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Low volt reading after start-up

Alex

Well-known member
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6 Mar 2014
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22,079
Noticed a few times recently that after a cold (morning) start-up, my volt meter is reading low, as though there's no juice coming from the alternator. After around 30 seconds, it springs into life and jumps up to the normal 14ish volts.

Convinced it's not the belt slipping or anything to do with the alternator not spinning. Do think though it's something to do with moisture on the armature or something similar.

Any ideas members?

Caught it (not) in action this morning setting off to work.

 
If you have a multimeter (even a cheapo one will do) then leave it in the car.

Connect it to the cigarette lighter socket just before you start it. When it shows low on the needle see what the multimeter says.

Then you can compare actual volts with the needle.
 
Thanks for the comments guys. Will keep my eye on it and try the multimeter in the fag lighter trick.
 
This is what's currently laying my 996 up. New battery, new alternator, 13.6 at the posts when running but showing sub 12v on the instrument cluster. Disco warning lights and flickering gauges, and stalls on occasion as soon as the brake lights are illuminated, at which point it won't restart despite it having 12v+ at battery.

It seems charging issues such as this are common.
 
Try an alternator load test ..

Turn on all electrical items , main beam , fog lights etc etc and test the charge rate with a meter at the battery , if under 13 volts then the alternator is starting to fail , if over then it is ok .

This might be just the gauge in the cluster , they are only an indication really ( Porsche had a gap to fill in the cluster lol ) so not 100% accurate and if it wasnt charging then you should have had a battery light or warning message depending on the system your car has displayed .
 
Had this problem

Had this exact same issue a few times over the past 3-4 months. Did it most reliably after rain or on cold mornings. I also had a slight shriek noise on start up, like a belt slip.

Had it into Northway and they said it could be the alternator packing up. I specified a new belt and it has not done it for the past month since I collected the car.
 
Sounds like damp getting into the Voltage regulator/rectifier to me Alex, quickly drying off when it warms up. Not too bad a job to swap the regulator/rectifier by the looks of things on youtube :thumb:

Mine is straight to 14V after firing up :grin:
 
Re: Had this problem

coopertrooper2001 said:
Had this exact same issue a few times over the past 3-4 months. Did it most reliably after rain or on cold mornings. I also had a slight shriek noise on start up, like a belt slip.

Mine is doing this now.

startup- 10.5v, then after a few seconds, a shreik from the back and up to 14v.

Did anyone get to the bottom of this?

Going to remove the belt and check all pulleys spin.
 
Theres another post which not the same but has a similar fault which turned out to be the ignition switch ..

http://911uk.com/viewtopic.php?t=112115&postdays=0&postorder=asc&start=0

Not sure if its relevant here but it wasn,t really what i was expecting for his symptoms .

i think if theres a noticable screetch from the rear then the alternator is faulty as thats the belt slipping or alternator not turning when first started .

Worth takeing off the belt and checking the alternator spins freely as well as the other pullys .
 
Shane911 said:
Dirty earth strap :D

:floor:

I just read that post :D

Not sure about the misfire bit , coils i expect .. think im going to have to start checking these more often .. :?:
 
Just been to the car, but it went dark on me so I didn't remove the belt.

Engine started: 10.5v on the dash

ran to back of car opened lid

looking at the pulley it is spinning looks OK

then "shreik" and a puff of smoke from the alternator pulley location

dash now reading just under 14v

----strange

I was reading about the alternators (exciting) and manual 996 2000 on started coming with clutched freewheeling pulleys, as the belts were jumping off on fixed pulley manual cars with abrupt rev changes.

Now mines an MY99 so would have shipped with a fixed pulley but it may have had an alternator in 128k miles.

Could it just be the clutch in the pulley thats gone perhaps?

Belt looks fine, doesn't look like its been slipping.
 
Does sound as though the clutch in your alternator pulley is failing and slips a bit before sorting itself out.

I'd remove the drive belt and try turning it by hand. It should be locked and turn the alternator when you turn it in the direction of normal running rotation and then if you abruptly stop it the alternator shaft and internals will free wheel.

It's not an expensive part

http://www.ebay.co.uk/itm/INA-Repla...en:996&hash=item2cb6aa0dfd:g:XH4AAOSw-RFYsfEW

Could also be a worn out drive belt or failing tensioner, all easily inspectable while your in there testing the alt pulley.

Good luck

:thumb:
 

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