Which alternator

Hi guys I'm planning to do the Martin Hill ekectric steering pump mod on my 996 (conversion to electric hydraulic steering pump) and also have an intermitent light screech coming from an ancilliary so thought I'd pre-handle the alternator to handle the extra load, so i started looking .. what a nightmare! there are literally endless alternator models available and it confused the hell out of me so having sorted out the problem I thought I may as well post it in case it helps someone else. 1) Ampage: I had no idea but there are two ampages, 120a and 150a, I didn't care enough to look into it deeply but my initial guess is that the 120a is for manual and the 150a for tiptronic as they tend tto be listed as manual/tip. It would have been so easy to have bought a 120a which would have been useless. 2) Models - I had to reasearch this with Porsche Norwich as nobody knew but it turned out: - 996/7 alternators are interchangeable and have part numbers: - - 996603012* - 997603012* Mine is a 996.1 cab and the original part is 99660301206, BUT from the Porsche parts perspective all versions have currently (18-02-2023) been superceded by the part: - 99760301212 Note and correction thanks to Griffter Please be aware that there are two types of alternator, one has an overrun clutch, by all accounts this was fitted to manual cars but the general concensus appears to be that this is the best choice for all. I also found a company near me in Colchester Universal who are specialists in these systems they were very professional and knowledgeable on the phone and say they can take a 150a alternator and rebuild it to close to 200a for a very reasonable cost so I may do that to the new alternator. I should also mention having searched the forums that there are posts here which talk about the cheaper solutions of replacing the regulator, rebuild kits etc.

Be aware that another difference between alternators is that some have an overrun clutch pulley and others do not. Seemingly manual cars have the overrun clutch. I ordered a cheap Eurocarparts pattern alternator for my 996.1 years ago. IIRC it was a 150amp model. The quality of the casting was awful (and broken) and the pulley was a solid one. I sent it back and replaced the regulator on my original alternator. You can find more info on the overrun clutch by searching the forum.

Thanks Griffter :thumbs: I believe you are correct, damn means I have ordered the wrong one, how can something so simple be so flippin' difficult to get information on! Hope you don't mind but I edited my initial post just in case somebody read it as "advice".

I am puzzled by this: Why do you need a bigger alternator to run the electric power steering when it is the battery which supplies the current and the alternator just keeps it topped up. If there was a problem then the answer would be a bigger battery not a bigger alternator. What you could do is to replace the regulator pack, and that costs between £12-15 and takes about an hour or so to remove the alternator, replace the regulator and put it back. i did a post with pictures on how to do it.

I find this very interesting and I'm far from an expert and others will benefit so I'd appreciate a discussion here to improve my lack of knowledge as i'm sure you are far more knowledgable on the matter. Why for example when I have air con on plus heated seats, roof going up/down etc. does the supplied ampage drop in all of my cars including the 996 if alternator is not affected by the draw? The battery must at least be being charged at a lower level to my mind. Also Porsche supply both a 120a and a 150a alternator for 996/997 so no reason not to have the greater ability to my mind, you also made me realise that it is indeed my plan to use an S10 rather than an S08 battery but i failed to mention it. (are those number correct? well you know what I mean.). Lastly in my "stupidity defence" I got the initial plan from a guy who was an engineer and swapped out air con in an air cooled by adding a purely electric system (quite common I understand) but to handle it he had to have a bespoke alternnator made (he didn't mention the battery), maybe I was mislead.

Now I'm really confused, this appears to differ

Have you/could you put a meter across your battery with the engine running and see what voltage is present. And yes with a dead battery an alternator or generator for that matter will run an engine/and ancilliaries once it has started. In your case perhaps you should look at the battery situation. I have had 2 x 996 cabriolets and have never experienced the problem you mentioned. Certainly raising the roof is a big drain on the battery which is why you should raise or lower it with the engine running according to the book and advice i recieved from an Indy.

Have you/could you put a meter across your battery with the engine running and see what voltage is present. And yes with a dead battery an alternator or generator for that matter will run an engine/and ancilliaries once it has started. In your case perhaps you should look at the battery situation. I have had 2 x 996 cabriolets and have never experienced the problem you mentioned. Certainly raising the roof is a big drain on the battery which is why you should raise or lower it with the engine running according to the book and advice i recieved from an Indy.

That's always been my understanding. The battery is just there to start the car (and in some cases provide electrical power under circumstances when the alternator isn't producing enough current, such as very low revs, or if the drive belt snaps). The rest of the time the alternator powers the car's electrics and keeps the battery topped up. A bit like a water wheel, the gravity fed flow of water (alternator) powers the wheel, but there's a mill pond (battery) to maintain a head of water in case the flow is disrupted, or varies. So if you increase the potential electrical power required by the car, you might need to uprate the alternator to provide it. Strictly speaking, unless you are increasing the cranking power required to start the car (e.g. cc increase, compression ratio increase or some other changes) you probably don't need a bigger battery - but I assume it can't hurt.

Well thanks for the replies guys, always appreciated :thumbs: I appear to be back where I started however (except I now know the alternator should be the clutched variety), as the electric steering pump will be using considerably more draw I am still headed towards a 150/200 amp alternator and the S5010 instead of the S5008. My perfect outcome would be the amps staying at about 13.4 when the car is ticking over and all systems are on.

Not amps, it is volts and it should be 14.3-14.8. at tickover, any less and your alternator regulator pack is duff. Possibly the brushes are worn away. Find my post about alternator bregulator replacement and you will see a picture of worn brushes against a new set.

My bad .. volts .. and i feel I have mislead you, currently the guage sits right near 14 until the heater and heated seats go on or the top is moving. What I am planning for is the best possible situation for when the electric steering pump is fitted, I also want to replace the alternator anyway as it is the only ancilliary not replaced bar the air con pump. I will indeed read your pos, thankst.