i dont know how long this problem has existed as the van is not used at night often. with the a/c kicking (florida in july) the head lights on the vehicle when moving works fine and idiot gauge shows the needle to the plus. at idle engine in gear at stop light the needle was dropping down and a few times it appeared the head lights got dim and some pulsing was noticeable. this does not happen every time. the battery is 18 months old and tested fine. removed alternator and had tested fine. inspected wires and battery terminals all appear fine. the belt is about 5000 miles old and appears to be fine. turning the blower motor to low (less electrical load) made a difference. turning the a/c off (less mechanical load) makes some difference but hard to tell more or less. after testing alt and batt and belt being good dont know where to look next.

 

The alternator might be putting out power, but that doesn't mean it's still capable of putting out 100% of it's rated power output. If it bothers you that much, then I would replace it and at the same time make sure you get the one that has the highest amps output for your model. Seeing as you didn't post what you have, we can't advise any further.

 

Also, this problem is common with dirty battery posts for the terminals being attached to them. Clean the terminal posts, terminals and re-tighten everything.

 

Alternators can fail in thirds. The blower motor on high draws more amps than the highbeams and parking lights. The AC reduces the idle speed which lowers the alternator speed, and might reduce it to beyond the threshhold where it provides more amps than the vehicle is using.

I have a 130 amp alternator and it seems incapable of providing more than 32 amps when hot, at idle. When I have the blower motor on high and the lights on, 10 amps are flowing FROM my 3 fully charged batteries, and my lights dim at idle. I have also greatly increased the wire sizes in the charging circuit which helps greatly at higher rpm's and lower states of battery charge, but does nothing when the alternator is hot.

That said, a higher amp alternator does not mean it will be capable of more amps at idle speeds. In fact while it might be capable of briefly meeting it's rating when cold, at high rpm, the smaller rated alternator might actually put out more amps at idle speeds, so if that is true in your case, there would be no benefit unless your alternator is indeed failing by 1/3's.

Alternators usually put out only a fraction of their 'rated' power, and even if it could put out 130 amps, a single battery could never accept that much current. You would need devices capable of pulling 100+ amps attached through cabling designed to carry such high current. The OEM charge circuit cabling is woefully undersized in this regard.

 

my biggest problem is it appears to be intermittent. although landyacht brings a valid point. hot and cold. maybe this is only happening when the alternator is hot? i have no way of knowing if i have a 90 or the 120 alternator. but it being a v6 cargo van with no accessories i have to assume 90. i will go thru all the wires and connectors again to make sure they are clean.

 

I beg to differ. Larger amp alternators have more windings and larger stators. Given the same RPM as a low amp alternator, the additional windings will produce more power.

 

I'm not saying every smaller alternator will perform better at lower rpm than it's higher rated counterpart, but Compare the output of these 2 higher amp alternators further down the page at 1500 and 2000 rpm, the 120 amp alternator outperforms the 150 amp alternator till higher rpm are reached, while the 70 vs 100 amp alternators back up what you say.

http://www.betamarinewest.com/balmar...-sheet-web.pdf

 

i checked the batt terminals (clean and tight) i turned everything on including lights and fans and a/c. the gauge dropped and the fans slowed. i slowly turned things off starting with a/c until enough things were off that the gauge moved to normal. i did this several times including shifting to gear and neutral. i also disconnected the battery with everything on and full blast. the gauge dropped way down but everything continued to run. what concerned me is the gauge did not go up and down the same every time. so as alloro pointed out earlier the alt may be worn but what the heck is causing it to act up intemittently? i just dont want to go waste 120 bucks for the 120 amp alt. if it was a worn unit wouldnt the load consistently drop and increase?

 

Never disconnect the battery with the engine running. It can cause voltage spikes which can ruin the electronics, and destroy the alternator diodes.

It was a valid test 45 years ago. no longer.

http://www.aa1car.com/library/charging_checks.htm

 

Odd, that is. That's probably the only combination out there that has it backwards.