Just replaced battery because everything just shut off on the way home last night. The battery has been getting old and we'd put off replacing it.

Anyway, new battery in there now but battery light comes on when idling or at lower rpms. When I rev the engine above 3k the light goes off.

The ground is solid. Connections to battery are tight.

Alternator?

 

Yup alternator.

 

Could be the serpentine belt tensioner.

 

Lol, no. As long as the auto tensioner did not fall off that's far from the cause of the charging issue. Even with a failing auto tensioner I've never seen it cause an alternator to not charge.

 

I have seen this problem many times on many different vehicles. When the spring gets weak the belt still stays in place and the alternator slips on the belt.

Check it to make sure that is not your issue. A simple quick check is to wet your finger and very briefly touch to pulley after the engine has been running for a minute or two. Better yet take an IR non-contact thermometer and point it at the pulley. If it is slipping the pulley will be VERY hot to the touch. Hot enough to quickly burn your finger.

You will be very unhappy if you replace the alternator and the problem was really a weak belt tensioner causing the pulley to slip. Gates makes an improved tensioner that uses a flat coiled spring rather than the OEM round one. They seem to last a lot longer. Gates #39182

 

I agree also release the tension and check for free rotation of the alternator....

 

what u said makes no sense at all. How does the belt stay in place but the alternator slips on the belt when the belt is what drives the alternator?!? Lol the alternator is free spinning, and if ur case is that the belt is loose from a bad tensioner and not driving the alternator, than how is it charging at high rpms? The belt wouldn't magically tighten up at high rpms. And I'm speaking from real world diagnosis expirence, not Internet reading expirence. Lol

 

Also from the owner if the vehicle there needs to be a little more information. How many miles on vehicle? Does vehicle make noises? Etc etc...

 

If the alternator doesn't rotate freely than the alternator would be bad, not the belt tensioner.

 

The good alternator will spin freely by hand with the belt removed, because at that very, very low rpm it is not generating any power. Up to speed and under load the alternator presents a drag on the belt and the engine. How do you think it generates power?. That power does not come for free. It come from converting mechanical power to electrical power

A 100 amp alternator at 14 volts with a full load can present 1400 watts of load on the belt and engine. 1400 watts = just under 2 horse power. The conversion is 746 watts = 1 horse power. If the belt is not tight, then it will slip under a 2 horse power load. Depending on how weak the tensioner is, it can slip at fractional horse power loads. Have you ever tried to stop a 1/4 horse motor by hand. I'll bet that you can not do it.

The alternator is usually the highest load on the serpentine belt drive. The AC compressor is not far behind it. The alternator is also the smallest pulley and has the least belt contact area. This smaller pulley makes it spin faster than engine rpm but also makes it the first item to slip under weak belt tension.

Whether it makes sense to you or not, just quit arguing, and check to see if that is your problem. If it isn't, then go on to checking the alternator.

BE CAREFUL NOT TO BURN YOUR FINGERS. If it is slipping it can get quite hot.

It is also common for the bearing to quit on the tensioner pulley, but this usually only makes noise or locks up and smokes the belt. It does not cause low alternator output.

Here is a quote taken from the following web site:

Alternator and Starter Diagnosis | KnowYourParts

A slipping drive belt is another common cause of undercharging, especially with V-belts on older vehicles. Serpentine belts usually provide a better grip, but if the automatic tensioner is weak or stuck it can allow the belt to slip under load. Glazed streaks on the belt or belt noise when high-load electrical accessories are turned on with the engine idling can be signs that the belt is slipping.

Good luck on finding your problem.

