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bill, jeep master tech
Category: Jeep
Satisfied Customers: 1632
Experience:  25 years with jeep drivability and electrical expert
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Jeep Cherokee Sport: a voltage drop that..defroster..alternator

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My 1999 4.0L Jeep Cherokee Sport shows a voltage drop that causes the serpentine belt to squeal when the lights or defroster are turned on high at idle. Car has both new alternator and battery, but problem has remained. Where should I look next?

There is more of a load on the engine when these accessories are turned on and more of a load on the belt. If the belt is slipping and making noise the voltage will drop. I would change the belt and tensioner as the tensioner is not keeping enough tension on the belt to stop it from slipping and making noise.


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Customer: replied 8 years ago.

Hello, thank you for your response. The belt is new, and this model jeep does not have an adjustable tensioner. The belt is adjusted correctly, and all the pulleys have been checked. This problem pops up on the jeep forums alot, but never seems to be solved. I have been driving Cherokees for my entire driving career (over 1,000,000 miles) and I have owned an '88, '89, '94, and now this '99. I have never had this issue before.


Some other info to digest: Before I replaced the alternator and battery, the voltage would be low when the car was started (12 or so) and stay that way until a clutch or something kicked in after idling for a couple minutes. (engine sound would change, but RPMs would actually go sounded like a fan came on) Then voltage would climb to 14...until the morning it dropped to 0 and the belt started to smoke, which i when I replaced the alternator, belt and battery.

Sorry, didn't know you already tried the belt as it was not mentioned in the original question. I will opt out for another expert to help.
Customer: replied 8 years ago.
I may not be clear here...the belt doesn't slip/make noise when the voltage drops...just when I step on the gas AFTER the voltage drops. Sorry...sometimes it's hard to describe these things correctly.

hello i'm bill

i have seen this many times the belt on your jeep needs to be VERY VERY tight or it will make noise on acceleration when it is "loaded up" like when the a/c is on or the rear defroster is on, i take a light and look for witness marks on the pulleys and jack screw and crank the belt till they are no longer visible then 1 more turn, the tension spec is 175 pounds belt tension measured with a belt tension gage, thats a hellof a lot of tension but is needed serpenine belts need a lot of tension or they slip and squeal under load but they last a long time compared to old style "v" belts which required little tension comparitivly

Customer: replied 8 years ago.

Hello Bill...this problem has persisted through 3 seperate belts, including a Gator Grip from's not the belt. As I stated, I've had a Cherokee my whole driving life and can replace these in my sleep. I have NEVER had a cherokee that even noticed when the headlights went on, even with the defroster, blower, and radio cranked. There is another issue here.


So, any other ideas?



have you checked the belt routing? here is how it is supposed to be,the routing was changed in 1997, if it is correct check pulley alignment, when did the noise start? before the alternator or after, also look to be sure the belt is on all the pulleys correctly and with the belt off check all the pulleys for excessive play pay close attention to the waterpump pulley and power steering pump they are common "failures" the powee steering pump USUALLY leaks but not always at the front bearing and seal and cause a squeal or noise



Customer: replied 8 years ago.

We are getting sidetracked by the noise here.....forget the noise, my issue is with the voltage drop...if I can solve this issue then there will be no noise. Turning on my headlights should not cause my voltage to drop from 14 to 12...and it's not just my headlights, defroster, blower, etc. all cause an above average drop. What is the onboard computer supposed to do when demand is high? are the RPMs supposed to go up?


For what it's worth, the belt is routed properly, and there is no fraying on the edges to indicate a misaligned pully.


the pcm (engine management computer) has the voltage regulator in it the engine rpm will go up very slightly if the pcm needs it to if there was an issue with the voltage regulator (VERY VERY UNCOMMON) the check engine light will illuminateand a fault code will setgenerally with poor charging system performance i start with a load test on the battery,the pcm uses outside temp, battery voltage at key on before the engine is cranked to start, and engine load to determine the proper voltage needed to keep the battery charged, the spec is 12.9 volts to 15.1 volts depending on temp the hotter it is out the lower the alternator output needs to be to maintain the battery, a weak or shorted(ing) battery will drive the pcm crazy, if the battery checks out ok then you need to look for excessive resistance in the charging system circuit most likely in the heavy guage red wire from the alternator to the battery and underhood fuse box, battery cable ends can cause excessive resistance also, and thus a voltage drop, if all that checks out remove the alternator and have it load tested a lot of socalled rebuilt alternators aren't done very well and cannot keep up with your demand, your alternator should be NO LESS than a 90 amp unit and we install 100 amp units at the dealer i work a 90 amp alternator should be able to produce at least 45 amps constantly with 90 plus amps peak
bill, jeep master tech
Category: Jeep
Satisfied Customers: 1632
Experience: 25 years with jeep drivability and electrical expert
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