the fuse for the injector circuit do not do it again.

Some have a fuseable link.

You could always remove the alt and have it tested.
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Unread 04-22-2011, 06:56 PM   #12
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I'm working on a WJ.

I had the alt tested and it tested good, I bought another and nothing changed. I did buy a new connector after all this and nothing changed then either.
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Unread 04-23-2011, 07:53 PM   #13
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hope this helps

VOLTAGE REGULATOR
DESCRIPTION
The Electronic Voltage Regulator (EVR) is not a
separate component. It is actually a voltage regulating
circuit located within the Powertrain Control
Module (PCM). The EVR is not serviced separately. If
replacement is necessary, the PCM must be replaced.

OPERATION
The amount of DC current produced by the generator
is controlled by EVR circuitry contained within
the PCM. This circuitry is connected in series with
the generators second rotor field terminal and its
ground.
Voltage is regulated by cycling the ground path to
control the strength of the rotor magnetic field. The
EVR circuitry monitors system line voltage (B+) and
battery temperature (refer to Battery Temperature
Sensor for more information). It then determines a
target charging voltage. If sensed battery voltage is
0.5 volts or lower than the target voltage, the PCM
grounds the field winding until sensed battery voltage
is 0.5 volts above target voltage. A circuit in the
PCM cycles the ground side of the generator field up
to 100 times per second (100Hz), but has the capability
to ground the field control wire 100% of the time
(full field) to achieve the target voltage. If the charging
rate cannot be monitored (limp-in), a duty cycle
of 25% is used by the PCM in order to have some
generator output. Also refer to Charging System
Operation for additional information.

You should be able to full field it acording to this.
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Unread 04-23-2011, 07:58 PM   #14
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have you cycled the ignition key 3 times without starting it and check for codes on the odometer display?

Does the dash volt meter also read low voltage when the engine is running?
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Unread 04-23-2011, 08:21 PM   #15
JeepManZJ
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Hi, no codes and yes the battery volt gauge reads low, nowhere near 14.
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