Possible Grounding Problem - Page 2 - Rubicon Owners Forum
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post #11 of 29 (permalink) Old 08-16-2019, 04:02 AM Thread Starter
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Originally Posted by johncanfield View Post
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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post #12 of 29 (permalink) Old 08-16-2019, 09:05 AM
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At ~1200 rpm I think we're about 14.5 volts give or take a tenth,

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post #13 of 29 (permalink) Old 08-16-2019, 11:18 AM
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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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post #14 of 29 (permalink) Old 08-16-2019, 03:14 PM Thread Starter
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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.
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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post #15 of 29 (permalink) Old 08-16-2019, 06:32 PM Thread Starter
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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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post #16 of 29 (permalink) Old 08-16-2019, 09:38 PM
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How did you wire up your amp, i.e. gauge of wire, where is the power taken from, where is the ground connected?
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post #17 of 29 (permalink) Old 08-16-2019, 09:40 PM
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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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post #18 of 29 (permalink) Old 08-17-2019, 02:23 AM Thread Starter
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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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post #19 of 29 (permalink) Old 08-17-2019, 02:44 AM Thread Starter
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Originally Posted by rlenglish View Post
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.

Last edited by Firemedic82; 08-17-2019 at 02:46 AM.
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post #20 of 29 (permalink) Old 08-17-2019, 09:36 AM
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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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