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Excessive Ripple = definitely Bad Alternator?

Hello! Are these facts about my electrical system enough to pinpoint the problem on my 99 Nissan Altima?

FACTS: In early April, the following were replaced: alternator/serpentine belt, a few pulleys, and compressor belt. In mid-May, I started using the A/C, and got a very short squeal from the belt each time. On May 21, the approx. 4.5-year old battery died. Next day, Advance Auto tested the battery, confirmed it was finished, so I got a new battery. June 8, while driving home, the radio turned on and off and so did the lighter (cell phone was charging) several times. As I pulled up at home, some dash lights flickered. Turned the car off, but it would not restart. Connections and cables were good.

Spoke by phone with my pro-mechanic and friend-mechanic; both insisted the alternator was fine, and battery was defective. Next day, jump started the car, Advance tested the battery serveral times, the read-out finally said the battery was bad, needed to be replaced, and alternator showed excessive ripple (764mV). They replaced the battery. Two days ago, my mechanic friend sprayed belt conditioner on both belts (alternator and compressor), then the squealing from A/C stopped. Yesterday, I had the alternator re-checked, still excessive ripple, but smaller than before (now 312mV).

QUESTIONS: Is the alternator the source of the problem here and thus needs to be replaced, or could the loose belt (compressor, I assume) or something else be causing the excessive ripple problem with the alternator? I just don’t want to replace the alternator and then still have electrical problems afterward because I missed the real problem.

And by the way, can a bad alternator damage the battery? I am wondering if the alternator has been bad for the past few months and is responsible for killing both the 4.5-year old battery and the new one that lasted three weeks.

Thank you for any attention you can give to this post! Please let me know if you need any more information.

I am wondering if the alternator has been bad for the past few months and is responsible for killing both the 4.5-year old battery and the new one that lasted three weeks.

I’m not sure. You made me less than confident with Autozone’s diagnostic prowess when you said the read out “finally” said the battery was bad. So, I’m not sure if their reading of the alternator is good or not.

The 4.5 year old battery was probably on its way out anyway. A bad voltage regulator can certainly kill a battery. Whether or not your voltage regulator (that’s part of the alternator) is bad, is up in the air until you have it diagnosed by a competent diagnostician, not an Autozone salesclerk.

could the loose belt (compressor, I assume) or something else be causing the excessive ripple problem with the alternator?

No. If the VR is working properly it can compensate for a slipping belt.

From your overall post, it sounds like your car has missed a few scheduled maintenance procedures. I’d recommend that you check the owners manual and replace what should have been replaced by now. And BTW, belt dressing is a temporary solution to a very cheap problem. Just replace the belt, and check the tensioner.

Thanks for your reply, Shadowfax. I’m feeling a little silly that I wasn’t even generally aware that an owner’s manual suggests replacing certain parts on a basic schedule. I’ll check that today.

Yes, it took Advance several consecutive tests before the read-out said the three-week battery was not simply uncharged, but needed replacement. I also didn’t mention another alternator test (in between the two I listed) with the current battery, in which Advance’s computer said the alternator was officially “good” but still generated excessive ripple. So I share your lack of confidence in their assessment.

I plan to get the compressor belt tightened a.s.a.p.

The item associated with the alternator that can cause ripple when bad does not cause squeal. Squeal is caused by slippage, and that’s caused by a worn/loose/stretched belt or a component starting to fail and binding. The component could be the tensioner, the AC compressor, the power steering pump, and yes, the alternator.

Ripple is caused by a bad alternator, specifically a diode in the bridge rectifier.

Since the sqeal was happening before the alternator was changed, I suspect you have something else failing. With the belt(s) removed, try turning and laterally wobblilng every driven pulley to see if you can find a component going bad. Check the tensioner (a Haynes manual will tell you how and what tool you need…it’s not expensive).

The alternator itself can be bench checked. Have that done by the place where you bought it.

The alternator will always have some “ripple.” The battery acts as a buffer/accumulator and will absorb that ripple IF and only IF the charging circuit is well connected. That means the large cable and the alternator body itself as that is part of the circuit. Battery cables, terminals etc are all suspect. The A/C ripple caused by a diode is too high frequency to see pulsing in things such lights but can be seen on a scope. A typical diode failure will show an asymetric wave. 300mV may be OK at the alternator’s output terminal but I would not expect it in a GOOD battery when measured at the terminals of the battery–not the battery cables. A sulfated or broken battery would invalidate the so-called ripple test. Voltage regulators are often responsible for low frequency, visible and rythmic voltage ripple…brushes that are worn out can also cause this.

