Bad battery or bad alternator?

[jpdaeschler]

Hi 996ers,

Can I have some tech assistance/guidance please.

I took my 996 TT to a local shopping centre yesterday and the car started fine and a strong 14v registering on the dash. Drove 8 miles, spent two hours there and on way home dashboard started to light up with loads of failures to systems and power dropped to 10v on gauge just 1 mile from home. Luckily limped back with alarm going too, tiptronic acting erratic and justa bout every warning light on the dash etc. BTW the car is on a trickle charger when not in use.

So, is this just the battery change time or alternator or both? I am comfortable replacing both myself.

Thanks,
J-PD

 

[Richard_Hamilton]

Jean-Pierre!

I don't think you will be comfortable with the price of the alternator. What is the battery voltage with the engine switched off? If it is still 10V, then I suspect a cell has failed. This is certainly the cheaper of the two options.

Before disconnecting the battery, switch the ignition on, then switch it off, but leave the key in the ignition. This stops the alarm siren going off when you disconnect.

Check out the battery thread in the Technical Articles section - there are a few other tips for battery checking.

https://www.porscheclubgb.com/forum/tm.aspx?m=331718

https://www.porscheclubgb.com/forum/tm.aspx?m=770120

And if it turns out to be the alternator, check this thread:

https://www.porscheclubgb.com/forum/tm.aspx?m=735206

 

[75pno]

I have very similar symptoms with my 99 c4 , I changed the battery but no change, dash lit up like a xmas tree, the alternator was the culprit, the chap who did the work sent the original alternator off for a refurbishment as opposed to a exchange or new one.
75pno

 

[jpdaeschler]

Thanks for the reply and the welcome, Richard. The battery condition is down to 10v on the gauge as I suppose the battery was being drained to keep the car going whilst the alternator was not supplying power. Luckily made it home before the car completely died.

 

[Pieman7]

You can repair the alternator your self there quite straight forward. By the sounds of it looks like the rectifier has gone these unsolder and unbolt. Google the alternator make and model no: and you should find a rectifier there around 30.00

 

[watlina]

When mine went I got a secondhand alternator from a breakers. eBay stevestrange007 (Douglas Valley Breakers). Cost £48 delivered and looked a lot better condition than the one I took off. Piece of cake to change.

 

[924nutter]

If a cell had gone down the battery would become a 5 cell 10v battery in which case a healthy alternator should easily charge a 10v battery with power to spare. You would know all about it if there was a short to earth capable of dragging nett battery voltage down to 10 from 14.2 with the engine running. That's twice the drop when drawing starter current so I think you can see what I am getting at. I experienced similar symptoms right down to the fly by wire not working and driving along with the alarm going, and keeping the revs down because the engine wouldn't rev, etc. It was the alternator and according to my indie it is a quite common on 996, so much so that he keeps at least one alternator in stock. When he gets a collection he sends them away to his local electrician who reconditions them. The service is very good, so much so that he could supply and fit for less than I could source an alternator. I think the alternator was £270 as a stand alone item, which makes labour £133 The bill was £557 including vat and an mot (£54) at the same time. Simon Hope (prop.) of Hillcrest Specialist Cars in Nettlebed in Oxfordshire, who is Porsche trained. The thing for me is that he is only 9 miles from me.

 

[jpdaeschler]

Thanks for all your replies and guidance. I retested the battery charge and drain and all checked out OK so confirmed my suspicions on the alternator.

As some of you indicated, it was the voltage regulator on the alternator that needed replacing and appears to be a common issue. New Bosch item cost £30 online and very straight forward to replace once you get the alternator out.

All back together and back to normal running order now.

Cheers,
J-PD