electrical and ignition Recipe for Troubleshooting a No-Charge (bad alternator? bad regulator? bad wiring?)

[thehackmechanic]

The "my alternator quit charging my battery" question comes up again and again. Someone just asked me about it on Facebook. I know there's already a very good article on it at http://www.bmw2002faq.com/articles.html/technical-articles/electrical-and-ignition/the-02-charging-system-r175/ but I thought I'd give my quickie recipe.

 

• Visually inspect the battery to B+ (the thick threaded post on the alternator) connection, clean and re-crimp as necessary.

 

• Visually inspect ground connections (battery to ground, battery to engine, alternator to engine), clean and re-crimp as necessary.

 

• If the car is still using the external regulator, visually inspect the three connections in the three-prong plug on the back of the alternator (D+, DF, D-) to make sure no wires are broken or frayed and that none of the terminals are pushing out of the back of the connector block. Crimp on new terminals if necessary. Do the same on the same three connections on the bottom of the external regulator. Verify, using a multimeter set to measure resistance, that the three wires between the alternator plug and the regulator plug aren't internally broken or shorted against each other.

 

• Crack the key to ignition. The dashboard alternator "L" light MUST come on. If it doesn't, it must be trouble-shot as to why. The light may be burned out, or the blue wire onto the "D+" terminal on the regulator (or, for an internally regulated alternator, onto the "D+" post on the back of the alternator) may be broken. This provides the excitation current for the alternator. It MUST work (though there's a way of bypassing it if it doesn't).

 

• Test the resting voltage of the battery with a multimeter. It should be about 12.6V. If it's really low, recharge or replace the battery before going further. Despite what you may have heard, alternators aren't designed to recharge deeply discharged batteries.

 

• If the battery is good and the "L" light comes on when the key is turned to ignition, start the car. The "L" light should go out. Test the battery voltage with the engine running. It should be at least 13.2V, and should rise to closer to 14V as you rev the engine up to 2500 rpm. If instead it stays at 12.6V with the engine running, and/or the light doesn't go out, and you've checked all the connections as I've described, then either the voltage regulator is bad (is likely stuck in the bypassed configuration) or the alternator is bad. You need to figure out which one it is. It could be both.

 

• You can "full field the alternator" (bypass the regulator's switching of the alternator on and off and instead set the alternator to dump current continuously into the battery) by unplugging the plug from the bottom of the regulator and jumpering across the DF and D- lines. NOTE THAT THIS MAY PUT OUT AS MUCH AS 17V SO DON'T DO IT FOR LONG. Put the voltmeter across the battery when you do this. The jumpering is described in the FAQ article linked to above, but it doesn't really say that what it's doing is full-fielding the alternator; it only says that it should make the warning light go out. If you apply the jumper when engine is idling, and the alternator is working, you should hear the alternator load down engine. If the warning light goes out and the voltage goes up at the battery, then the alternator is probably fine and the regulator is probably at fault. Replace the regulator with a new one or a known-good one, measure the voltage at the battery, and see if you've got charging voltage.

 

• But if the full-field jumper test does NOT load the engine down, does NOT cause the light to go out, and does NOT produce extra voltage at the battery, the alternator is probably bad. Replace the alternator and retest.

 

Note that, as was the case with Jeff Caplan's car that I trouble-shot at The Vintage, both the alternator AND the regulator can be bad. In Jeff's case, it was a newly-rebuilt Bosch alternator and a new Hella solid-state regulator, and they BOTH appeared to be bad, and the system charged fine when they were BOTH replaced with good components.

 

It can be frustrating, but other than the alternator, regulator, warning light, and connecting wires, there's really nothing else to the system. 

 

 

Note that all of this covered in my electrical book, with photographs, but because the chapter has to cover a wire range of European makes and years, the description I've written here is more concise and 2002-specific.

 

Note that the book also covers the voltage drop test you can do on both the positive and negative sides of the alternator and battery, but a visual inspection is usually good enough.

 

Note that the method for troubleshooting an internally-regulated retrofitted alternator is to:

 

• Inspect the positive and negative cables and the B+ line and alternator warning light as described above.

 

• Remove the brush pack / internal voltage regulator and inspect the brushes. Often you'll find the brushes are worn down to nubs, or a brush is broken. Or the regulator is simply bad. Replace the pack and retest.

 

• If that doesn't fix it, replace the alternator.

 

 

Rob

 

 

Edited September 20, 2016 by thehackmechanic

[First02]

This post still adding value in 2022.  In my case it was no continuity on D-

[Son of Marty]

Really nice Rob clear and concise, one thing to add is the l  light must be a incandescent bulb no led's.

 

Please include this in the tech section.

[thehackmechanic]

Thanks.

 

I had a guy come by the house just last week with a no-charge situation. He had a recently-installed in internally-regulated alternator. The wires seemed fine. I pulled the regulator and looked at brush pack and it looked okay. Then I noticed that the alternator light on the dashboard wasn't working—it wasn't on with the key cracked to ignition. I manually jumped the B+ wire to the battery for a second and verified with a voltmeter that the alternator then began charging the battery. I pulled the cluster and swapped bulbs with the working oil pressure bulb but it made no difference. I was stumped. Then the owner noticed that the little push-on B+ connector was only held onto the blue wire by a single tiny strand. I crimped on a new connector, the alternator light came on with ignition, went out when we started the car, and the battery began charging. Never saw that one before.

[Dugan27]

Just wanted to say thank you!