mattsimis

Battery light came on 2 days ago. There was some corrosion (as usual) on the junction box alternator to battery wiring. I cleaned the corrosion, replaced all the bolts and nuts and added a second heavy gauge cable running from the alternator to the junction.

No change.

Battery reads 11.59 when idling. Reading straight off alternator is similar.
Punching accelerator peddle battery light disappears and voltage increases (to 12.6 etc).
I have my DRL leds wired through this junction too and they were acting strange in the last few days, they have circuitry that detects when the car is running (voltage I guess) and they came on a couple of times in the dead of night (car off).

Im wondering if this suggests the Voltage Regulator on the Alternator was putting out odd volts?

silverd2

I think many just assume the regulator (relatively cheap...non-dealer) and easy to install...and very often the problem. Other than a regulator, the only repair I've ever had to do to an alternator was a bearing replacement...never a diode problem.

Bentley doesn't seem to have a specific test, but I found this link (below...not Audi specific). Page 2 explains a test to isolate the alternator output from the regulator...and other info on this type alternator/regulator in general.

http://www.bronek.org/sailing/zamind...leshooting.pdf

mattsimis

Alright, Ill try the alterantor regulator first too then.
Easy to replace.. so just pop off the plastic back on the alternator in-situ ?


FWIW to anyone else, the stock (assumed original) alternator on this 32v 1998 S8 is:

Bosch Part: 0123520003
VAG Part: 077903015f
Its the 150amp version.

silverd2

Haven't done it on mine, but looks pretty simple:

mattsimis

Hmm, its not possible to do in-situ, Alternator has to come out as the Oil Filter housing blocks lots of parts.

Annoyingly I mistook a torx fitting for a hex and sheared the bolt on the Belt Tensioner, meaning I cannot get the serpentine belt back on. Any alternative to taking the Rad, Condenser, Front end off to fix this?

silverd2

?? What exactly did you shear (or strip)? ...the head of a bolt or the connection to the motor...and which bolt? (see drawing). Or did you break it off? ...and which one?

Is the entire tensioner still on the motor, but you can't detension it for belt installation?

mattsimis

Lol from your reply it was clear I was doing something fundamentally incorrect. Based on past experience I was twisting the belt tensioner on bolt #28 in your image. This worked till it striped the inside.

However from your image I could see there is a 13mm nut on the tensioner and twisted on that instead.. which was a lot easier done.

Anyhow, all back together now. Sanded and cleaned every connector from the alternator to the battery (and I doubled up the cable as stock was corroded) and Im reading 14v at idle (vs 11.56). It was clear the old VRM was broken as the bushings were all the way at their lowest point of travel (ie not making contact anymore).

Im actually kinda thinking 14v at idle is kinda high, though it is only on the battery/alternator, power to lights etc is 13.5v.

silverd2

14V is good...well within the limit...won't overcharge or boil your battery.

Glad you got it workin right.

pocketchange

Voltage should be in the 14.5V range.

Turn on a few major items and check the voltage at the battery.
Make sure the battery is fully charged since alternators are not designed to charge dead batteries.

The regulator for the OE alternator is from Bosch BTW, don't try to find one as it's a wasted effort. A reputable rebuilder will install quality parts since they don't want to deal with returns and it's not positive for business to peddle junk electronics.

Anything more than $125 would get me looking for another rebuilder..

silverd2

14.5 volts might be tolerable, but totally unnecessary. I've never had a German alternator read that high. 13.5 volts is plenty strong for any battery and any load you can put on it.

My present D2 has never read higher than 13.8V...have had the car since 35K miles (now at 116K) and still has the original OEM battery put in the car Nov 2000....never a problem...and never a change in alternator output, with age.

Alternators are absolutely made to charge a battery, while simultaneous putting out enough for any reasonable power drain. An alternator puts out DC, just like a generator...just much more at low idle speeds. That's what the diodes are for...converting AC to DC.

13V+ put into a 12V battery will charge it up willingly...if the battery was low or "dead" due to a faulty alternator (or reg.), not a bad battery, a properly working alternator will charge a working battery in short order, regardless of the battery's starting voltage reading.