Alternator or something else

"hedbutter" (hedbutter) 01/16/2014 at 07:30 • Filed to: None

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Oppo- Recently my Jeep Cherokee has been having an issue with the electric system right when I start the jeep. For approximately 2-3 minutes, the voltage gauge will read anywhere from 9-11 V (should be reading 13.75), and periodically dropping to 9V (aka far right and 0) and causing a check gauges light. If I ride through the light for a few minutes, I can hear the fan kick up a little bit higher and the gauge lights get brighter. If the check gauges light is on and the reading is at "0", it will stay like that until restart, and then its fine. If the light is not on, the voltage reads as it should. Either scenario, I can finish the drive with no issues. The belt doesn't have any markings on it that indicate slipping (new belt 2 months ago) and a new battery a month ago. Could it be a grounding issue or is it most likely the alternator? There aren't any odd squeals coming from the alt, and everything I've read states that the big box auto part store testing is useless.

tl;dr

Start jeep, voltage is low (9-11V)

After driving, sometimes drops to 0 (Causing a check gauges light)

After a couple minutes, its runs fine and jumps to normal

Does it sound like a bad alternator or a bad ground?

KB Garage > hedbutter 01/16/2014 at 08:19

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Sounds like a couple diodes in the alternator have gone bad. Any local parts shop can test that for you. New one should run you $100-150 (plus a core charge) if I had to guess.

Meatcoma > hedbutter 01/16/2014 at 10:01

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start car > Remove the negative battery cable, if it dies it's the alternator

You can tell a Finn but you can't tell him much > hedbutter 01/16/2014 at 11:12

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Best guess would be alternator, though it wouldn't hurt to check the main ground. I'm assuming there is a ground lead that goes from the battery to either the block or sheetmetal somewhere nearby. Check that to see if its corroded or loose. If that checks OK it is most likely the alternator.

Does the voltage go up and down when you are parked and rev the engine? If the voltage follows the rpm's it is almost definitely the alternator, specifically the voltage regulator. Not sure if the voltage regulator is integral to the alternator on those or if it is separate, though most vehicles are integral.

If the bearings on the alternator are good there are basically three parts to replace, the voltage regulator, brushes & diode trio. If you can get those parts it is much cheaper than a new or even rebuilt alternator. And replacing those parts is pretty easy to do.

NaturallyAspirated > hedbutter 01/16/2014 at 13:43

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I'm assuming that you are referring to an XJ Cherokee and not one of the new Jeeps called a "Cherokee" or one of the FSJs with the fake wood sides.

It sounds to me like a bad gauge. Here's why.

The engine idle speed on a Cherokee is below the coming-in speed of the alternator. Coming-in speed is the RPMs at which an alternator or generator will produce it's rated power output. In my experience, at idle the alternator in a Cherokee will not produce enough power to charge the battery, or even power the XJ's ECU (classic bad battery in an XJ or ZJ is that, when jump-started, it runs fine when the RPMs are kept above idle, but immediately dies when you let the engine go to idle). In fact, postal XJs had a switch on the dash labelled "Extended Idle" that would raise the idle speed to 1000 RPM for better cooling and to spin the alternator fast enough to charge the battery.

This means that it sounds perfectly normal to me for a just-started XJ to have a lower voltage battery than expected. The fact that the lights get brighter when the fan kicks up is good too, as the XJ will raise the idle speed slightly when running the fans (thus raising the amount of power the alternator is capable of providing). Revving the engine above 1000 RPM with the gas pedal should have the same effect.

If the actual voltage in your XJ's electrical system ever dropped to 0, the engine would immediately die.

So I'm guessing it's a bad gauge. It's probably reading correctly most of the time, but when it drops off sharply I would guess it's not reading correctly. I'd suggest reading the voltage across the battery terminals with a voltmeter next time it happens to check this.

TwoFortified > Meatcoma 01/16/2014 at 15:11

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I...wait, that works??

Noted. Dually. Didn't know that trick.

Meatcoma > TwoFortified 01/16/2014 at 16:19

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With low voltage it could potentially keep it running even with the negative battery cable disconnected, but more than likely it will cause enough strain on the alternator that it will kill the engine. I've done this to prove idiots from O'Reilly's wrong with their machine that told me it was a battery when I knew it was the alternator. Kept me from having to jump a brand new battery days later because the alternator wasn't recharging it.

hedbutter > Meatcoma 01/17/2014 at 10:08

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I saw this mentioned on a few forums when researching the problem, but it was always followed up with how terrible it is for the PCM and there is a major chance of frying it?

hedbutter > You can tell a Finn but you can't tell him much 01/17/2014 at 10:09

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Thanks. I'm going to clear the terminals and check some grounds today to see if that fixes the issue. If not, Im hoping i can make it another couple weeks so I can get back to my parents place and change it out (garage is way better for this than the gravel driveway at my apartment).

hedbutter > NaturallyAspirated 01/17/2014 at 10:12

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Correct, it is an XJ. I did notice that the voltage/fan speed/lights do not change when the engine is revved, just once the initial down period is done, it kicks up. Also, it only pegs to 0 after sitting for more than 10 hours, as overnight, it typically pegs to 0, but after I leave work, it'll be low and then jump to normal. I'm thinking you may be on to something with the gauge, but i will get the alt tested by the advanced auto parts near me to confirm (even though those tests don't prove a ton). Im working remotely today, so I'm going to clean the terminal/grounds and see if that does anything.

hedbutter > KB Garage 01/17/2014 at 10:13

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Thanks, I'm going to get it tested tomorrow after cleaning up the grounds/terminal today

Meatcoma > hedbutter 01/17/2014 at 10:51

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That may be true, but I personally have never had an issue. I've done this on my 2003 Ford F-250 with the V10 as well as older vehicles and have not had any issues. I have not done it on my 2010 Camaro, but have not had the need to either. I can assume that if you pulled the positive cable and grounded it, that could have some issues.

But I do know for a fact that 75% of the guys working at O'Reilly's, Advance, Pep Boys, Auto Zone etc. have no clue what they are doing and don't really give 2 shits about 'your' car.

hedbutter > hedbutter 01/17/2014 at 13:05

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Quick Update:

I cleaned some corrosion off of the battery connections and made sure they were completely tight (the ground was a bit loose on the terminal and the positive clamp wasn't completely tight). I tested the voltage, and it was around 12.25 with the engine off. After restarting the jeep, the voltage meter was still low and the voltmeter read 11.90 on battery terminals. After a minute or so, the voltmeter jumped back to normal, and reading across the terminals was at around 14. It looks like the voltmeter is reading correctly.

You can tell a Finn but you can't tell him much > hedbutter 01/22/2014 at 13:49

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Did you ever get this figured out?

hedbutter > You can tell a Finn but you can't tell him much 01/22/2014 at 15:37

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I was able to figure it out. The voltage gauge checked out and some of the cables were loose, but i was still seeing the issue. I had a new alternator put in yesterday (i was hoping to wait until the weekend so I could drive to my parents place and use their garage, but the temperatures in Pittsburgh were going to get into the negatives today/tomorrow and didn't want to risk it) and it fixed the issue.