Alternator over charging

UncleDave

UncleDave

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Joined
Nov 26, 2019
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Location
Atlanta Ga
Hear me out-
Previous alternator (2006 4.0) would charge, but with all the accessories on resulted in the ‘check gauges’ lamp to come on until I gave it some RPMs. I had ordered a new PCM On order from Wranglerfix but decided to go ahead and order a new alternator as well (the one in the TJ was a reman of unknown origin).

New PCM comes, install takes 5 minutes, trans issue fixed. But old alternator still won’t handle load. New alternator comes, instant 14.8 volts charging. Problem is, it’s around 14.8 all the time, took the TJ on a 2 hour drive, still hitting 14.8. Battery is showing 12.8 volts sitting. Swapped old PCM back in- same action; 14.8 volts all the time unless I disconnect the trigger wire from the PCM.

is this indicative of a bad new alternator (sadly I don’t have the old one to try with the new PCM)? Is there a way without sophisticated meters (I have your basic cheap voltmeter) to test the trigger wire? FYI I have cleaned engine grounds as well as contact points on alternator bracket.

Your ideas are appreciated. I don’t want to destroy the battery.
 
Would have to check the FSM but I thought the voltage regulator was in the PCM.
 
The alternator charge voltage is set by the PCM, which sends a signal to the alternator. If there is a problem in the PCM, the voltage could be wrong.

If an internal failure happens in the alternator, it will usually fail to charge or charge 100% all the time. This would likely result in a much lower or higher voltage than 14.8. Some aftermarket alternators will bypass the voltage regulator and self-regulate, but this is not common on non-HO alternators.

If you have an AGM battery in good condition, it should be able to survive 14.8; however, it will have less of a lifespan. Many car audio enthusiasts charge at 14.8 on 12 volt systems for a little extra power. 14.4 is a much more common charge voltage, and is standard for AGM batteries in normal temperatures.

You can install an external voltage regulator that can be set to any desired voltage. This would be a last resort.

Make sure you clean the contacts anywhere from the alternator to the PCM, especially on the battery terminals, engine block ground, PCM supply/ground, etc. If the PCM is seeing a reduced voltage due to a bad connection, it will think the voltage is too low and overcharge the system.
 
The battery measuring 12.8 with the engine off means it still has a surface charge. Turn then headlights on for ~30 seconds, what does the battery measure?
 
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Check that both of your battery connections are wire-brushed clean and tight! Don't just look at them and assume they're ok because they can look clean but still have a bad/corroded connection between the battery posts and connectors.
 
Lou said:
The battery measuring 12.8 with the engine off means it still has a surface charge. Turn then headlights on for ~30 seconds, what does the battery measure?
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I agree to try this. Do it with the vehicle off and the lights and/or radio on. I've seen many batteries that check perfectly fine, but drop to half or even no voltage as soon as you put a load on them. If that is the case, as soon as you are running the vehicle and the battery is getting a load, it's dropping down, the system is detecting that and trying to charge it. At that point you are mostly running off the alternator, with just enough actual battery juice to keep the system going.
 
AdventureWill said:
I agree to try this. Do it with the vehicle off and the lights and/or radio on. I've seen many batteries that check perfectly fine, but drop to half or even no voltage as soon as you put a load on them. If that is the case, as soon as you are running the vehicle and the battery is getting a load, it's dropping down, the system is detecting that and trying to charge it. At that point you are mostly running off the alternator, with just enough actual battery juice to keep the system going.
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Lou said:
The battery measuring 12.8 with the engine off means it still has a surface charge. Turn then headlights on for ~30 seconds, what does the battery measure?
Click to expand...

done-
headlights and underhood lights on 30 seconds - 12.6V
headlights turned off (Under hood lamp still on) voltage returns to 12.85V


Also, I did a voltage drop test:
Alternator Positive to Battery Positive- 34mv
Alternator case to Battery negative- 10.7mv
 
Jerry Bransford said:
Check that both of your battery connections are wire-brushed clean and tight! Don't just look at them and assume they're ok because they can look clean but still have a bad/corroded connection between the battery posts and connectors.
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Thanks- yes, new terminals/ wire brushed posts. Also note voltage drop test would seem to validate good connection (as I understand it).
 
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CharlesHS said:
Check the condition of the temperature sensor inset in the battery tray; this sensor tells the ECM if the battery is cold or hot and at what rate to charge at.
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As I understand it 2006 models do not have the battery tray sensor. I also do not recall seeing a hole or a sender in the battery tray when I had it out a couple days ago.
 
UncleDave said:
As I understand it 2006 models do not have the battery tray sensor. I also do not recall seeing a hole or a sender in the battery tray when I had it out a couple days ago.
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Didn't realize Mopar discontinued the use of the sensor...
I wonder how they make charge rate adjustments due to battery temperature...
 
I assume you are reading the 14.8V across the battery? If you are then sounds like you charging system is working fine, mine puts out 14.2 while running and no accessories on.
 
John Scoggins said:
I assume you are reading the 14.8V across the battery? If you are then sounds like you charging system is working fine, mine puts out 14.2 while running and no accessories on.
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Yes, when running I’m showing 14.8 with no accessories. Maybe 14.3 with full load. Way different than the 11.8 I was hitting with the old alternator at idle.

I guess it just makes no sense to me that the alternator would be pumping out 14.8 continually with a fully charged battery, but if yours is pushing 14.2 maybe it’s not that big a deal.
 
UncleDave said:
Yes, when running I’m showing 14.8 with no accessories. Maybe 14.3 with full load. Way different than the 11.8 I was hitting with the old alternator at idle.

I guess it just makes no sense to me that the alternator would be pumping out 14.8 continually with a fully charged battery, but if yours is pushing 14.2 maybe it’s not that big a deal.
Click to expand...

I know this is an old thread but did you mange to get this resolved? having a similar issue
 
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