dodgy battery or alternator? 85' cq

andya · 28-05-14, 07:22 PM

Used my car saturday. Went to go out last night and the battery was as flat as a pancake. Charged it overnight. It started first time. Used a voltmeter - 12v off, 13 volt on, and when I rev it, the volts hover no more than 13v. Am I right in thinking the volts should increase with higher rpm? And if so, could it be a dodgy alternator?

Any thoughts would be appreciated.
Cheers
Andy

John. · 28-05-14, 07:35 PM

It should be closer to 14 volts when running but the revs shouldn't matter.
Are you handy with a meter? Volts drop test under load wouldn't be a bad idea.

BIGGYIN · 28-05-14, 07:44 PM

Engine running at 3000 rpm,all electrics off and volts should be between 13.5-14.5.

If not,could be voltage regulator?

andya · 28-05-14, 08:15 PM

Cheers for those tips everyone. Will be tomorrow evening now before I can have another go. Perhaps the persistent rain may have gone by then!!

Isimmo · 28-05-14, 10:32 PM

As Karl says, volts should be about 14.5V with the engine running, it'll only drop with electrical load, but the voltage regulator should prevent the volts changing as the revs change.

My guess is a failing alternator output or an earthing problem.

andya · 30-05-14, 08:41 PM

Well a bit odd. Charged the battery tuesday night and all is fine. Was beginning to think maybe I had left the radio on on the night the battery went flat. But just come back from a nights drive, and a few things I noticed. The volts display dropped to below 12v with the lights on - is that normal, never noticed that before, as soon as I put the indicators on, the volts drop down once again by 1 notch. Off they go back, switch my lights off, and its back up to 12v. Am I being paranoid? Ive dug out a spare alternator just in case

Isimmo · 30-05-14, 08:49 PM

Is the battery charging light faintly illuminating when the lights are on and you are indicating....? If it is, its an earthing issue, quite possibly off the starter motor area....

andya · 30-05-14, 08:52 PM

Quote:

Originally Posted by Isimmo

Is the battery charging light faintly illuminating when the lights are on and you are indicating....? If it is, its an earthing issue, quite possibly off the starter motor area....

Yes when the indicators are on, I can see the battery charging light faintly flickering

Isimmo · 30-05-14, 09:03 PM

Quote:

Originally Posted by andya

Yes when the indicators are on, I can see the battery charging light faintly flickering

Right, its an earthing issue... First job, check/clean all the earthing points for the heavy stuff... So think starter, alternator, engine gubbins etc....

andya · 30-05-14, 09:31 PM

Right, will get onto that tomorrow morning.

Cheers
Andy

28-05-14, 07:44 PM	#3
BIGGYIN 4 ring whore! Classic Audi Club Member Join Date: Mar 2009 Location: Merseyside Posts: 7,976	Bosch 75/90 A Engine running at 3000 rpm,all electrics off and volts should be between 13.5-14.5. If not,could be voltage regulator? __________________ q90 B2 Type 85 - q90 B2 Type 85-PROJECT-ABY 20vt KARL. Injection is nice but I'd rather be blown

28-05-14, 08:15 PM	#4
andya Senior Member Classic Audi Club Member Join Date: Aug 2010 Location: Hampshire Posts: 1,149	Cheers for those tips everyone. Will be tomorrow evening now before I can have another go. Perhaps the persistent rain may have gone by then!! __________________ 1985 Coupe Quattro 1986 Coupe 1.8 weber carb 1987 Coupe 2.2 GT

28-05-14, 10:32 PM	#5
Isimmo Super Moderator Classic Audi Club Member Join Date: Jul 2007 Location: Sissix-by-the-sea Posts: 15,218	As Karl says, volts should be about 14.5V with the engine running, it'll only drop with electrical load, but the voltage regulator should prevent the volts changing as the revs change. My guess is a failing alternator output or an earthing problem. __________________ 1985 WR quattro, 1985 20vT RallyRep, 1993 MTM ABY S2 Avant. 2010 S3 Sportback.

30-05-14, 08:41 PM	#6
andya Senior Member Classic Audi Club Member Join Date: Aug 2010 Location: Hampshire Posts: 1,149	Well a bit odd. Charged the battery tuesday night and all is fine. Was beginning to think maybe I had left the radio on on the night the battery went flat. But just come back from a nights drive, and a few things I noticed. The volts display dropped to below 12v with the lights on - is that normal, never noticed that before, as soon as I put the indicators on, the volts drop down once again by 1 notch. Off they go back, switch my lights off, and its back up to 12v. Am I being paranoid? Ive dug out a spare alternator just in case __________________ 1985 Coupe Quattro 1986 Coupe 1.8 weber carb 1987 Coupe 2.2 GT

30-05-14, 08:49 PM	#7
Isimmo Super Moderator Classic Audi Club Member Join Date: Jul 2007 Location: Sissix-by-the-sea Posts: 15,218	Is the battery charging light faintly illuminating when the lights are on and you are indicating....? If it is, its an earthing issue, quite possibly off the starter motor area.... __________________ 1985 WR quattro, 1985 20vT RallyRep, 1993 MTM ABY S2 Avant. 2010 S3 Sportback.

28-05-14, 07:35 PM	#2
John. 4 ring whore! Classic Audi Club Member Join Date: Mar 2011 Location: Scottish Borders. Posts: 7,520	It should be closer to 14 volts when running but the revs shouldn't matter. Are you handy with a meter? Volts drop test under load wouldn't be a bad idea.

30-05-14, 09:31 PM	#10
andya Senior Member Classic Audi Club Member Join Date: Aug 2010 Location: Hampshire Posts: 1,149	Right, will get onto that tomorrow morning. Cheers Andy __________________ 1985 Coupe Quattro 1986 Coupe 1.8 weber carb 1987 Coupe 2.2 GT