996 voltmeter reading

996 voltmeter reading

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Discussion

griffter

Original Poster:

3,981 posts

254 months

Saturday 10th October 2015
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I've googled this and understand dodgy cabling, earth points and heat-soak affected regulators can play a part, but my question is, is anything really wrong or does my voltmeter just under-read? Anyone had similar symptoms and done anything - or not?

So my 996 has a new big powerful battery, and the voltmeter reads under 12v with the ignition on. When I start the engine the reading goes up to over 14, but over the next minutes/miles drops to just over 12v. This is pretty much where it stays, with a little gradual fluctuation. Hitting traffic and sitting idling, the reading will drop below 12 and sometimes as low as 11 with lights/ac on, but always a bit above the ignition on / engine off reading.

I've done 500 miles at least half with the lights on and the car starts with a healthy turnover each time.

Is the voltmeter reading the battery voltage or the alternator output?
Is there some sort of intelligent regulation of battery charge?
Am I just seeing the effects of car lengths of cabling between the battery and alternator heating and cooling and carrying different currents depending the state of charge and the electrical demand?

Any opinions or experience welcome. The bottom line is that I'm not really concerned because the system seems to be working OK - if it wasn't for the voltmeter I wouldn't have noticed anything. There have never been any warning lights.

Thanks.

jkh112

21,888 posts

157 months

Saturday 10th October 2015
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My 996 doesn't do this. Why not buy a cheap voltmeter and hook it up the battery then go for a drive and see what it shows?

Pope

2,633 posts

246 months

Saturday 10th October 2015
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Measure the voltage at the battery, then at the power pont in the engine bay

Or

Measure the volt drop in the +ve cable - connect a voltmeter on the +ve terminal of the battery and the power point in the engine bay; spec is <0.3v drop per cable.

BG and The Bear

123 posts

141 months

Saturday 10th October 2015
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1st problem I ever had with my 996 was an alternator fail
The volt meter was reading near 12V for several days, thanks to a high energy battery. But it soon failed

Certainly worth checking with an external meter for reassurance. I think you can get a free battery/alternator check from Halfords if you don't want to buy a meter

pete

1,586 posts

283 months

Saturday 10th October 2015
quotequote all
It does sound like the alternator voltage regulator, a common 996 failure. You get the full 14v when cold, then as it warms up the output falls off. I had exactly what you describe, and drove hundreds of miles with no major issues, but it did start to get gradually worse until I caved in and bought a new regulator. I also didn't get any charge warning lights, but it was starting to worry me, especially when sitting in traffic with lots of heat soak and the cooling fans on full blast.

A replacement is about £40 from Woods for the correct Bosch part, and a few hours' work to fit - you have to remove the alternator to get to it.

Pete

Edited by pete on Saturday 10th October 22:48

BertBert

18,953 posts

210 months

Saturday 10th October 2015
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What problem are you trying to solve?
Bert

DasChin

609 posts

215 months

Sunday 11th October 2015
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i would say your alternator is on the way out chap.

its a common fault on the 996 and its a gradual thing rather than total failure like coil pack.

I tried with a new bosch battery but it was no good and eventually the Alt gave up.

got a Bosch one on the car now new and its never run better with new battery and new coil packs/plugs earlier this year.

I have a GT3 but Alt is a common 996 part

griffter

Original Poster:

3,981 posts

254 months

Monday 12th October 2015
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Thanks all. It sounds like it'll be difficult to convince the warranty company anything's wrong without actually breaking down. The dealer's said tadts.

But if I can diy for £40 I may as well save the argument and a "warranty approved" garage breaking bits on my car.

Interestingly with a multimeter, battery and engine bay +ve read 11.6v - more or less exactly what the voltmeter does. But I guess if that's all the alternator's putting out, that's all the battery will produce. I've tried a trickle charger too but I need to give that a bit longer to see if it'll increase the voltage on the battery.

DasChin

609 posts

215 months

Tuesday 13th October 2015
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If battery says that with engine running then its defo the alt.

It should be reading 14 and it should say that on the internal dash dial.

Take a picture of that and send to warranty company.

Get it sorted before winter as the cold will defo kill and could lead to other problems like totally killing the battery and that's just more money.

You can get an alt for like £200 from euros so not bad if you are handy with spanners.

griffter

Original Poster:

3,981 posts

254 months

Friday 13th November 2015
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Thanks all for your helpful guidance.

Update: I bought a bosch regulator, part no. ending 350 (top) instead of 255 (bottom) from eBay. According to pelican parts thread (see also below) and other sources these are equivalents. In fact the 350 is listed for a 1.9 tdi 110 Skoda!:



I fitted as per pelican parts thread:

http://forums.pelicanparts.com/porsche-996-997-991...

It's straight forward enough, but you need a variety of tools Eg to lever the alternator out, and I took my time cleaning and checking things like the air flow meter, connections and wiring along the way. I also cleaned and silicone greased all the alternator / starter / 12v post connections.

The only new tip I can add is that hitting the bush in the alternator is never going to budge it. You've got to lever and jiggle the alternator out. But then on the bench you can use a threaded bar or bolt, washers and sockets to press it back (in my case with lots of plusgas and copper grease as well):



Then in my best Haynes manual voice,

"refitting is the reversal of removal".

Job done. 14v all the way.

nebpor

3,753 posts

234 months

Friday 20th April 2018
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pete said:
It does sound like the alternator voltage regulator, a common 996 failure. You get the full 14v when cold, then as it warms up the output falls off. I had exactly what you describe, and drove hundreds of miles with no major issues, but it did start to get gradually worse until I caved in and bought a new regulator. I also didn't get any charge warning lights, but it was starting to worry me, especially when sitting in traffic with lots of heat soak and the cooling fans on full blast.

A replacement is about £40 from Woods for the correct Bosch part, and a few hours' work to fit - you have to remove the alternator to get to it.

Pete

Edited by pete on Saturday 10th October 22:48
I had exactly this - 14 when cold, then over the course of a drive it steadily drops to about 13.5 or just under.

Measured all the voltages (at battery, at engine etc) and all within spec.

Made it through a long cold winter of short drives without ever going flat, but decided to change the voltage regulator today for a new Bosch part.

Made zero difference, still the same behaviour! I'll suck it up, given it took a beating and came out of winter unscathed ....

pete

1,586 posts

283 months

Friday 20th April 2018
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Holy thread resurrection, batman! ;-)

When I had this problem it would drop to well under 12v when warm, hence my nervousness. Now it does exactly as you describe, which I think is completely normal.