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Discussion Starter #1
Hey guys. A recent project (audio install) has got me trying to figure out whether or not I have a poor ground somewhere.

Here are the two things that I have seen/noticed:

- When starting the engine, I'm seeing the voltage drop as low as 10.5 volts before climbing back up and spiking at 15 volts before falling back down to 14.7 - 14.9.

- I haven't been able to test this one with the multimeter, but I have been noticing what feels like another voltage drop when the Jeep is coming to a stop. Occasionally, it will lead to a rough idle, almost as if it's about to stall.

I've checked my connections at the battery and alternator. Both are great. I plan to check the ground connection on the engine block tonight, but that's as far as I've gotten. Does this sound like a grounding issue or something else?
 

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You may want to check for any DTC's
The charge rate of the alternator is managed by the PCM based on various inputs.
You could have a DTC stored that is not severe enough to illuminate the light in the dash.
Here are other things to check:
 

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Discussion Starter #3
You may want to check for any DTC's
The charge rate of the alternator is managed by the PCM based on various inputs.
You could have a DTC stored that is not severe enough to illuminate the light in the dash.
Here are other things to check:
Just checked and it's not flagging anything.
 

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failing rectifier bridge in the alternator?
 
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Do you have another battery you can test with ? My TJ would have a hard time idling with a "shot" battery. Shot meaning a failing cell, if driven regularly you can get it to start (in the summer at least) but the charging system can't deal with it.
 

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Discussion Starter #7 (Edited)
failing rectifier bridge in the alternator?
I'm not 100% up on my game when it comes to automotive mechanics , so the rectifier being a potential problem, I'm not sure of, but I will look into this or open it up and take a look. Any suggestions on how I need to go about testing for this?

If this does happen to be the case, would it be something that I can just order up a diode and solder the new one in? Aside from that, I'm guessing I'd have to buy a whole new alternator, correct?
 

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Discussion Starter #8 (Edited)
As far as the battery, I'm not sure if a cell has crapped out on me or not. I (ignorantly) didn't think too much about the battery since it is only about a year and a half old, but we all know where assuming can lead us. I haven't been able to do any testing on it yet, but I will check the battery when I do get to it.

My gut is telling me the "voltage regulator" (if that's even what it's actually called), is failing on me, but ruling out these other potential suspects before I order up an external regulator. To my surprise, it really isn't too expensive though, which is a good thing. Far better than the price for a PCM I'm sure.
 

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Discussion Starter #9
I did forget to mention that I have now checked the engine block and chassis ground. Still need to do a voltage drop test on the others.
 

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Back to the original problem - did this all of a sudden start after you did something? You're assuming an idling problem is battery related? Did you load test the battery?
 

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Discussion Starter #11
Back to the original problem - did this all of a sudden start after you did something? You're assuming an idling problem is battery related? Did you load test the battery?
Having no digital voltage gauge in the dash cluster, I can't say for certain if it started at the time I noticed it or if it has been this way. What caused me to take notice was the amplifier that I was installing kept throwing itself into protection mode. What I came to realize later was that it does this when the input reaches 15 volts.

As far as the idling, yeah that was a blind assumption. My only reason for going that direction was essentially the same issue that sent me down this rabbit hole.
Occasionally when coming off of the throttle, the amplifier will throw itself into protection, just as it does after starting the engine. I haven't been able to test to see what the voltage is doing at that point in time, so that is still unknown. I'm guessing maybe the voltage is dropping a little, then briefly spiking. I really don't know.

I have now gone through and done a voltage drop test and found no problems. The load test brought it down to 10 volts.

When checking voltage right off of the battery, I'm getting fluctuations between 14.9 and 15.1. Mostly hovering between 15 and 15.1. Not a huge concern, but that's more than I expected to see. So now I'm just curious more than anything. Whether or not it's beginning to fail at regulating the voltage, or if this voltage level is normal for the Jeep. What idling voltage is normal for the TJs?
 

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15 volts is too high. I believe the OEM alternators have a temperature sensor which adjusts charging voltage (lower temp gets more voltage), but it shouldn't see 15 volts. Again, this is a symptom of a failing diode in the rectifier bridge - possibly a short. The easiest troubleshooting of the alternator would be to take it to an auto parts store and have them bench test it for you.
 
