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Alternator -- Diode Ripple not detected?

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Drained my '01 Box Non-S battery with a radar detector in the garage by accident for an entire week (didn't drive it, didn't maintain the battery, left the detector in there after a trip to Road America).

Went to a P-car function last weekend and after the function, noticed an audibly slower crank on start up. Took the battery to the seller and had them trickle charge and test it. At this point, no battery light was on.

Tech at the seller said it was a bad battery, no Cold Cranking Amps, that it should be replaced under warranty. Store manager said he was reading the machine wrong and that there were plenty cold cranking amps. Put it in car, drove it 30 miles, battery light came on but it was starting the car fine.

The NAPA dealer concurred (without me telling them about previous diagnosis) that battery had no cold cranking amps, but didn't ask if I wanted to check the charging system until after I purchased a $100 battery (didn't want to fight with the previous seller-- they had already replaced the battery under warranty once in February 05).

Then, he told me to start up the car with the alligator clip machine attached to the battery and he tested the system. His diagnosis: "No diode ripple detected-- you need a new alternator."

I asked if it could be a relay, if it could be another component of the charging system, a bad ground strap, or something, and he said it was probably a faulty diode in the alternator as that was the machine's reading.

Please advise. I know some archive posts suggest looking at the ground strap. On my 928, I had many such straps. I kept them clean and the connectors clean and lubed up with dielectric grease. I used vaseline on the bat straps on this car and the ground strap in the bat compartment appears clean and attached well.

I have not examined the underside of the car.

I have priced alternators-- I see Pelican distinguished between 5 speeds and Tips. Mine is a 5 speed. I understand there is a fixed pulley on tips and a free wheeling pulling on the 5 speed. I'm not sure of the difference. Many thanks in advance. :(

Looks like I will get some garage floor time under the Box soon. I am not an electrician, so please go easy on me regarding techincal descriptions. My question is whether I should go straight to the alternator, straight to the Irish whiskey, or a combination of relay, multimeter, harness, booze work.

David Cmelik

'01 Boxster Non-S 5 speed 2.7L

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From what I understand, there is a wire coming off the back of the alternator that goes right to the instrument cluster. When the alternator stops generating current it becomes a ground which grounds out the battery light and makes it light up on the dash. I would say you need a new alternator.

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'01 Boxster Non-S 5 speed 2.7L

Any chance you have a couple of months left on your 4 year warrantee (meaning a free replacement?)

None, but the thought had occurred to me. I actually found a discussion of possible bad alternator/voltage regulator posted by BoxsterSS November 26, 2004.

It discusses examining the wire winding (smells burnt?), checking to see that the pulley rotates smoothly (issue with tip versus 5 speed-- I had heard there is) to see if the bearing is sound, and ultimately the voltage regulator or rectifier is bad.

Replacing the alternator is apparently not necessary if the regulator is bad-- and there is a list of other-mfg'er cross reference part numbers in that post, including: VW Golf, New Beetle, Mercedes, and Audi A3.

I am going to check this out and keep the list posted on my progress.

Many thanks for all your help. :cheers:

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  • 2 weeks later...

I was in the battery business for 15 years, I realize that technology moves on, but I've never heard of "No diode ripple detected-- you need a new alternator."

I would measure the battery voltage, at rest. 12.6 is good.

I would measure the battery voltage running with the car running, 13.6 - 14.8 is good.

I would attempt to measure the miliamps draw with the car off. A couple miliamps is fine, more than a couple hundred isn't.

AFAIK... Diodes, should have no ripple. They take AC and turn it into DC. I would think if they have a ripple, then they are bad.

Generally speaking, if the alternator light is on, that means the battery voltage exceeds the alternator voltage. If the light glows dim, that means some AC is leaking in the electrical system, big hint, bad diode (there are generally three) If the light is off, with the car running, the output voltage good, but the battery discharges with the key off, then there is a draw, and sometimes they can be hard to find.

If you remove the alternator and take it to a good shop, they can test it, there is a really good one in Lancaster county PA. WEAVER'S REPAIR SHOP

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I agree with you jnc. I would run the tests that you described and make a decision based on those results. BTW Moose, the regulator is built into the alternator. The car does not have a separate regulator.

The purpose of the regulator is to convert the alternator AC to DC and to limit the amount of DC current to the battery. AC ripple is something the regulator is suppose to eliminate. I also do not understand what the guy means by “No diode ripple detected”. To me that would be a good thing, however, the regulator cannot totally suppress the AC ripple so perhaps his instrument is capable of detecting a very low AC ripple reading which would be present on a properly functioning alternator/regulator. Since none was present this meant the alternator/regulator was not functioning.