Regulators also contain capacitive/resistive filters to modulate the ripple. According to the OP, the ripple was out of spec.

I agree with the rest of your post. Actually, my commendt isn’t a disagreement as much as an explanation as to why I think the alternator should be bench tested. Especially if it’s a rebuilt.

I’m a technical / engineering scientific kind of person that engineers automation controls–computer science major. I used to work on heavy trucks for a living. I look at it from this perspective: The vehicle is the best place to test the alternator–the environment is perfect. A bench test is useful for an unknown alternator or repaired etc. I believe it is foolish to remove an alternator to bench test it as it is a less conclusive test than just leaving it in the car. To a person that has little knowledge of how it works, they might feel accomplished by taking it out and giving to an “expert” but that is really an ill-informed method of repair. But, if time permits and that person enjoys the the greasy cut knuckles, then removing it serves a greater purpose of entertainment. I don’t get much enjoyment out tinkering

I’m in agreement about not just the bench testing of alternators but starter motors also. The results on a bench can be somewhat iffy at best.

Maybe there’s a problem between the alt. and the battery terminal; as in the solder in a fusible link trying to give up the ghost.

Alternators generate 3 phase A/C which is rectified into DC current. There is very little ripple. If one of the three big diodes that are performing the rectification gets weak or fails, there will be a LOT of “ripple” (A/C ) in the output. The output of the alternator will also be greatly reduced…

As Caddyman pointed out, the alternator is a three phase generator with a diode rectifier bridge. One of the reasons that three phases are generated is that the cosine of 60° is .5. That means that as one phase is 60° past its peak, the next phase is reaching 60° before its peak. Both are at their half power points. .5 and .5 added together = 1, the same as peak for one phase.

This makes for a very smooth output, but not perfect. At each 30° point in a rotation of the generator, the sum of all powers equal zero. At all the other points, there is a small unbalance that results in the ripple. I can’t remember what the unbuffered theoretical ripple is in a three phase generator is but I think the 312 mv in a 12 volt system is not anything to worry about. In fact, 764 mv would not be alarming either.

If you had a diode burn out, the ripple would be a lot greater, on the order of a couple of volts.

The amount of ripple voltage you are seeing along with belt squeal indicates to me that you do indeed have a bad alternator. The ripple voltage should be less than .1 volt AC. The squeal is occuring because there is a problem with some the diodes and it is causing an excessive current load to the alternator. Another indicator you can check is the DC output of the alternator. My guess is you will find it slightly on the low side due to the bad diodes.

This problem won’t cause a no start condition so if that is still happening then check the voltage getting to the fuse panel under the hood. Clean the battery connections and check the wire running to the main panel from the battery along with cleaning the chassis grounds.

To bring closure to this post, after I posted my first message I risked leaving the questionable alternator in and carrying on with the second replacement battery, but was not very comfortable doing so. (My semi-pro friend-mechanic kept assuring me the alternator was not the problem.) On Thursday, July 7, I got my annual state inspection, and the auto mechanic told me my car almost didn’t start and my alternator was “acting up.” At that point I had enough, so I immediately headed straight for Advance to get a new alternator and my car died (AC was running) along the way. I got a jump and turned off my AC, then picked up the new alternator and went home.

I replaced the alternator Friday (myself), then the ripple tested 55mV (0.055V), putting out 13.9V under no load, and 13.6V loaded. All of the previous alternator tests in the last month showed around 12V loaded or not loaded, with only one test (at night) showing around 13.6V. On a 12V system, I’m guessing I’ve been running too close to draining the battery under load, so given the higher voltage output now, and healthy ripple, I’m guessing the alternator was indeed the problem (whatever the ultimate failure inside the alternator was). If anything indicating a different problem occurs, I will update this post. Otherwise, that’s it. Thanks for everyone’s input, and feel free to comment further if you like.

Thanks for taking the time to come back and close up your thread…Glad to hear you have cured the problem.

Well looks like you got your answer and replaced the alternator. I have argued with NAPA and Walmart on testing alternators and batteries and just plain don’t trust those tests anymore. I had two alternators under warranty that tested good but would would give me a fluctuating headlight at idle. I finally just threw the warranty away and bought a Delco and problem solved. I also had an 18 month old battery that wouldn’t take a charge but took over an hour for Wally’s to agree to replace it since it tested good. Might tell you if they are totally bad but not borderline stuff.

Good job. The ripple voltage you are now seeing is normal. The fact that the DC output voltage was lower and the AC ripple voltage was higher on the bad ones means some of the output diodes were bad. That causes less DC output and higher AC ripple. Just as you discovered.