Voltage Fluctuation

[scam75]

Hello all

I've had this issue the whole time I've owned the car. Had umpteen alternators and batteries and it never changes. It's not the end of the world but I'm thinking something is not quite right. I get about 13.8 volts across the battery on start up. Gauge in dash agrees so pretty sure it works ok. The voltage will drop when I switch things on, to the extent of being barely above 12 when lights, full ac and demister is on. The gauge pings back up when these things switched off, but not always back to 13.8v. It's not uncommon for it to only climb back to 13v for example after switching things off, sometimes it does go back up to 13.8v. Also see a small dip when brake pedal applied.

So I've changed alternators, second hand, refurb, new, no difference. Different batteries make no odds. Earth strap at battery is sound. Main earth in engine bay is sound. Connections to alternator are sound. I can't help thinking it must be earth related but if so, where else should I check? Also it is advisable to grease the battery terminals or spray some de-oxit or the likes on the battery connectors? As I say it's no massive deal but I don't think I should be seeing only 12v at idle with all my things switched on?? I understand some fluctuation is normal but I think I have too much?

Any thoughts welcome.

Cheers

Stuart

 

[Cater_Racer]

I'm not sure what you expect Stuart, when the engine is running without further loading the electrics it's 13.8v when you load beyond the circuit it drops to 12v (indicating lights etc are drawing probably more than the alternate itself can supply hence draining the battery) and immediately afterwards it goes t 13.00 (indicating it's charging the battery back up).

Whats the problem? I would think your observations are perfectly normal.....

 

[scam75]

Well I tend to get about a year, year and a half, out a new battery before it starts getting puffed out. This has been 4 x Bosch S3 70AH. So I'm thinking although it's charging it's maybe not charging as well as it could? Not sure, hence why I ask. Just wasn't sure the battery should go into a state of not actually charging when I have a full suite of electrics switched on. Would have thought Porsche would have specced the alternator and battery to remain charging on a cold winter morning when you have all guns blazing? Hence why I thought a weak earth somewhere was maybe the cause but if this is normal then although slightly surprised I'll shall just accept it!

Stuart

 

[Eldavo]

The voltage dips when you push the brake pedal because the brake lights come on

 

[bmnelsc]

My S2 behaves much as you describe although I often don't get 13.8 either - usually will for a short time when running after its sat for a week or so in the summer months. I have my "alternate" headlights always on as Daytime Running Lights as required in Canada so I'm always drawing against the charge rate. Running with AC and headlights brings it down to 12ish volts. During winter its parked with a maintainer on it. I got the car in 2008 with a marginal battery (Bosch) which died the next year (I had no maintainer then so parked it with the battery disconnected but it didn't like sitting for extended periods). A new Interstate battery was installed in 09 and I've used a maintainer since during winter "sleep time" and just let it live on the alternator during the summer usage period. I had one instance of low battery this year where the OPC did not tighten the positive terminal after a belt change but I cleaned and tightened the post and charged the battery and all seems fine again. Battery electrolyte levels are right up and I've never had to add. So I think you are seeing "normal" behavior for our cars and I wouldn't take the gauge readings as gospel either...e.g. my speedometer is under 10% with correct tires which is interesting - close enough at town speeds but I have to "adjust" for highway motoring

 

[AndrewCS]

Clutching a straws ... []

At idle ... do the engine revs reduce / recover when you switch things on ?
Is there a voltage regulator unit in the system, other than that in the alternator ?
Did the issue arise post the modifications ?
Voltage dipping when braking will relate to the engine braking, does it ever stall under braking ?

 

[Geoff_Fields]

Question: Do 944's have freewheel clutch drive pulleys on the alternator?
If so when the alternator load increases, maybe the pulley slips and effectively undercharges. But when the load is removed then stops slipping and returns to normal charging.
Long shot I know.

 

[scam75]

Clutching a straws ... []

At idle ... do the engine revs reduce / recover when you switch things on ? No
Is there a voltage regulator unit in the system, other than that in the alternator ? Don't think so
Did the issue arise post the modifications ? No
Voltage dipping when braking will relate to the engine braking, does it ever stall under braking ? No

Answers above

Thanks

 

[vitesse]

I always use petroleum jelly (vaseline ) on both battery terminals after cleaning them

Unusual voltage drops are down to unusual resistance(s) somewhere-all things being equal -even terminal connections -but your quoted figures don't seem excessive.have you checked the wiring connections on the ammeter itself?

