Volts & Charging

[Fat Albert]

I have done a search and can't find any references to the following issue on the forum:
Normally my Voltmeter shows just under 14V and never deviates.

Last week I had to do a bit of shopping and stopped/started the car 3 times in quick succession. Driving home the car felt very blunt, mis-fired when i floored it and the Volt meter was showing just over 10 volts. There were no other warning lights.
The following morning it was back to normal.
The next afternoon the Volt Meter was showing just over 11 volts, but the car drove normally
Saturday Morning fine
Saturday Afternoon showing just over 11 volts, and the car drove normally
Sunday fine
[&:]
Woss goin' on?

It seems to be if the car has sat in hot weather it shows a lower voltage. I haven't had the chance to get a multimeter on it, but when I checked the oil yesterday I looked for the Alternator and couldn't find it! (I will have to peel open the pages of my Haynes manual!)

Any Ideas?

Edited to say that this behaviour has co-incided with my alarm plipper running out of battery, so I am having to lock/unlock the car manually and use the immobiliser key in the slot before starting, so unless the alarm/immobiliser is doing something odd, I shouldn't think it would have any effect.....would it?

 

[zcacogp]

Fattie,

14v is about as much as you will ever see on the dash gauge - mine read 14v when I fitted a new alternator last summer, but has dropped a little since. Anything over about 12.5v is OK in general, but be warned that the meter on the dashboard is not the most accurate - if in doubt then open the boot and connect a DVM across the battery.

It does sound like the poor running could have been associated with lack of volts. Why did it happen? Loose fanbelt, perhaps? Did you have anything else on - lights/heated rear window (in summer?)/something else that gobbled juice?


Oli.

 

[Fat Albert]

Thanks Oli, the only thing I would have turned on on those days would be the air-con.
What I'll do the next time I am out and the Voltmeter looks OK is to put the aircon on and see if I then get a drop.
When the issues happened I turned everything off that I could but it seemed to make no difference. I will also have a poke around with my Multimeter

 

[shammy1971]

14 volts is the optimal voltage rate to charge your battery - this is controlled by a voltage regulator in the alternator. So this is what everyone should see, 12.5 volts will not fully charge a battery which has 13.2 volts fully charged. Seeing as you are getting a varying rate of voltage this would suggest a bad connection between the alternator and the battery - most likely to be a bad earth.

 

[944 man]

Possibly a poor earth.

 

[Neil Haughey]

On mine it was the point on the starter where 3 different wires all bond together. I had very eractic volt readings and poor starting. Turned out the nut had come loose and one of the wires was literally flopping about. Very heavily corroded as well. A good clean up with wet N dry sand paper then coat with vaseline cured my problems. I also found that I tended to see less then 14v on the dash for many years. It slowly got lower over about a year then went crazy and resulted in the battery not being charged so I replaced the regulator (about 20 quid from local autofactors) and was back to a steady 14v on the dash again.

 

[sawood12]

Are you sure you have no electrical items on when you are seeing the lower voltage? Though your volt meter will read the highest when everything if off, it will drop everytime you switch on an electrical device. Ideally you should still be reading over 12v if every electrical thing is on. However i've found that generally the voltmeters are not very accurate and should be taken as a pinch of salt. The best thing to do is to get a multi meter across the battery terminals and read what you're getting - 12v with the engine off and between 13 - 14v with the engine on at idle with all the electical devices off. Mine reads about 12.5v on the gauge at idle, but 13.5v across the battery terminals. And with all the electrics on the gauge will read about 10v but it will be about 12ish v across the terminals.

However votage is only one parameter that can indicate battery health. Another test is to show how 'strong' your battery is. i.e. what it's capacity to deliver high amps for a sustained period of time. This test is done by a tool that basically short circuits your +ve and -ve terminals and reads the ampere hours (I think that's the units) the battery is delivering. It is held on the terminals for about 5 seconds and it should deliver the spec'd amps consistently over that time. If it reduces over that time period or struggles to reach the necessary level then the battery could be cream crackered.

By the way you'll struggle to see the alternator from above on a 951. It is down the right hand side at the bottom, under the A/C compressor and all the air intake ducting, AFM and all that. You'll see it from underneath once the undertrays are removed. One thing to check is that the conical shroud screwed to the rear of the alternator with the flexible hose attached to it is intact. This delivers cooling air directly to the alternator as being situated where it is - under the turbo and the A/C compressor - it gets hot and its life will be drastically reduced if it is not being fed with this cooling air. Unless you or a previous owner have replaced the flexible hose I can guarantee that by now it will be cream crackered. I replaced mine about 3 years ago as it was just falling apart.

 

[Fat Albert]

Thanks for your answers chaps, I will have a poke around with my multimeter at the weekend and try and locate the Alternator to look at the shroud.

Yesterday I took my Son's Mate home (30 mins each way) and on the way there it was showing 13.5 V, but when I started up again to come home it was showing about 12.2V initially and then climbed to 12.7V over then next 2-3 mins. It stayed on that then all the way home. There was no difference in electrical load when I stopped and started the car.
I started the Air-con as I got near home and there was no change, so that eliminates that from the equation.

I get the feeling it is temperature related, so the Voltage Regulator and the cooling shroud sound plausible explanations