Alternator problem - Troubleshooting - VWDiesel.net The IDI, TDI, and mTDI source.

• Alternator problem by Powjetta on 12 Mar, 2007 20:30
• When I got this car more than a year ago it wouldn't charge so the first thing I did was put a new rebuilt alt in from Autozone, their expensive with the lifetime warranty.  It was working good until today when we had a heat wave and it got up to 96 F while I was running errands.  I have put the big passat fan that really draws current and that has worked good, the coolant doesn't get too hot.  But today as I was coming home the alt light came on and I had a burning smell at the alternator.  I have AC and between the AC and the cooling fan I think the alternator got cooked somehow.  I assume I got the 90 amp one and I see the passat has a 120 amp so I am thinking I should look for a 120 amp alt.  
  Heres my question - The light never went out, but on the way home (nice and warm with no current draws) my volt meter slowly crept up to 14 or 14.5 range.  When I stopped it would drop back down to 12 or so.  Did I cook a diode or brushes?  I know some guys here know more about these than me.  
  thanks,  Dave
• #1 by jtanguay on 12 Mar, 2007 20:55
• well if its making extra volts, i'd say its a regulator thats blown... could be diode, but they only limit the flow of electrons to one direction right?
  
  so you installed the dual passat fans? yea they will suck extra juice, but the 90 amp should be just fine.  The problem is that the alternator doesn't get enough cooling half the time... it does have the fan on the belt side, but not on the other side.  I've seen the 120 amp on ebay etc... probably not a bad upgrade!
• #2 by Doug on 13 Mar, 2007 06:02
• Low charge rate is indicative of a partial charge due to an open circuit diode in the rectifier or winding on the stator. Most automotive applications of that vintage are 3 phase alternators with a 6 diode full wave bridge rectifier. If it came with a guarantee take it back.
• #3 by jtanguay on 13 Mar, 2007 09:53
• Quote from: "Doug"

  Low charge rate is indicative of a partial charge due to an open circuit diode in the rectifier or winding on the stator. Most automotive applications of that vintage are 3 phase alternators with a 6 diode full wave bridge rectifier. If it came with a guarantee take it back.

  
  
  would that apply to his alternator problem though? making more volts & higher rpm's and at idle nearly none?  I've got the same problem, and it has to do with the belt slipping on the alternator pulley... makes a hell of a chirping sound when it slips and i gun it!!!  :lol:
• #4 by Doug on 13 Mar, 2007 11:06
• I am assuming too much that the belt slippage has been eliminated? He noted that there was a burnt electrical odour.
  
  JT, why don't you spring for the correct size metric belt and scuff the inside of the alternator pulley surfaces with an abrasive. That would cure your slippage problem.
• #5 by Powjetta on 13 Mar, 2007 14:41
• I took it to Autozone and it failed the bench test and they are getting me a new one (at least a different one) tomorrow.
  My belt wasn't slipping but I don't know symptoms of a partial diode failure, with the light staying on and the voltage behaving differently than before I am glad to get a new reman unit.  I am out the labor and time to replace it and I'd rather not do that the next time.  Are there some units better than others?  If I bought it from VW whats that get me?  Other than a lighter pocket.
  thanks guys, Dave
• #6 by Doug on 13 Mar, 2007 15:37
• I think that there are OEM rebuilders like Bosch that may produce a better product when you are buying from the big chain distributors. I wouldn't have any qualms about buying a rebuilt from a reputable local shop that does the work themselves. There is no doubt though that this stuff can fail at any time. Most electronics on the alternator are built to size, with a modest overcapacity. You know the old adage about saving a nickle on each unit adds up over the long run. It seems that everything about the mass produced automobile echoes that idea.
• #7 by Powjetta on 25 Mar, 2007 21:57
• My replacement alt went out today while I was running the AC.  The big two blade fan is rated 450, 250, 80Watt which is a little higher than the old single blade fan.  Any ideas if this is the problem?  It wasn't that hot and the old one seemed to last a lot longer.  I think I will go down to the local rebuilders and talk to them.
  Dave
• #8 by jtanguay on 26 Mar, 2007 04:33
• you mean the rad fan you put on is bigger? hmmm...
  
  i couldn't see that as being the main culprit for the failure...  the resistor will limit the amount of juice going to it... you didn't bypass the resistor did you??? that would make either the fan fail, or the alternator due to the constant load.
• #9 by Powjetta on 26 Mar, 2007 09:10
• My local rebuilder says they won't touch VW, they like the big industrial stuff.  I guess I need a new rebuilder.  
  I have not bypassed the resistor, but this fan is a definite load on the system.  I have a fan controller coming and will wire that into the system.
  At 450 watts that should only be 37 amps, and the AC clutch and blower fan shouldn't get to 90 amps.  I may have another short some where in the system or a bad alternator.