Battery light/ Voltmeter strange fault

Slimbo9 · 28-08-16, 10:50 PM

Driving home the other day, 70mph, my 1994 Audi Coupe 2.0 16v.
I got a sudden judder, and the ABS light came on, for a second, then back to normal.
A few minutes later, the same happened, except this time the speedo dropped to zero, and then back to normal.
I noticed the voltmeter was reading 12v, but wavered to 8/9 then back up to 12v.

I have just replaced the Alternator, after the old one failed- in a nice normal fashion. The replacement was an exchange unit, I also noticed at the time, the voltmeter was only showing 13v, instead of 14v when running.

With the engine off, turning on the ignition to pos1. The battery light pulses dim to bright, and the voltmeter shows 8/9v.

I put a meter across the battery, and from the alternator to ground (engine running), and both check out fine, at 12.4v and 14.2v respectively.

Do any of you guys know what might be going on?

chalkyh · 29-08-16, 12:09 PM

Do a volt drop between the alternator + and the battery +, anything greater that a 0.5 volt reading on the meter is a concern - there's a resistance in the wire. Also battery - to engine block, same test; but checking continuity of the main batt - to engine earth strap. Anything greater than 0.5 volt on the meter again is showing a resistance in that ground strap. Also try engine block to alternator body, again looking for less than 0.5 volts on the meter. That could be paint between the alternator body and the fixing brackets.

You're missing near 2v on the system somewhere between the alternator (14.2v) and battery (12.4v), both should be about the same. Remember that can be on the + cabling side or the ground side! A battery ideally sits at 12.7 volts or so static and fully charged (without a surface charge) and needs another 1.5v or so to successfully charge.

Slimbo9 · 29-08-16, 06:41 PM

Ok.... I did some tests, and didn't find anything conclusive. But I will try again tomorrow.

I tried starting her, and she normally fires first time. This time she kept turning over, and twice the whole system died completely, with a jolt - possibly the starter motor??

After about four tries- she fired, and ran smoothly- with the voltmeter wavering around 14V this time.

No idea what is going on, my thoughts are maybe an intermittent short??

Thanks again.

29-08-16, 12:09 PM	#2
chalkyh Grown up member Classic Audi Club Member Join Date: Feb 2012 Posts: 116	Do a volt drop between the alternator + and the battery +, anything greater that a 0.5 volt reading on the meter is a concern - there's a resistance in the wire. Also battery - to engine block, same test; but checking continuity of the main batt - to engine earth strap. Anything greater than 0.5 volt on the meter again is showing a resistance in that ground strap. Also try engine block to alternator body, again looking for less than 0.5 volts on the meter. That could be paint between the alternator body and the fixing brackets. You're missing near 2v on the system somewhere between the alternator (14.2v) and battery (12.4v), both should be about the same. Remember that can be on the + cabling side or the ground side! A battery ideally sits at 12.7 volts or so static and fully charged (without a surface charge) and needs another 1.5v or so to successfully charge. Last edited by chalkyh; 29-08-16 at 12:11 PM. Reason: additiona info

29-08-16, 06:41 PM	#3
Slimbo9 Newbie! Join Date: Aug 2016 Posts: 4	Ok.... I did some tests, and didn't find anything conclusive. But I will try again tomorrow. I tried starting her, and she normally fires first time. This time she kept turning over, and twice the whole system died completely, with a jolt - possibly the starter motor?? After about four tries- she fired, and ran smoothly- with the voltmeter wavering around 14V this time. No idea what is going on, my thoughts are maybe an intermittent short?? Thanks again.