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Unread 03-20-2010, 10:54 AM   #1
I8AHUMR
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I'm guessing alternator for 4.0

Check engine light comes on and flashes 12 and 41 for trouble codes. Dash voltmeter reads no more than 11v when running (not real accurate I'm guessing?) and hand voltmeter reads 12.5v when running and 12.65v when off. I'm guessing alternator? Anybody disagree with that? I'm thinking time to upgrade to the 136amp from junkyard...

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Unread 03-20-2010, 12:37 PM   #2
TheRealOldjeep
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Could also be the ECM since that is where the regulator is
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Unread 03-20-2010, 04:48 PM   #3
I8AHUMR
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Had the alternator checked and supposebly it's good. Guy said the voltage regulator is internal to the alternator. Can someone veriify internal or in ECM? How much faith does everyone put in these testing machines? Read somewhere that it shouldn't be much. Anyone have any comments on that?
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Unread 03-20-2010, 11:16 PM   #4
sentinal02
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Quote:
Originally Posted by I8AHUMR View Post
Had the alternator checked and supposebly it's good. Guy said the voltage regulator is internal to the alternator. Can someone veriify internal or in ECM? How much faith does everyone put in these testing machines? Read somewhere that it shouldn't be much. Anyone have any comments on that?
some are internal, but not the 91+ alts found in the jeep. ours are externally reg'ed by the ECU. check your fuses though. if the alt really is good then my money is on the two 50 amp maxi fuses under the hood are bad. as far as the machine goes, it all comes down to the operator. the machine itself works just fine (all it needs to do is read the output of the alt after all). but if the guy hooking it up doesn't know what they're doing then who knows what you'll get. as the old saying goes, garbage in = garbage out.
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Unread 03-21-2010, 09:12 AM   #5
I8AHUMR
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Ok I did some t-shooting this morning. Followed this page....

Diagnosing Your Charging System

Without the battery hooked up, the battery read 12.93.
With the battery hooked up and engine not running, the battery read 12.74.
With the battery hooked up and the engine running (only load was the light in engine compartment) 12.34.
With heater, radio, lights, and two 55W lights on the battery read 12.05.

With everything turned off and running the measurement across the alt output the reading was 12.4.
With everything turned on (lights, radio, etc) and running the measurement across the alt output was 12.0.
(Ok so I'm assuming no short in the connections as the output of the alt is consistent with the battery readings)

With everything turned off again the reading accross the field coil terminals was inconsistently jumping from 11.5-12.1. Occasionally it would drop to 8-9. According to write-up this should be roughly 5.

With everything turned on again the reading across the field coil terminals was very consistent at 11.8.

So something is occuring when there is no load. After hooking the battery up the voltage there was slightly below the threshold of 12.8 stated in write-up. I read 12.74. In addition, the field coil readings were sporatic and above the stated value of roughly 5. Any thoughts on this? Write-up suggests alternator.
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Unread 03-21-2010, 09:31 AM   #6
I8AHUMR
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Checked fuses and they're good.
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Unread 03-21-2010, 09:32 AM   #7
sentinal02
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Quote:
Originally Posted by I8AHUMR View Post
Ok I did some t-shooting this morning. Followed this page....

Diagnosing Your Charging System

Without the battery hooked up, the battery read 12.93.
With the battery hooked up and engine not running, the battery read 12.74.
With the battery hooked up and the engine running (only load was the light in engine compartment) 12.34.
With heater, radio, lights, and two 55W lights on the battery read 12.05.

With everything turned off and running the measurement across the alt output the reading was 12.4.
With everything turned on (lights, radio, etc) and running the measurement across the alt output was 12.0.
(Ok so I'm assuming no short in the connections as the output of the alt is consistent with the battery readings)

With everything turned off again the reading accross the field coil terminals was inconsistently jumping from 11.5-12.1. Occasionally it would drop to 8-9. According to write-up this should be roughly 5.

With everything turned on again the reading across the field coil terminals was very consistent at 11.8.

So something is occuring when there is no load. After hooking the battery up the voltage there was slightly below the threshold of 12.8 stated in write-up. I read 12.74. In addition, the field coil readings were sporatic and above the stated value of roughly 5. Any thoughts on this? Write-up suggests alternator.
your regulator sounds fine to me. the reason you're seeing it jump at a high voltage is because the computer is seeing an under voltage state at the battery and is trying to bring the alt's output up to compensate. if you're not seeing upwards of 14 volts at the alt output terminal under those conditions, then likely your alt is bad.
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Unread 03-21-2010, 02:27 PM   #8
I8AHUMR
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Just an update. Alternator was bad. Swapped in new one and it's good to go. 14.3V when running.
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