Continuing E30 Charging problems

Review: Car runs until battery goes dead, when jumped starts and runs again... for a while. Battery tests bad and is replaced, alternator tests good. Problem remains.
Just got the alternator back in the car and got some volt readings. Before starting battery reads 12.62v, after starting battery reads 12.3v. Same voltage at the alternator as at the battery regardless of whether or not the car is running. The field wire (small wire on the alternator... don't know if that;s the proper term) read about 1v less than the battery. System voltage slowly declines as the car runs, so it obviously isn't charging. Now, I would still think that I'm looking at an alternator replacment, even though it bench tested as good, as I have had that happen before, but was wondering if there is something else in the system to check first. It's the Bosch alternator w/ internal regulator that showed good in the testing. Thanks to all.
Matt
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Tough one. Sounds like something is different when the alternator is in the car so it would probably be a good idea to check the ground path resistance from the alternator to the battery. Also does the alternator light on the dash behave normally?? It is in the current path from the battery voltage to the field winding so if it's only dropping 1 volt with the key on and the engine not running then it's probably not illuminated which would be consistant with the bad ground theory. I checked the resistance on my car (91 318is) from the alternator body to the battery ground cable end that's just aft of the front right hand shock tower (where the battery would normally be on an e30 318i). I got 0.3 ohms.

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I read 0.2 ohm from the alt body to that ground, which was the same reading I get with the probes touched together. Thanks
Matt
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Many DVMs ain't that accurate at low ohms. A better way is to measure the voltage drop across the lead under load. Switch on headlamps and rear window etc with the engine stopped. Should be less than about 0.2V
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The alternator should be charging the battery at over 13 volts when running at idle, according to my experiences. Even if you load it up by running the heated window and lights it shouldn't stray much below that if it were healthy. I would suggest then that your alternator is definitely no good, whether that be the diodes/regulator/something else I wouldn't like to guess. When mine went I experienced similar issues, and changed the whole alternator for a second hand (but known to be good) unit and all has been well since. I should add that I also had to replace the battery at the same time because the drain of running the car with a bad alternator killed one of the cells, but it was at least a few years old anyway so hopefully you won't find that necessary.
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When you say "BOSCH alternator with an internal regulator" do you mean it has the plastic regulaor that bolts onto the back with two 10mm nuts?
If so that part is cheap and very easy to replace. It's about $18 or so.
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But how do I know that this is the problem and not the alternator itself? I suppose the point is that 18<200.
Thanks Matt
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You don't. But, it's stupid cheap, easy to do, the part that usually weirds out and/or fails and you can use it on your next alternator if it's not the problem.
Modulo some weird-ass wiring problem I'd guess this was the root of all your evils.
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RR News wrote:

Is that a typo? I ask because after starting, it should read about 13.2V. If not, and it's really 12.3V, there's definitely a problem with the charging system and you have narrowed down the problem.
Ed
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Nope, no typo, the voltage was just slightly lower after starting the car than it was off... I guess it looks like alternator replacement time...
Matt
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No, it's regulator replacment time. The alrternaor is just wire, magnets and bearings, it's not screaming like a banshee or making zero volts it's just the (cheap, easy to replace) regulator. If you have a 10mm wrench you have all the tools and expertiase you need.
I've seen guys pay $700 for a new alternator installed at the deaeler for want of $2 worth of brushes. Sick.
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wrote:

True enough, I'll start with that little gem. Thanks
Matt
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Let us know how it turns out, Matt. I'm still wondering why your alternator tests OK out of the car but doesn't work when it's installed.
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I will... I have a voltage reg on the way.
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