My alternator came from the factory with to much AC ripple. It was fine when the ambient temperature was cold but on hot days, all lights on the car would flicker lightly, the stereo and AC blower would pulse on and off at a very fast rate. You could see the problem by connecting an analog voltmeter across the battery. The needle would fluctuate at a rate base on the engine RPM. The higher the RPM, the faster the fluctuations. The alternator was replaced under warranty.

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The purpose of the regulator is to convert the alternator AC to DC and to limit the amount of DC current to the battery.  AC ripple is something the regulator is suppose to eliminate.  I also do not understand what the guy means by “No diode ripple detected”.

Thanks for the wisdom on this list. :cheers:

Update: Removed alternator before reading this post. I earlier bought a new battery. Bat light remained on.

The Napa guy used his tester to indicate that the car needed a new alternator. I agree that he is probably trying to sell parts-- but I thought I'd have the alternator and the Bosch volt regulator on the back of the alt tested by a reputable electric motor shop here in Cedar Rapids, Iowa. Hope it is the volt regulator, which I can replace with a Bosch/Mercedes/VW part for, as I understand it, $20-80. Many thanks again-- I will keep the list posted.

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  • 2 weeks later...


There are a lot of smart guys on this forum (much smarter than me), so I thought someone would have jumped in to explain what was really happening and how to simply test your Alternator. Since no one has, I thought I'd give it a go.

To begin, an Alternator is not a very efficient way to produce electricity for a car. First, it produces Alternating Current (AC), which the car cannot use, so Alternators must rely on diodes to rectify their AC output and convert it to Direct Current (DC) which the Car can use. Second, an Alternator requires two watts of energy to spin it for every one watt that it produces. It's advantages are that it can produce large amounts of current relative to it's size, and it can produce large amounts of current at idle, things a generator does not do very well.

The Lundell Alternator, technically the proper name for your Alternator, is really three Alternators in one body. Each of the three sections of the alternator generates Alternating Current (AC). In one revolution of the alternator it puts out three separate currents. These currents are each out of phase with the other two sections. Since the complete cycle (one revolution) of the Alternator is 360 degrees, each of the three phases are shifted by 120 degrees from the next phase. Each of the three phases has its own windings in the Alternator and each of the windings has its own pair of Diodes. Because the Alternator generates AC current, each of the Diodes in a pair have their own polarity - one positive (+) and one negative (-). The negative bridge of the Diode connects directly to the Alternator housing, which is of course grounded to the Engine Block, while the positive bridge connects to the output stud, which then goes back, via a cable, to the positive (+) terminal of the Battery.

The Diodes function is to block the current from one polarity while passing through the current from the opposite polarity (this is why you have two of them for each phase - they each function in opposite ways to each other). This is how they convert the Alternating Current (AC) to Direct Current (DC). The Diodes (or Rectifiers, because they Rectify the current), are usually arranged in pairs with all six being located on a single IC board or chip. Because of this, you get some rippling DC in the Alternator's output if everything is operating properly (this was the test the Auto Store conducted). But, the Battery smooths this out to regular or smooth DC.

These Diodes last only so many thermal cycles. Some of the newer Avalanche Diodes are even designed to be self-sacrificial by shunting large output spikes to Ground in order to save the sophisticated on-board electronics found on today's cars. Modern Alternators also produce high output from a very small package, which means that the case is not a large enough to act as a Heat Sink. The high heat which consequently builds up helps shorten the life of the Diode chips significantly.

Now, since each of the three phases has its own windings in the Alternator and each of the windings has its own pair of Diodes. Each of these windings and/or Diodes can fail, one set at a time.

If this happens the Alternator can still run the Car, it's Accessories and charge the Battery, but only with a limited current, approximately 2/3 of its original capacity if one system fails. If two systems fail, then it puts out only 1/3 of its rated capacity. What this means is that a failing Alternator can go unnoticed for a long time, because unless you are using the bulk of the Car's Accessories at one time, the limited output of the failing Alternator may still be sufficient to meet the Car's needs.

The Alternator will continue to fail until finally, there is insufficient output to run the Car, it's Accessories and charge the Battery. The first you may notice is the Headlights are dim, or the Dash Indicator lights, or the Stereo starts to whine, or worse, the Car fails to start (as Luck usually has it, typically late at night and in the rain).

Before suspecting a failed Alternator, you need to eliminate other possible causes. Check the Accessory Belt to insure that it is tensioned properly, usually no more than .5" movement when you push it with your finger (car OFF of course). Next, with the car OFF, check the voltage across the Battery terminal with a Voltimeter/Multimeter (set to DC scale), it should read at least 12 volts, more is better.

