Battery: Intermittant idiot light at hot idle

[Six_in_a_row]

Hello,

My 2006 Liberty had the battery replaced under factory warranty at ~ 30,000 miles in August, 2008. The dealer replaced it with another MOPAR Group 34 with 600 CCA. This is the size recommended for the car. The battery warranty was for 18 months. The car and battery are now out of factory warranty.

The following happened three times during the past two weeks. Two of these events occurred yesterday. After driving at highway speeds during hot weather, the battery idiot light went on while waiting at a stop light. The idiot light immediately went off when I either drove off or lightly touched the gas.

I cannot duplicate this idiot light event on my own. However, when running all of the accessories, I can see the headlights brighten somewhat against the garage wall when I touch the gas at hot running idle.

The shop manual states that the idiot light shows when the system is either below 9 volts or above 16 volts.

The car has a duty cycle of one week of two miles per day followed by a week of mixed in-town and highway driving.

I posted this question because some of the symptoms (headlights at idle) appear to be from a dying battery. However the battery is not that old. Of course, the factory battery wasn't that old when replaced under warranty either.

The car starts strongly. The electrolyte level appears full in all the cells. When I connect the battery to my charger, the "100%" light comes on after a few seconds.

I appreciate any advice so I can either avoid tying up the car or just go out and buy a new battery.

Best wishes and thanks again for your help!

 

[tmorris1]

The alternator is bad, not the battery.

 

[Chris Meutsch]

Alternator 100%.

 

[Trav]

Check the belt tension first before replacing the alternator,many times a slighly loose belt can cause symtoms like this.

 

[HTSS_TR]

Originally Posted By: Trav
Check the belt tension first before replacing the alternator, many times a slightly loose belt can cause symptoms like this.

+1

 

[Craig in Canada]

As others have said, these "battery" idiot lights are all about the alternator. If you take the belt right off the idiot light will be on even though the battery would keep the system at around 12V.

Different brands have different triggers for the light (no alt output, low alt current, problem with the field current, system voltage out of spec as you quoted) but it's pretty much never about the battery.

I believe in the old days where each accessory had a V-belt these lights were in place to tell you that your alternator belt broke. That's before my (driving) time.

 

[jim302]

Do you have any after market accessories? It could be that the alternator isn't quite up to the task to run everything when it is hot.

Or, as others said, it could be a slowly failing alternator. You may want to check for any TSB regarding this issue just in case there is an upgraded or improved alternator available.

The next time your warning light comes on, turn off all of your accessories (fan, AC, radio, etc) and see if it goes out.

Most part stores will test your alternator for free, so that may be worth trying. If it is only failing under certain conditions, they may or may not find a fault... so if it tests good, it could still be failing.

As always, check your wiring and connections just in case that is your problem.

 

[440Magnum]

+1 on likely being the alternator. Its probably got a dead diode or two and is operating at 2/3 its original capability. Or it may be that the brushes/slip rings are worn and the field current can't stay high enough. Either way, its got excess capacity and that allows it to work well enough for most conditions, but when its hot with a full electrical load, it can't keep up.

 

[mechtech2]

Check the battery voltage with the car off with any common multimeter.
Now start it and check the voltage when you rev it up a bit.
You should go from 12.5 or so to 14.5 or so .
This is a good quick test for the alternator.

Like the guys said, this is most likely an alternator problem.
But who knows? You may simply have faulty wires/cables or their end connections and where they seat may be imperfect.

 

[Six_in_a_row]

tmorris1,
Chris Meutsch,
Trav,
HTSS_TR,
Craig in Canada,
jim302,
440Magnum,

Wow! What a concensus! I'll make an appointment with my mechanic and have the Liberty looked over with the expectation that the original Denso alternator may have to be replaced. I hope that he can get a new one reasonably priced. From various threads here and elsewhere, a rebuilt unit may not be a great value if I have to have it reinstalled again.

Thanks again! I hope this thread helpful to someone else too!

