Voltage drop whilst revving-up THE engine

[boeykew]

The voltmeter from my 996 3.6C4 Tiptronic. from 2002 shows since 3 weeks a strange behavior. The engine starts easily when cold in the morning, the voltmeter indicating 12,5V before cranking the engine, when it runs warm it indicates nearly 14V. Once warm and at idle the voltmeter stabilizes slightly below 13,5V. Until there everything is normal. Now it comes: when running the engine flat out, it will shift @ 7000rpm and then immediately the central info panel turns red with the message that the battery tension is critically low! 5 to 10 sec later it disappears and the voltmeter is showing 13,5V. It happened regularly so I started to check the voltmeter whilst accelerating flat out ( one eye on the road and one on the dash...), well during acceleration the voltmeter shows a steady tension drop: at 4000rpm the needle is gone below 12V and at 6000rpm below 11V. Shortly afterwards the alarm message appears. Whilst lowering the revs, the tension seems to pick up again. The battery is 6 years old, but showing 12,35V when disconnected after use. I left the battery on a microprocessor controlled charger for a night and the charger tells me the battery is fine and not broken. So faulty alternator or regulator? Many thanks in advance.

[JFP in PA]

Couple of possibilities: Some of these cars were equipped with a decoupling front alternator pulley (called an overrunning alternator pulley, acts like a clutch and allows the unit to free wheel under certain RPM circumstances), which may be the source of what you are seeing. Secondly, you could have either a diode bridge or voltage regulator on the way out. Easy enough for an equipped shop to load test the unit and verify what needs to be done.

[wvicary]

Also inspect the tensioner assembly for range of motion and condition of the belt but it does sound like the alternator is on it's way out.

[boxster-s-dk]

Also inspect the tensioner assembly for range of motion and condition of the belt but it does sound like the alternator is on it's way out.

+1

[boeykew]

Many thanks, the alternator is equipped with a simple straight pulley, so it must be the alternator or regulator. I'll take it out and have it exchanged. There is a specialized shop in our village and you can swap a faulty alternator for a reconditioned one for less then 100€. Maybe I change the battery too. 6 yrs is already something for a lead-acid battery. They now offer "Calcium-calcium" batteries, without lead. Anybody any experience? Grz...

[JFP in PA]

Many thanks, the alternator is equipped with a simple straight pulley, so it must be the alternator or regulator. I'll take it out and have it exchanged. There is a specialized shop in our village and you can swap a faulty alternator for a reconditioned one for less then 100€. Maybe I change the battery too. 6 yrs is already something for a lead-acid battery. They now offer "Calcium-calcium" batteries, without lead. Anybody any experience? Grz...

Sorry, but calcium-calcium batteries do contain lead; the difference is in a standard battery the metal grid is a lead antimony alloy, in a calcium battery it is lead calcium. This changes the battery's characteristics. Lead antimony batteries are far superior in their ability to deep cycle. And while lead calcium units tend to have lower self discharge rates, the grid plates tend to grow over time, causing internal shorts. Calcium batteries also tend to suffer from severe internal electrolyte stratification (when the specific gravity at the top of the cell is totally different than that at the bottom), which requires reconditioning (a special type of charge cycle that causes the electrolyte to remix) which is only found on higher end battery maintainers like the Ctek 7002.

[fpb111]

If you are going to replace the battery anyway, why not do that first to see if the 6 year old battery is the cause of your problem?

[logray]

If it were me, and you suspect the alternator/regulator. I would simply remove the alternator, battery, and starter. Take them all to a parts store or shop. In the USA, most parts stores bench test these using a sophisticated machine for free, and give you a detailed report on each item. I'm not sure if they offer this in Belgium but I can't imagine it would cost very much to have these parts load tested on a bench.

[boeykew]

Many thanks for the constructive feedback. First sensible explanation of what Calcium batteries are. Thanks for that!

Got a new battery and new (revised) alternator. Read the 997 DiY alternator replacement on this site first and was apprehensive about the amount of stuff that has to be removed before accessing the alternator. Well not on a 996: took me one hour and 15 min to replace both. Straight forward. Now I notice the difference: althought the old battery was not faulty ( no shot circuit in one of the cells) the starting power (revs made by the start motor) is much higher. The battery must have been tired. Another noticeable thing: since a while the idling of the engine was not clean, with noticeable vibrations and hesitations regularly. I did change the spark plugs (30k old) and the 6 ignition coils (11yrs old, 133k and some rusty...) but it didn't improve idling smoothness. Guess what, with the new alternator the idling fault is solved! It probably did send tension peaks into the car's electrics....

Grz...

[JFP in PA]

Many thanks for the constructive feedback. First sensible explanation of what Calcium batteries are. Thanks for that!Got a new battery and new (revised) alternator. Read the 997 DiY alternator replacement on this site first and was apprehensive about the amount of stuff that has to be removed before accessing the alternator. Well not on a 996: took me one hour and 15 min to replace both. Straight forward. Now I notice the difference: althought the old battery was not faulty ( no shot circuit in one of the cells) the starting power (revs made by the start motor) is much higher. The battery must have been tired. Another noticeable thing: since a while the idling of the engine was not clean, with noticeable vibrations and hesitations regularly. I did change the spark plugs (30k old) and the 6 ignition coils (11yrs old, 133k and some rusty...) but it didn't improve idling smoothness. Guess what, with the new alternator the idling fault is solved! It probably did send tension peaks into the car's electrics....Grz...

Glad you got it worked out.

Your comments point to a common misconception about the battery and charging system in modern cars; despite what many say, they do age and lose significant output over time, particularly the battery. This why the bench mark tests for batteries and alternators is how they perform under load. To that end, the standard testing procedure to check out either unit is by using what is called (not surprisingly) a load tester, which puts a heavy drain on each one separately to see how they respond. While the test only takes a few min. to run, it is a fail safe "go/no go" way to determine if it is time for a new battery or alternator. Most competent shops have the equipment to do this, and many serious "DIY" own them as well as they work on anything with a battery and/or a charging system. A decent hand held digital 130 Amp unit sells for around $80-100 over here.