Otherwise you would need to use a suitable multimeter & systematically go through all the Consumers 1 by 1 as each is switched on & off--AC compressor motors have a different load characteristic when switched on to when switched off until the engine stabilises .

 

[scam75]

Thanks Vitesse. I'll have a go at cleaning the connectors to the battery and applying some Vaseline and see how we go!

Stuart

 

[edh]

944 Dash guages aren't always accurate...my oil pressure used to drop slightly with high boost readings. Conclusion was just that age had taken it's toll on the dash electrical tracks & wiring

How old is the alternator? - should be quite cheap to get a bosch agent to inspect/refurb it as long as it's not completely knackered.

 

[zcacogp]

I'd second what Ed said; don't rely on the gauges as they aren't accurate. Apply a digital voltmeter across the battery terminals to find a true reading.

Oli.

 

[brummie]

When I acquired my S2, it had a new battery from the supplier but it did not read a high enough figure on the dash. I confirmed that the battery was fully charged by testing the open circuit voltage ( ie engine off and all things switched off ) , should be about 13.5 volts. I then took out the dash instrument cluster and set about " cleaning the RUBY BALLS as per the article in Clarks Garage. This cured the low reading and also gave steadier readings.These connections on the back of the PCB are now 25 years old and benefit from some TLC to restore accuracy. If you decide to clean the connections I strongly recommend only using a contact cleaning glass fibre pencil.

 

[scam75]

Thanks guys. Alternator is fine, had a few and always same thing happens. Rugby balls already cleaned and dash gauge, although not 100% accurate has been calibrated to match battery and seems pretty accurate compared to meter readings now.

Stuart

 

[scam75]

The main issue is not really the dash readings. It's the fact I'm consistently getting about 18 months from a brand new battery which makes me think although charging it's not charging 100% as it should. I must have swapped the alternator 6 times now chasing a separate fault and this behaviour is always constant which allows me to rule out the alternator.

Stuart

 

[Waylander]

Check the main power leads from the alternator to starter then if it's the same as the 968 the cables all the way back to the battery, these cables are old now and maybe starting to break down causing a high resistance,

there are two more earth points for the dash one each side check and clean those as well,

 

[thewildblue]

I had a simliar issue. My car would be fine then a flash of the lights and the volt gauge would drop drastically, stereo would reset etc. It would be intermittant, some journeys would be fine others it would do it all the time...even so much as on the ABS self test it would try to cut out. I check all my earths and in the end sent off the alternator for a rebuild. They stripped it apart and found it full of corrosion, replaced the VR, brushes and installed a new bearing. I refitted on Monday this week....and wow what a difference...lights are brighter and the car runs better and idles evenly now as well.

 

[scam75]

Cheers again chaps.

 

[scam75]

So a small update. Although the earth strap at battery looked good, I took it off and cleaned it up and applied some De-Oxit. Also took off the battery terminals for a clean up and also applied De-Oxit. This morning I can report 13.9 volts on start up and notably smaller fluctuations on voltage, for example applying brake pedal now notches the dial down about 1mm as opposed to maybe 2/3 mm before. I also now have the car still charging with lights, full a/c, demister and radio on. Showing about 12.4v as opposed to a flat 12v before at idle. So I'm on to something I think. One nagging concern though was that after turning everything off again she only returned to 13.5v despite showing 13.9v before switching everything on. However this again is improved as before it could sometimes only return to 13v. Also noticed this morning on way to work, with fan on 4, voltage stayed constant around 13.8v whereas previous this could end up around 13v after driving for half an hour. So although progress made still not sure why voltage sometimes only returns to 13.5v when everything switched off. Can only be resistance in the main cable? Anyway will continue to monitor. I think I'll De-Oxit main earth under bonnet next and see how that goes.

Stuart

 

[scam75]

So as the De-Oxit continues to do its job there is now no discernable drop in voltage from the brake pedal and I'm maintaining 14v whilst driving with only fan on at position 3. Oxidisation of battery connections may well be the culprit here.

Stuart