 

[Craig in Canada]

Six: you should still check the simplest recommendation - see that the belt isn't slipping when it's really hot out. The combination of pulley contamination, a worn and stretched belt and high heat might mean that your tensioner is running out of adjustment room to keep things snug.

That's by far the cheapest/easiest.

Good luck

 

[Six_in_a_row]

Originally Posted By: Craig in Canada
Six: you should still check the simplest recommendation - see that the belt isn't slipping when it's really hot out. The combination of pulley contamination, a worn and stretched belt and high heat might mean that your tensioner is running out of adjustment room to keep things snug.

That's by far the cheapest/easiest.

Good luck


Craig,

I agree 100%!

I used the Gates serpentine belt gauge discussed recently on this forum on both of my Jeeps and the belt depth was OK and there were no cracks. The belt tension also seemed very stiff. There is no way to tell for certain without a belt tension tool and the tension pulley is not adjustable.

If it were a V-belt, I'd be more confident in making a belt tension assessment, at least based upon belt depth/deflection. At least there is no squealing at start up or under load such as happened with my older Jeep. That was for me a simple repair. These self-tensioning pulleys are a truly a mixed blessing.

Craig, thanks again for the encouragement! I will report back when I learn more from my repair shop.

 

[Six_in_a_row]

Originally Posted By: Craig in Canada
...snip...

Good luck


Craig in Canada and,
tmorris1 and,
Chris Meutsch and,
Trav and,
HTSS_TR and,
jim302 and,
440Magnum,

I had some good luck thanks to the helpful swarm of advice suggesting that I had an alternator problem.

The folks at my regular garage could not get a failing reading on the battery or alternator, even when warmed up. So the owner took my car home that evening and when he returned to work the next day the car was good and hot. He read the sytem voltage and found it fluctuating at idle between 11v and 12v with the accessories on.

Since my idiot light only reads below 9v or above 16v, there would have been no indication of any problem because I don't have a voltmeter on this car. He concluded that the voltage regulator failed to work properly when hot.

He replaced my OEM Denso with a rebuilt alternator. Any extended stop and go driving time with 11v or so would not be good. I'm glad they persevered with this.

Thank you all for steering me in the right direction!

 

[Craig in Canada]

Good to hear you've got it fixed. Make sure you check in on the situation in the same conditions to make sure that it's actually fixed.

I'll assume that your mechanic is honest, and made sure there was no belt slippage. All of the things he read with his meter could still be true if the belt is slipping.

Just out of interest, I had a car with a Bosch alternator (I think it was a 120A or 140A variant and was installed in a 1992 Porsche) which dropped system voltage when hot. I changed the regulator and it didn't fix it, and the belt wasn't slipping. I suspect it may have been a rectifier problem. The car left my possession before it was totally figured out. That car did have a voltmeter and it wasn't anywhere near as serious as 11V. More like 13.1 instead of 14.

 

[Six_in_a_row]

Originally Posted By: Craig in Canada
Good to hear you've got it fixed. Make sure you check in on the situation in the same conditions to make sure that it's actually fixed.

I'll assume that your mechanic is honest, and made sure there was no belt slippage. All of the things he read with his meter could still be true if the belt is slipping.

Just out of interest, I had a car with a Bosch alternator (I think it was a 120A or 140A variant and was installed in a 1992 Porsche) which dropped system voltage when hot. I changed the regulator and it didn't fix it, and the belt wasn't slipping. I suspect it may have been a rectifier problem. The car left my possession before it was totally figured out. That car did have a voltmeter and it wasn't anywhere near as serious as 11V. More like 13.1 instead of 14.


I was reluctant to give up on a bolt on repair such as this, but the Jeep shop manual didn't have any tests and swap outs for the individual Denso alternator components. For instance, the voltage regulator is built into the PCM (Powertrain Control Module). There is plenty of hot weather remaining this summer and I'll be driving the highway stretch where the idiot light events occurred quite often before it starts cooling off.

BTW, my olden '60 VW had a simple voltage regulator adjustment with a screw driver!!!

Thanks again, Craig! I really went to school on this one!