Idle Question...

I have a 97 Explorer 4x4 4.0 and I have a question about the idle. The truck idles at about 750 rpm and very smooth but as it idles the battery/charge gauge drops very low and all the lights exterior and interior go very dim, when I accelerate from a stop light everything goes back to normal. My question, is this normal and is there any problem this could cause. I replaced the alternator and battery last year, belt and tensioner 3 months ago. Just wondering if any damage can be done to the computer or emission controls, sometimes I have to idle for a long period in traffic.

The idle speed seems fine, but if the lights get dim at an idle, something with the charging system is not keeping up. I'd suggest you have the charging system checked. Many of the chain auto parts stores will offer this service at no cost. Or, if you have a multimeter, you can check the charging system voltage yourself. Do you have a battery charger? If so, charge the battery. Once the battery is fully charged, turn on the headlights and leave them on for 15 minutes. After 15 minutes, turn off the headlights and try to start the car. If it won't start, your battery is not holding capacity.

If it starts fine after leaving the headlights on, then the alternator is probably not providing enough charge current. Either way, you want to determine the problem and get it fixed. If you have a problem with the battery, it can shorten the life of the alternator and vice versa.

Of course you should also verify that the battery cables are clean and tight as well.

-Rod

The idle speed seems fine, but if the lights get dim at an idle, something with the charging system is not keeping up. I'd suggest you have the charging system checked. Many of the chain auto parts stores will offer this service at no cost. Or, if you have a multimeter, you can check the charging system voltage yourself. Do you have a battery charger? If so, charge the battery. Once the battery is fully charged, turn on the headlights and leave them on for 15 minutes. After 15 minutes, turn off the headlights and try to start the car. If it won't start, your battery is not holding capacity.

If it starts fine after leaving the headlights on, then the alternator is probably not providing enough charge current. Either way, you want to determine the problem and get it fixed. If you have a problem with the battery, it can shorten the life of the alternator and vice versa.

Of course you should also verify that the battery cables are clean and tight as well.

-Rod

Thanks for the response Rod, I checked the voltage and at idle it just over 12 volts and when I speed the engine up it goes to 15 volts. The terminals are clean and tight and the load test on the battery itself is fine.

Just over 12 Vdc at idle, especially with little load on the charging system, is too low. At idle, you should see something closer to 13.5 Vdc and when speeding up the engine, you should get not more than 14.5 Vdc.

The next thing to try may be to use the Vac setting of you meter and see how much AC voltage is on the battery. You should see very little. If you have more than probably a volt, you might have a rectifier diode issue.

How did you load test the battery, at a parts store or using the headlight method? If at a parts store, I wouldn't necessarily consider that gospel yet. I've seen some load testers claim a battery is just fine when the battery definitely had problems.

-Rod

Just over 12 Vdc at idle, especially with little load on the charging system, is too low. At idle, you should see something closer to 13.5 Vdc and when speeding up the engine, you should get not more than 14.5 Vdc.

The next thing to try may be to use the Vac setting of you meter and see how much AC voltage is on the battery. You should see very little. If you have more than probably a volt, you might have a rectifier diode issue.

How did you load test the battery, at a parts store or using the headlight method? If at a parts store, I wouldn't necessarily consider that gospel yet. I've seen some load testers claim a battery is just fine when the battery definitely had problems.

-Rod

Ok, I checked the Vac on my multimeter and it is ranging from .103 - .003 and the load test was performed on a Matco tester at a quick lube. The battery has 12.8 Vdc when turned off/not running.

Okay, 12.8 Vdc when the engine is turned off is good. Sounds like that's probably just after the engine was shut off.

The Vac is reasonable, so probably not a problem. Hmmm....

I'd still like to see you try the headlights left on with engine off test suggested in post #2 above. If you pass that, the battery is most likely not an issue.

-Rod

Okay, 12.8 Vdc when the engine is turned off is good. Sounds like that's probably just after the engine was shut off.

The Vac is reasonable, so probably not a problem. Hmmm....

I'd still like to see you try the headlights left on with engine off test suggested in post #2 above. If you pass that, the battery is most likely not an issue.

-Rod

Left headlights on for 20 minutes, cranked right up. I dunno man, everything seems ok. One thing I forgot about that may or may not have anything to do with this is, I accidentally left the key switched on all night about a month ago and the battery was dead the next morning, I jumped it off and been fine ever since. Could that have damaged anything, I know the manual says not to leave the key on for a long period of time. BTW the alternator was installed 6/06, was a brand new one from Advance Auto.

If the battery was run down to completely dead due to the key being left on, that would have made the battery more susceptible to freezing. However, if the battery didn't freeze and was allowed to charge up completely the next day, it probably didn't damage anything. However, if it was just jumpstarted and you drove the Explorer right away, that would have put a pretty good load on the alternator. That could have accelerated wear on the alternator.

I'd have a hard time condemning the battery based on the checks you've performed. Sounds like the alternator or the wiring of the alternator is a bit screwy. The fact that your voltage goes as high as 15 volts when the engine is running and believing that your battery is fine would start to point to the voltage regulator. I believe on your '97 the regulator is part of the alternator.

-Rod

If the battery was run down to completely dead due to the key being left on, that would have made the battery more susceptible to freezing. However, if the battery didn't freeze and was allowed to charge up completely the next day, it probably didn't damage anything. However, if it was just jumpstarted and you drove the Explorer right away, that would have put a pretty good load on the alternator. That could have accelerated wear on the alternator.

I'd have a hard time condemning the battery based on the checks you've performed. Sounds like the alternator or the wiring of the alternator is a bit screwy. The fact that your voltage goes as high as 15 volts when the engine is running and believing that your battery is fine would start to point to the voltage regulator. I believe on your '97 the regulator is part of the alternator.

-Rod

Well I took the alternator back to AAP, exchanged for a new one and what do ya know...all is well now. The voltage is holding steady at 14.5 volts even at idle. I dont know if this is wishful thinking or whatever but the truck even seems to run smoother and shift gears smoother. I appreciate all your help Rod.

Great, I'm glad to hear that your Explorer seems to be fine now. Thank you for the follow-up as well. There's a good chance this will help someone in the future as well.

-Rod