Faulty battery or alternator ?

[8690q]

I left my lights on overnight last week and came out in the morning to a car that wouldn't start, first for me

I charged the fairly new battery and refitted it and all seemed well.

Car is showing 12v when ignition is on but only max of 13v when engine is running and this drops below 13v when anything electrical is switched on. It's also having a wee cough and fart to itself when the volts drop.

Car is a 1995 80 quattro avant with a 7a engine fitted.

How would I test to see if it's battery or alternator at fault ?

Thanks

Michael

[driftin jo]

you should be getting 13.8 volts from the alternator.

[diesel des]

Yep using a proper multimeter you should have 13.8 to 14.4 volts at the battery with engine running and everything off. Any less and alternator or connections could be bad. Check voltage at alternator too.

Sent from my GT-N7105 using Tapatalk 2

[8690q]

I have the available funds so I'm going to stick a new alternator on.

I've found an alternator http://www.mister-auto.co.uk/en/alte...021436303.html

Is this the right alternator and his anyone ever ordered from this company before ?

[BrianSmith_S2]

Before you replace the whole alternator it might be worth you replacing the carbon brushes first as they only cost about £15 and take less than 5 minutes to replace

[caterpillar]

you should be getting 13.8 volts from the alternator. WRONG... it should be 14,5 volt... change the regulator/buschkit... and all be ok...( im serious abot the volt)

[8690q]

New alternator fitted and it doesn't seem much better voltage wise. Ticking over like a new car now though.

I'm only seeing 13.83 at best across the battery posts with the car idling, drops if revved or anything electrical used and doesn't come back up. Should be seeing higher ?

Battery failing ?

I had to swap a wee thing over from old alternator to new one. Spade connector with wire from black box which screws onto alternator body. I swapped a couple of these and same results every time ?

I've had a look about and can't find an earth strap in engine bay going to bodywork ? Should there be and where about ?

[rynaqui]

I know there's an earth strap round the turbo side by the head somewhere, can't remember where, coil related earth strap that one was. If you turn off anything electric, voltage should go back up to about 13.8 in a few seconds or so.

[8690q]

I found the earth straps I think, down where the battery used to live in v6 form (behind passenger headlight. My battery is in the boot now).

[8690q]

New battery and alternator fitted and it's still the same Wiring between battery and alternator have been checked.

Most noticeable at full temp when the fan kicks in the idle has a wobbly.

Not sure if it's a problem with the fan or some other full temp related fault.

No fault codes, that'd be too easy.

[johne]

drops if revved or anything electrical used and doesn't come back up. Should be seeing higher ?

I believe you've got a poor connection between the alternator and the battery, most probably in the earth line.

As a check, use a good voltmeter and a long length of wire on one of the leads to connect the voltmeter between the alternator body and the battery -ve post. You should see zero volts or close to. Anything higher than zero or a voltage that goes up as you put electrical loads on means you've got a poor earth.

John

[5cylinderman]

Grab your jump leads, connect them in series and make a temporary connection between the alternator body and the battery negative terminal. Check your voltage then. That will rule out bad earths on the body shell.

[8690q]

Thanks guys,

I've done that and readings were the same.

I'll try it again just be sure as I might have missed something.

[8690q]

I've been back at it tonight.
I cleaned and checked all earths again, measured the alternator output as 13.85 ?, put my multimeter between the jumpleads running from battery to alternator and got varying readouts but highest was during starting then dropped to a stable 0.50v once idling.

Excuse my stupidity but testing the earth points under load ? Where would you put the multimeter leads ? Alternator casing or charge post ? other lead to starter etc with electrics switched on ?

[ManiacuV6]

So you need to measure the output from the alternator ( the M8 bolt and the casing for ground) and after that you measure o n the battery terminals . The difference should be maximum 0.3-0.5 V . Second check is with the high beam on and engine at 2000 rpm and the same 2 checkings ( on alternator and battery) .