How to adjust PCM alternator charge voltage


Steel City 06

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Edit: Scroll to post #13 to learn how to change the charging voltage.

Out of curiosity, has anyone figured out a way to adjust the PCM charge voltage?

I currently charge at 13.50 volts regardless of alternator demand and temperature.

Is it possible to adjust it to 14.4 volts, for example? That is the standard charging voltage to an AGM battery (2.40 volts per cell), and a typical charge voltage for many cars. It would also give me an extra 14% power on all my resistive loads.
 
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Jerry Bransford

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The correct charge voltage is based on the battery's current level of charge and its temperature. There is no one single correct charge voltage.
 
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Steel City 06

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I've never measured it past about 13.65 volts, and the voltmeter in the dash is always below 14.

Even if it still adjusts for temperature, I wouldn't mind a higher charge voltage comparable to a typical car.

I've thought about an external regulator, but I doubt it's worth the effort. If I were to go to a HO alternator, then I could get the standard 14.8 volt output, but I plan to stick with the Durango alternator for now.
 

CharlesHS

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I suspect that the only way to increase the charge voltage would be to go into the ECM program to increase the setpoint OR somehow trick the PCM input signal for the PCM to charge at a higher rate.
 

Jerry Bransford

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Even if it still adjusts for temperature, I wouldn't mind a higher charge voltage comparable to a typical car.
You might not mind it but the battery certainly would. The battery sits on top of a temperature sensor that tells the PCM when to slow down the rate of charge when the battery starts getting hot during charging so as to not damage the battery. My best advice is to not eff with it.
 
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Steel City 06

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You might not mind it but the battery certainly would. The battery sits on top of a temperature sensor that tells the PCM when to slow down the rate of charge when the battery starts getting hot during charging so as to not damage the battery. My best advice is to not eff with it.
Ultimately, that's the safest solution. If I can't figure out a safe and easy way to adjust it, I plan to leave it.

The lower voltages won't harm the AGM, but also won't charge it at the designed rate. The PCM charge voltage is set at a voltage and temperature-adjusted rate appropriate for a standard lead-acid battery.

AGM batteries are typically charged at up to 14.8 volts, and floated at up to 13.8. If I could get closer to a 14.4 standard voltage, then theoretically it would charge faster without harming the battery. I have no intention of going to the full 14.8 that HO alternators provide.
 

Jerry Bransford

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Ultimately, that's the safest solution. If I can't figure out a safe and easy way to adjust it, I plan to leave it.

The lower voltages won't harm the AGM, but also won't charge it at the designed rate. The PCM charge voltage is set at a voltage and temperature-adjusted rate appropriate for a standard lead-acid battery.

AGM batteries are typically charged at up to 14.8 volts, and floated at up to 13.8. If I could get closer to a 14.4 standard voltage, then theoretically it would charge faster without harming the battery.
You're overthinking this whole subject, the standard Jeep charging system works fine with AGM batteries. Your battery is normally at or very close to full charge anyway so raising the alternator's charge rate is just not needed. I've run nothing but AGM batteries in my Jeeps and never once felt like the OE charging system was inadequate. Such modifications are more likely to either overheat the battery or overcharge it, something the OE charging system will never do.
 
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Grant Lasson

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Out of curiosity, has anyone figured out a way to adjust the PCM charge voltage?

I currently charge at 13.50 volts regardless of alternator demand and temperature.

Is it possible to adjust it to 14.4 volts, for example? That is the standard charging voltage to an AGM battery (2.40 volts per cell), and a typical charge voltage for many cars. It would also give me an extra 14% power on all my resistive loads.
I agree with the advice not to mess with the PCM. I do use an AGM specific battery charger/desulfator for my AGM batteries. See the Amazon link below. I use it for my 6V deep cycle house batteries on my motorhome and my 12 V AGM Jeep batteries (Sear Platinum Die Hard and NorthStar). It revived my Sears from dead until it will hold a 12.6 V charge (can't comment on CCA). I'm not driving the Jeep a lot and the Battery Minder has noticeably improved cold cranking performance. I attached a permanent set of leads to the battery so it is very easy to plug in. I haven't used it during trips. I think the OEM battery charging is fine when the Jeep is being driven frequently. My hope is that battery life will be extended by years if I regularly use it though I do not know how I would measure that outcome.

https://www.amazon.com/dp/B005EKY20K/?tag=wranglerorg-20
 
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Steel City 06

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I ended up using an external voltage regulator to bump up the charge voltage to 14.2 volts, which is more common. I used a quick disconnect harness on the alternator leads so I can quickly swap between the external regulator and the PCM regulator if there is ever an issue with one or the other.

The external regulator is very good at responding to changing loads (as is the PCM), though it did very slowly lose about 100mV as it warmed up, which might actually be a good thing. I did have to run a dedicated relay for it, as it cycled when I tied in the reference leg to the fuel pump relay output. The dedicated relay eliminated any cycling.
 
