Voltage Regulator Test? - JeepForum.com
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post #1 of 10 Old 12-08-2012, 02:27 PM Thread Starter
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Voltage Regulator Test?

Hi everyone,

I'm struggling to figure out what's wrong with my Jeep charging system. Is there any way to test the voltage regulator in a 2000 Cherokee 4.0 L ? I know it's in the PCM, or at least I think it is...

Here's a description of the problem I've experienced:

As I drive down the highway my battery volt meter spikes to 19v, stays there for a while, then drops back down to 14v. Later, I'll stop the vehicle and it won't restart.

I took the battery to be tested, it tested fine. I took the alternator to be tested, and it test fine twice and bad twice. I would just go ahead and replace the alternator, but I want to be sure that the voltage regulator is not causing this problem.

I'm not sure why the alternator would test perfect twice and then fail twice, but I'm hoping to determine the cause of this problem before shelling out any cash.

Any help is greatly appreciated.



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post #2 of 10 Old 12-08-2012, 02:46 PM
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I had this problem once and as silly as it may seem it was just a loose battery cable. I assume since you have take the battery out to have it tested then you tightened it down well afterwards. Is it doing this all the time or is it sporatic? Unfortunantly I don't know much on the voltage regulator so all I have is dummy advice.

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post #3 of 10 Old 12-08-2012, 03:20 PM Thread Starter
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Hi Cowboy,

Thanks for the advice. Dummy advice is perfect, because that's right at my level. I have tightened the cables down, but I suppose they could use a good cleaning with some Coke and a wire brush. Would bad connections explain why the charge voltage spiked up to 19v on my dash meter? Could a bad alternator cause this?

I had it test 4 times today. It tested good twice (at the same store), then bad twice (at two separate locations). I'm wondering if I should replace the old alternator back in the vehicle, or if I should go to the parts store and pick up a new one.

Any advice would be awesome.

Thanks again,

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post #4 of 10 Old 12-08-2012, 03:21 PM
AZ Jeff
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Testing the voltage regulator function is pretty straightforward, but you need to use a REAL VOM, not the junky gauge on the instrument panel is only approximately correct.

Unless stated otherwise, all the measurements below should be taken with the probes of the VOM in contact with the battery POSTS, not the battery clamps.

Charge the battery up with a battery charger until battery voltage measures 12.5 volts or better after the charger is disconnected.

Start the vehicle, and leave all accessories, lights, etc. OFF. Immediately after starting, measure the voltage at the battery. It should be somewhere over 14.0V, probably closer to 14.5V. If it's NOT, measure the voltage on the battery CLAMPS, and see what they read.

After a few minutes of running (at least 10), remeasure the battery voltage with the engine idling. It should measure around 13.5 to 13.8V. If it does not, repeat the measurement on the clamps.

Any noticeable difference in voltage between the clamps and the posts indicates bad battery connections.

I don't own an XJ any longer, but I still think they are the best of all the Jeep products ever made. My XJ was my favorite vehicle in my 50+ years of driving.
So...I stick around and give advice.
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post #5 of 10 Old 12-08-2012, 03:45 PM
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^^^ what Jeff said
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post #6 of 10 Old 12-08-2012, 03:49 PM
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Well there ya go, and to answer your question when I had that issue with my cables being loose the voltage (on the gauge) dropped and spike and all gauges shut down for a bit. It did a lot of squirrely stuff.

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post #7 of 10 Old 12-08-2012, 05:24 PM
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The dashboard voltage gauge is pretty good at showing charging system issues. Yours is indicating that have a charging system problem or battery problem. A failing alternator would be a good guess. A faulty PCM/voltage regulator would be very highly unlikely.

Perform routine maintenance of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Do the same for the grounding wires from the starter to engine block, and from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.


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post #8 of 10 Old 12-08-2012, 08:09 PM
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^^^^ what all others said ^^^^^
found this on the web...
I deal with 12/24/ & 36vdc in my job, everyday....regarding the charging issue, VOM your posts and verify voltage is 14vdc +,
regarding the battery, have it load tested, when able...
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post #9 of 10 Old 12-09-2012, 08:55 AM
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2x on loose or dirty battery connection (had this problem friday night just after I put in a new diehard), but I believe this would only happen on the positive battery connection.
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post #10 of 10 Old 12-09-2012, 09:39 AM
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Originally Posted by jeeplessinseatl View Post
...but I suppose they could use a good cleaning with some Coke and a wire brush.
I don't know how this myth got started but it's not a good idea. Your battery is acid. Coke is acid. Acid does not clean acid. What you need is something that will neutralize acid... plain ol' baking soda. (Not baking powder, baking soda. They are not the same.)

Mix up a thick paste of baking soda and water in a small plastic bowl, and scrub it over the battery and cable clamps with a old toothbrush. Scrub up inside the clamps and over any exposed wire. It will foam when it touches any acid. Don't get it inside the battery (pretty hard to do with today's sealed batteries.) Then rinse it all off with plain water from a hose. Don't worry; it won't hurt anything.

I've been doing this for over 40 years and it really works. Plus it's cheap and easy.

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