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Discussion Starter #14
15 volts is too high. I believe the OEM alternators have a temperature sensor which adjusts charging voltage (lower temp gets more voltage), but it shouldn't see 15 volts. Again, this is a symptom of a failing diode in the rectifier bridge - possibly a short. The easiest troubleshooting of the alternator would be to take it to an auto parts store and have them bench test it for you.
Ok thanks again, Mouse. Just so I'm understanding, the rectifier bridge is a secondary form of regulating voltage? I was only aware of the regulating done by the PCM. I tried doing a quick search before asking, but I'm not getting helpful results.
 

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Discussion Starter #15
I stopped in at O'Reilly's and had the alternator tested. He went ahead and tested the battery and starter while he was there. He said that everything checked out good. I'm not sure what to think. Aside from the amplifier, I'm having no problems overall, so now I'm questioning going further. What do you guys think?
 

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Do you have a scanner? If not buy a cheap one that can pick up some data. Access your target and actual charging voltage in your pcm data. They should be really close to each other. At the same time check your charging voltage with a multimeter at the battery.
If the pcm or battery wiring has any resistance then the pcm will be trying to up the charging voltage to compensate.


If you are having a low idle dip because of a idle motor or otherwise than charging voltage can dip also. You may be trying to chase two things at once.
 
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Discussion Starter #18
How did you wire up your amp, i.e. gauge of wire, where is the power taken from, where is the ground connected?
It's power is being pulled directly from the battery using 8 gauge wire. The ground is mounted to the chassis at one of the two rear seat bolts. The ground is using the same 8 gauge wire. Later tested it using a temporary run of 0 gauge straight from the battery negative.
 

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Discussion Starter #19 (Edited)
Do you have a scanner? If not buy a cheap one that can pick up some data. Access your target and actual charging voltage in your pcm data. They should be really close to each other. At the same time check your charging voltage with a multimeter at the battery.
If the pcm or battery wiring has any resistance then the pcm will be trying to up the charging voltage to compensate.


If you are having a low idle dip because of a idle motor or otherwise than charging voltage can dip also. You may be trying to chase two things at once.
I don't have a scanner, unfortunately. Is there anything specific that I need to look for when locating a cheap scanner? One that will access this specific data I mean.

For the PCM to be having resistance, is there anything other than damaged wiring or bad connections that I should look for?

I'm not really sure what is causing it to occasionally go into protection when I come off of the throttle. This problem isn't limited to coming off the throttle, it's happening at random times, but the thing they all have in common is a change in RPM.
Between the 8 foot of 8 gauge and the inline circuit breaker, the voltage is reduced just enough so that the amplifier is seeing an idle voltage of 14.8 to 14.9 with the occasional 15v spike. At the battery the voltage is primarily 15 to 15.1. The amplifier isn't a concern though. Really just trying to figure out what problems the Jeep may be having.

Thanks for the replies thus far. I appreciate the help.
 

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So backing up again to the beginning - everything was fine until you ran the heavy gauge wire and installed the amp?

The alternator produces an AC voltage which is rectified (probably a bridge rectifier - four diodes) and the PCM regulates the voltage. The alternator output voltage will increase or decrease based on engine rpm and load, it's the job of the PCM to adapt by keeping the voltage constant (above a certain rpm.)

The only way I can see the voltage increasing to >=15v is the PCM isn't regulating or there is an inductance in series with your amp. From the Wiki on inductance:

One intuitive explanation as to why a potential difference is induced on a change of current in an inductor goes as follows:
When there is a change in current through an inductor there is a change in the strength of the magnetic field. For example, if the current is increased, the magnetic field increases. This, however, does not come without a price. The magnetic field contains potential energy, and increasing the field strength requires more energy to be stored in the field. This energy comes from the electric current through the inductor.

The increase in the magnetic potential energy of the field is provided by a corresponding drop in the electric potential energy of the charges flowing through the windings. This appears as a voltage drop across the windings as long as the current increases. Once the current is no longer increased and is held constant, the energy in the magnetic field is constant and no additional energy must be supplied, so the voltage drop across the windings disappears.

Similarly, if the current through the inductor decreases, the magnetic field strength decreases, and the energy in the magnetic field decreases. This energy is returned to the circuit in the form of an increase in the electrical potential energy of the moving charges, causing a voltage rise across the windings.
 
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