 

Lmao are u trying to teach me a lesson on how an alternator works? Cuz apparently u have no clue, and ur wrong. First off an alternator makes the same amount of voltage at low rpm and high rpm, it don't change and it's regulated. U make no sense what so ever. So if the tensioner is weak and the belt is loose why is his cars electrical system charging at higher rpms when there is more load like u just explained? More load with a lose belt? U obviously don't know what ur talking about. Ur contradicting ur diagnosis. It's people like u that search on Google or give the wrong advice to people that make forums so unreliable. I am no where saying his tensioner isn't bad, but the point from the little information he has told us points to that the tensioner is doing its job. I should be telling u to quit arguing with a Honda Technician. By the way there's an easier way to check if a tensioner is bad then to touch it or whatever ur backyard method is. Look at the tensioner indicator pointer. Like I said, I'm speaking from real world expirence, ur speaking from, well, copy and paste. I could sit here and explain to u all day the mecanics of a proper tensioner and tight belt vs a bad tensioner and loose serpentine belt but u prop still won't understand it because u have no real world expirence. End of story.

 

ur a Honda tk? I am em pressed.....

 

Sounds like the tone here has gotten more personal than information sharing like.

Guys, Im a professional tech for over 25 years and run a 17,000 square foot vehicle shop and manufacturing facility. I estimate Ive serviced ver 50,000 cars over the last 25 years. Many shops Ive run have pushed over 400 vehicles a month through the doors. Both of you make valid points but at this point are more consumed with the tit for tat than helping here. Shake and make up here so we can help the poster please.

In my own experience, yes, a belt can slip on occassion and cause a charging problem. Could a weak tensioner do it? Perhaps. I have seen some tensioners tighten as you rev the engine on some vehicles. You could actually watch the tensioner move and tighten as the throttle was snapped. Was that a weak spring or design? Who knows but the replacements never moved like the ones that did. Never looked at this one while reving the engine so im not saying it will move. Its possible that higher rpms may spin the alternator fast enough that it still slips but now spins fast enough to charge. A bad alternator can also cause this and has historically been the problem way more times than a loose belt or bad tensioner.

Either way it would be a good idea to check it and see if the tensioner or slipping is potentially the problem.

Good luck!

 

Ive had an alternator in the past that bench-tested 100% good, spun freely by hand, charged great while driving, but was useless at idle like OP.


Even with a garbage battery, once started, the alternator is supposed to be big enough to keep the car going. If it shutoff, I would definitely grab a new alternator asap, before it kills your freshly replaced battery.


If the OP had belt noises, whether squeaking or whining, Im confident he would have posted as such.



Also, if belt tensioner was bad, he would have more issues

 

Auto tensioner? We don't go no stinking auto tensioner!


Don't know how it is for you boys in the Great White North, but my '08 USDM Fit has a manual serpentine belt tensioner.

 

WHAT ISSUES WOULD THOSE HAVE BEEN?


The first time I ran into this issue many years ago, I felt the same way. I had a low output alternator with no noise or other symptoms. A 23 year GM mechanic friend told me "replace the tensioner". I argued with him but I also had enough sense to listen to him. It indeed turned out to be a weak tensioner spring and a simple replacement (one bolt). That vehicle is still on the road today with over 200,000 miles on the same original alternator. I have seen several other failures like this over the years.

Serpentine belts, unlike v-belts, tend not to squeal under load. They will squeek if they pulleys are not aligned correctly. This happens frequently after a power steering pump replacement where the mechanic fails to install the pulley in the correct position on the shaft.

If a serpentine belt it is slipping, the slipping pulley will get hot and you will see glaze on the slipping side of the belt which in this case is the grooved side. Yes, the back side also drives some of the lower power consuming components. Usually that is the water pump and mechanical fan, if so equipped.

I'm almost sorry at this point that I tried to help the OP. It was a simple suggestion of a possibility that it might be, and an easy thing to check before pulling an alternator.

I'm sorry that you have taken a simple suggestion from many years of experience and stretched it this far. If you disagree with a posting try to understand the issue from the posters point of view. He might might be right and you will learn something, or then he also may be completely wrong and you can state why.

I'm done with this topic. I have wasted too much time with these detractors. I have to go out and replace a 6 year old battery in a snow plow today before the cold weather sets in tonight.

OP, check your tensioner. It probably is not your problem, but check it any way. If you feel I can assist you more send me a PM off net.

 

You sir are correct. I didnt relize we are talking about a GD fit, which does not have an auto tensioner, furthrrmore proves this IS NOT a tensioner issue.