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Old 11-15-2017, 02:35 PM
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andrewwynn andrewwynn is offline
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Location: Racine, WI
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The test for the alternator is free at an auto parts store; (but you need to remove it). You can test the running voltage with the hidden test 9 on the dash. if it dips below 13V ever it's having problems. If you had an oil leak that got into the alternator and it's original or over 100k miles I would bet that is the problem. If it's not terribly old you can refurb yourself pretty easily; the regulator comes off pretty easy and not terribly difficult to re-install (just a bit tricky to push in the brushes to get over the slip rings). You can clean the slip rings and de-grease the brushes/regulator etc. If the slip rings aren't terribly worn and it's just a matter of too much oil, simply cleaning them will get you back to perfectly functional.

The slip rings only carry enough current to power the magnetic field, they couldn't push 1/ 20th the current of the actual output, there is a special burnishing tool for properly cleaning slip rings you can find it searching 'contact burnishing tool' but fine sand paper will work, you just don't want to create grooves that the brushes need to smooth off it will take 1000s of miles off the lifespan.

If you slip rings are shot and/or the brushes are shot, you can buy a kit for $20 that includes both. It took me maybe 90 minutes to disassemble, clean in my kitchen sink using dish soap in a spray bottle and rinsing, re-build the slip rings and re-assemble. Heat was my friend to remove the old slip rings to release the potted connections. The YouTube video i followed chiseled it out but with some heat from a micro torch it released the whole potted chunk in one piece.

The regulator can be bought as a kit as well and will have new brushes included, saving some work and if the slip rings are ok but brushes are worn that will likely fix your problem.

First get the voltage measurements while running; if they fluctuate a lot and especially can't stay above 13v it's the alternator that's the problem. Make sure your output nut is tight; i forgot to tighten mine on re-assembly and it fell off giving me all sorts of fits including transmission safe mode.

If the voltage dips below about 12v while running you should see secondary failures like DSC/brake (yellow) failure.

A properly operating alternator won't vary more than about 0.4v ever. 13.4 to 13.8 volts for example. from idle to redline and it shouldn't take long to stabilize either. if the voltage wanders or dips/spikes the regulator is not doing its job. The primary reason will be dirty/worn brush/slipring but some other parts can fail in the regulator which likely can be determined with the actual bench test (e.g. regulator or stator the cause).

FYI; many don't realize but the way that alternators can output constant voltage independent of RPM is that they take some of the output power, feed that back into the armature through the slip rings; the regulator monitors the output voltage and changes the current instantaneously going into the armature to boost or lower the output. Since the alternator is not outputting any power until it's spinning it gets a little help from the battery to get voltage/current to the armature until it can self-power and get the regulator operating. To eliminate the high-current path from the commutator the power is generated in the non-moving stator as 3-phase AC, that 3-Phase AC goes into a diode-bank of six diodes to convert the output to bumpy DC that is smoothed by being connected to a ginormous battery. If one of the diodes breaks you'll get erratic output. if the output will maintain at higher revs but not at lower revs it's almost always the slip-rings and brushes at fault (it takes less current at higher RPM to maintain voltage, at low RPM when higher current is needed, the brushes can't push enough current at only 14V so output falls).

The feedback loop for the regulator is completely internal you can't hook up anything to test it it works or doesn't, but you can replace the whole part. There are no moving parts in the power generating part of the alternator so it makes them crazy reliable.

I was very happy to have the Valeo brand in my x5 because that was the one i could find the $20 kit to refurbish the slip rings and brushes. The bosch i didn't see the slip ring kit and the regulators were more expensive also. The guy at Advance Auto was pretty blown away by my 150,000mile alternator testing like new.

It's so easy to remove/replace the regulator if it was my car i would take the alternator back out, remove the regulator and clean the brushes/slip rings. If they aren't terribly worn, re-assemble and take to Advance Auto to test before re-assembly. I would first determine if voltage is staying at 13.6 ± 0.4, if it is then it's doing it's job and the problem is elsewhere.

The previous post is perfectly detailed how to determine if the battery is properly holding a charge. Most people will not have a way to pull half the current out of a battery for 5 minutes so they won't be able to complete the test shown above, however you can take the car to a battery store for an under load test. They may be able to do the test shown (half load test after removing surface charge), they usually just do a full load test, but you can probably ask for a proper half load test like described above.
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