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Steel City 06

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Did you use a "kit" or just DIY?
I used one of these kits.
They work as advertised, and so far no issues. The heat sink popped off, so I'll have to re-glue it at some point, though it didn't get terribly hot.

No CEL, even on the 2006 model. The voltage meter in the instrument cluster does show the change in voltage, so it appears to not just be a dummy gage.

The kit is adjustable within a pretty significant range. I didn't see what that range was, as I didn't want to blow my car up, but it definitely will go at least 1.5 volts up or down.
 
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Steel City 06

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Well...

It turns out if you have a HPTuners Tuner, it only takes a few minutes to change your PCM charge setpoint. No external regulator needed.

You merely load your factory tune (or aftermarket, if you have one) onto the computer, use VCM Editor to change the setpoints, save, flash the PCM, and you're good to go!


Here's a bit more detailed guide:

You'll need a HPTuners tuner plus 2 credits to be able to program anything in the PCM. If you purchased a tune through Flyin' Ryan Performance @Flyin' Ryan Performance like me, then you already have the tuner and presumably have bought the credits.


First off, you'll want to collect two pieces of information.

1st, you need to find out what the jeep is charging at currently. For this, I recommend a dual approach. Stick a voltmeter at the battery, and use VCM Scanner to watch/graph the voltage simultaneously. Doing this, I found that the Control Module Voltage and the Battery voltage according to a relatively high end multimeter were about 0.08V apart, but otherwise fully in tune with one another. Let the jeep idle for a while, and see if there is any change. I let it run about 10 minutes. I saw no change after the starting voltage. Be sure to rev it up to 1500 and 2000 RPM. If your regulator and alternator are good, there should be no change in voltage at any steady-state RPM.

2nd, you need to find out if your jeep compensates for the temperature of the battery. Some have this sensor, some don't. I am 95% sure I don't. If as the underhood air temperatures heat up, the voltage begins to drop, this is a good sign. Alternatively, you can pull the battery and check underneath for a sensor. Thirdly, there are ways to check the battery temperature sensor temperature and voltage through HPTuners. But as mine showed up blank I have no way of saying if that's because the tuner can't access those values or because the sensor doesn't exist.

1623360684157.png


Next, after you shut off the engine, you will load your stock tune (or aftermarket tune) into VCM Editor, and view the factory PCM alternator charge setpoints.

1623360777667.png

As you can see, the factory attempts to adjust the voltage according to the temperature. I suspect the axis called "ambient air temperature" is actually the battery temp sensor. As you can see in my case, the highest temperature voltage is suspiciously close to the voltage the jeep always charges at (see above). Hence making me think there is no battery temperature sensor in my jeep.


So now it's time to change your voltage.

You'll want to choose a voltage that is safe and effective, and will protect the battery you have installed. If you have an AGM battery like a Northstar or Odyssey, the charge voltage should be between 14.1 and 14.8 volts. The hotter it gets, the lower you want it to be. Theoretically, if you did see 158 degrees under the hood, you would want 13.8 at that point. Really cold temperatures can tolerate higher charging temperatures. Note that even the factory set the freezing charging temp at 14.7. You could probably go as high as 15, but personally I would not risk anything above 14.8.

Since I have no temperature compensation whatsoever, it appears that the PCM defaults to the worst case scenario, or 13.47 volts, which is simply too low for an AGM battery to have a full lifespan. So theoretically I could just adjust all the temperatures up to the same with no effect. If your PCM actually does compensate for the temperature, you will want to make sure there is a decent slope as the temperature increases.

1623361225355.png


Knowing mine was going to default to the worst case scenario, I chose to set the lowest charge voltage to a relatively safe 14.3 volts. 14.4 is a very common flat rate charging voltage, and is not known to harm good batteries. I did slope everything else up a bit, just for completeness, but in my case this was likely unnecessary. If I actually did have temperature compensation, I would have made the slope a bit steeper.

At this point you'll want to save the modified tune as a new file, and flash it onto the PCM.
Repeat the test.

1623361435290.png


As expected, the charge voltage defaults to the worst case scenario, and the voltage remains constant regardless of any temperature changes.

And now I'm charging at a more normal voltage!
 
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Steel City 06

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This was the voltage measured at the battery terminal. Like the Control Module voltage, it did fluctuate a few times a second but it always tracked 0.08V over the PCM voltage. This number fluctuated between about 14.34V and 14.38V, which is very close to where I wanted to be (14.4, or slightly under).

20210610_170017.jpg

I feel very safe running this voltage, as it is well within the alternator charging voltage specification for the battery (14.1 to 14.8 volts). I'm not worried about the temperature being a big issue. On another (Toyota) forum, one member posted a conversation with a Northstar tech who stated that 14.6-14.8 was ideal, and seemed to hint that anything starting with 13 was too low. So this should protect my battery far more than the previous voltage of 13.4-13.5.
 
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