From: CAPTEARLG@aol.com Sent: Friday, February 18, 2000 8:53 AM To: 928 Subject: [928] Alternators and Brains Alternators should be considered a maintenance item. Alternator brushes and slip rings wear out and must be replaced. On older American alternators I usually replace the brushes at 100,000 miles for a few dollars and get another 100,000 miles out of the alternator. On Bosch alternators the slip rings wear out with the brushes and must be replaced also. This is not an easy DIY project. The least expensive way to have this maintenance done is to remove it and take it to your local auto electrical shop and have it rebuilt. The brushes and regulator on the Bosch alternators are one assembly and must be replaced together, and the slip rings must be replaced also, so it is more expensive to rebuild than other alternators. My local shop charged me $110 for my '88. The frequency for this maintenance is approximately 100,000 miles. I don't know if the amount of air conditioning use affects this figure but if you live in a hot climate maybe you should do it at 80,000 miles. One of the symptoms of worn brushes and slip rings are "voltage spikes" that destroy the regulator. This sounds OK since the regulator is replaced during the rebuild anyway, but are these "voltage spikes" responsible for some of our fuel injection "brain" failures? Rebuilding the alternator may be good preventative maintenance for the fuel injection "brain" also. It may be good for your sanity as well, since it will keep you from being stranded with a dead battery when the brushes wear out. I don't have experience with the earlier Paris Rohne alternators but I would think that the 100,000 mile frequency would hold true. YMMV. Earl Gillstrom '88 S4 5 Speed -----Original Message----- Date: Thu, 12 Sep 2002 05:12:09 +0000 From: "Harvey Hughett" Subject: Alternator Puzzle I'm pretty bummed out. Alternator quit on my 1985 928S and I bought a new (rebuilt) one and installed it. It worked great for about 8 minutes...then it quit too and all the dash warning lights came on! The Alt gauge shows no activity now. Other than this it still runs fine but I have a problem. I'm not aware of any shorts, haven't unhooked battery cable while it was running, etc. It's a new battery and fully charged. Only thing that's questionable is that I had a fuel pump relay quit earlier, replaced it and a couple of others (for igition and jetronic). That's all that's new. Maybe I jiggled something in the fuse panel? Is the alternator fused? Could the alternator be working and just my dash warning lights gone crazy? If there is a short somewhere, would it have burned out the complete alternator or just part of it? Where should I look for a short? The alternator is not hot to the touch. I've checked the battery and engine grounding cables to make sure they're clean and snug. Or is there something else that I'm overlooking? There are no mechanics close by so I'm really kind of stuck. If I were to drive it on the battery power only...how far could I get? (qualified mechanic is 100 miles away). Would I be OK to drive it with the alternator not working? It concerns me that all the dash warning lights are on. It was not this way the first time the alternator quit. Your ideas will be much appreciated. Thank you, Harvey 1985 928S Guards Red From: Dan [mailto:dbrindle@kondratyev.com] Sent: Friday, September 13, 2002 1:19 AM To: 928 Subject: [928] RE: Alternator Puzzle Harvey, The quickest way I know of to fry an alternator is to lift the field. This drives the alternator into a self feed back loop causing it to over charge and burn up the diodes. The field is wired through the alternator light on the gauge cluster. In parallel with the alternator light is a resistor that prevents the field from opening when the light bulb burns out. When this happens the alternator will still charge but you have to rev the engine to 3000 RPMs first to get it to kick in. This does not sound like your problem, but still something to check. The harness to the alternator runs across the front of the engine. Oil, heat and vibration can easily wear down the insulation on this harness and cause alternator problems. Sadly if the field is open then you have probably fried the new alternator. Hope this helps. Dan the Pod Guy Portia's Parts -----Original Message----- From: Wally Plumley [mailto:wplumley@bellsouth.net] Sent: Friday, June 13, 2003 8:57 AM To: 928 Subject: [928] Re: Alternator woes At 11:23 PM 6/12/2003, Bill Shaw wrote: >I'm on my second alternator since BeaveRun. The first rebuilt ($220 local >auto parts store) self-destructed after about 15 minutes. The replacement >doesn't start charging until I rev it over 4000 rpm. Turn the key on and it >shows about 12V, start it and it shows about 11v, rev it to 4000 and it >starts charging, 13+ volts and stable until I shut it off again. Fiddled >with the connections in the pod and on the alt, all seems OK, no change in >behavior. Wierd, definately RPM related, not time or temp. Anyone seen >this before? Regulator problem? An alternator converts rotary energy into electrical energy by moving a magnetic field (the armature or rotor) thru the coils in the housing. The magnetic field MUST be present for the alternator to generate electricity. This magnetic field in the rotor is initially created by the excitation current that is supplied thru the charging warning light and the parallel resistor. If the excitation current is too weak (bad resistor, bad bulb, bad connection), but present, the alternator will START charging only at higher-than-normal RPM levels. If the excitation current is not present at all, the alternator will never charge. Once the alternator starts charging, it supplies its own field current. Since there is now voltage on both sides of the light/resistor combo, the light goes off. Once the alternator starts charging, the light/resistor combo has no effect. Wally Plumley 928 Specialists