If all these things check out properly, then the Alternator becomes suspect. To begin, start the car, make sure all the Accessories are off and increase the RPMs to a fast idle. Set the Voltmeter/Multimeter to the DC scale (not AC or Ohms). Measure the voltage across the Battery terminals - red (+) lead of the Voltmeter/Multimeter on the positive (+)terminal, black (-) on the negative (-) terminal. The voltage should, read between 13.0 - 14.2 volts. If it reads less than 12 volts you most probably have a failed Alternator.

If the voltage reads OK, (with Car still running) turn on the the Stereo, the Headlights, the Rear Window Defroster, the Heater, and anything else that draws power. Increase the RPMs and watch the Voltmeter/Multimeter. It should still be reading around 14 - 14.2 volts. If it reads lower than 13 volts the chances are that your Alternator is not operating properly (or fully, as described above).

Finally, you want to check the Field Voltage. In order for the Alternator to generate electricity (because it lacks the permanent magnets of a Generator) it must first be supplied with a Field Voltage. If you know which wire is the one that supplies the Field (normally labeled 'F') then simply check with a Voltmeter/Multimeter to see if there is at least 12 volts at the Field. Another way to check this is to use a paper clip, small screwdriver, or anything Ferrous (iron, Steel). Hold it near the side of the Alternator with the ignition switch turned in the ON position, engine OFF because you don't want to have your hand near the spinning pulley/belt. If there is a Field Voltage present then the metal will be attracted magnetically to the side of the Alternator, not very strongly, but you will feel the magnetic attraction.

One last thing to check is the Bearing. The rotor inside the Alternator rotates on Bearings, and these can fail. When they do you will hear a loud grinding noise associated with the Alternator. To isolate the noise take a length of tubing, Heater Hose will do fine, put one end to your ear and move the other around in the vicinity of the Alternator. The noise will be much louder when you point it at the Alternator if that is the culprit. To further confirm this, you may disconnect the Accessory Belt and spin the Alternator by hand. If you hear, or feel a rumble or grinding then the Bearings have failed. Even If you don't hear a noise it may still be the Alternator Bearings since the Bearing might be smooth without the tension of the Accessory Belt. Also, check for side play in the Pulley.

NOTE: NEVER operate the car without having the Battery connected. Without the Battery, or if either the positive or negative cables are not hooked up, voltage can rise to over 40 volts!. This will cause extreme damage to the Car's very expensive electronics.

FYI, in an effort to increase their reliability, Alternators are becoming water-cooled. A 5/8" take-off connects to the Car's cooling system. Keeping them cooler will extend the life of the heat-sensitive Diodes. The 4.6L Cadillac NorthStar Engine currently uses a water-cooled Alternator. This is going to become more prevelant as Manufacturer's move to 36 Volt and 42 Volt electrical systems, because of the greater efficiency they provide (this is exactly why 12 Volt systems were adopted over 6 Volt systems). This switchover will occur before the 2010 model year. Hope this helps...

Happy Motoring!...Jim'99

Edited by MNBoxster
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To clarify, you stated that "AFAIK... Diodes, should have no ripple. They take AC and turn it into DC. I would think if they have a ripple, then they are bad."

This isn't exactly correct. True a Diode should not have a ripple. But, as in my description of how the Alternator works explains, essentially, you have three (3) separate pulses (for lack of a better term) of current being produced. There is a slight delay between these pulses and this is the ripple which is being detected. So, there should be a ripple detected, if not, then one or more of the 6 Diodes (because they exist in pairs), not the 3 which you mentioned would be Bad.

Also, you mention that "Generally speaking, if the Alternator light is on, that means the battery voltage exceeds the alternator voltage... "

Again, not exactly. You need to understand that unlike a Car with a Generator, in a Car equipped with an Alternator, all of the Car's electrical needs are satisfied only by the Battery. The Alternator only supplies power to the Battery, no other Accessories. This is necessary to smooth out the effect of the current pulses or ripples which the Alternator produces (The Battery can be charged in Pulses. But, other Accessories, such as the Stereo would not operate very well with a pulsed current flow.).

Also, the Warning Indicator is not the Alternator Light, it is a Battery Warning Indicator and means that the Battery is Low or won't accept a charge. There are many reasons for this including a Short Circuit to the Battery, Cracked Cells in the Battery, Low Electrolyte level in the Battery, loose or missing Accessory Belt, and possibly a failed Alternator. In other words, the fault isn't automatically with the Alternator.

Finally, you state "If the light glows dim, that means some AC is leaking in the electrical system..."

Again, nope. AC and DC are incompatible. AC cannot leak into the system. If a Diode fails, the current flow through that Diode either cancels out, or stops flowing.

Hope this helps...

Happy Motoring!...Jim'99

Edited by MNBoxster
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