Suggestions for diagnosing intermittent charge warning light

Vehicle: 2004 BMW 325iT, 262k miles, Bosch 120A alternator
History: At 260k miles, I decided that the brushes must be about shot, so I replaced the voltage regulator/brush assembly pro-actively. Brushes were about shot. Turned out that my part is no longer manufactured, but auto electric shop sold me a Bosch regulator for Mercedes that fit and seemed to work perfectly.
2000 miles later, the charge light is going on and off randomly. Not related to RPM, electrical load, bumps, or anything else I can detect.
Today I will examine it for loose connections and run a wire from the alternator to the passenger compartment so I can monitor voltage at the alternator as the light goes off and on.
I have never understood the circuit that controls the charge light, so I am not confident trying to diagnose that.
Anyone seen a similar problem?
Oh, and I pulled out the instrument cluster to fix the clock set button when daylight savings started, so it is conceivable but unlikely that I damaged one of the connections to the cluster a couple of weeks ago.

did you have your battery tested? check to see if your ground cable is clean and tight where it connects to the engine side. also battery cables can look clean on the ends but there can be corrosion on the inside under the rubber coating.

Pretty confident that battery is good and has good connection. Engine cranks briskly.

Have not examined alternator yet, but voltage at the OBC port is 12.7 volts before starting and drops to 12.2 volts after I start it. Insensitive to RPM. Wish I had a tester that would check diodes and so forth.

The sticky part of this is that my voltage regulator has an oddball plug shape where it plugs to the car. That was why I had such a hard time finding a voltage regulator that would work. If I buy a whole new alternator, It almost certainly won’t plug into the car until I go to a junk yard and find the correct shaped plug and solder it onto my car. I fear electrical incompatibility, otherwise why would there be two different shaped plugs on 2004 BMWs?

You could start w/ the basic alternator battery test: Before first start of the day the battery should measure about 12.6 volts. Then immediately after starting the engine, 13.5 - 15.5 volts. Measured at the battery terminals using you lab volt meter. What do you measure?

Running wires to the passenger compartment and monitoring voltages might make debugging sense depending on what the above results are. The alternator’s output is roughly proportional to the product of the engine rpm and the stator’s current. On older cars (like mine) the stator current is determined by a voltage regulator feedback circuit; it increases the stator current if the alternator output voltage decreases. The stator’s current on a 2004 BMW is probably decided by the ecm’s charging algorithm.

On my cars the charge light turns on (meaning there’s a problem) when the alternator’s voltage output is less than the battery voltage.

Is yours a late E46 or an early E90?

Late e46.
Still have not gotten the car in out of the weather to work on it, but I have learned more.
Bentley manual notes that there were several different charging systems installed on e46s, but it describes and shows wiring diagram for only the system “with multifunction controller” which mine does not appear to be. Based on my original voltage regulator part number, my charging system is shared with a 2004 Z4 and a few 525i and 530i vehicles. Searching on-line in ShopKey Pro using my VIN returns what appears to be the correct wiring diagram. I see now that my thought of cutting a plug from another car to fit a new voltage regulator or whole new alternator would likely be a really BAD idea because I would be putting on hardware that would be incompatible with the field signal coming from the car.


I’d say your right, they went to different charging systems in 2004. The alternator charge output design was changed as the started to go on to externally controlled lines. If you have the last seven characters of your vin I can confirm it for you.

end of the VIN is EZ15555
I examined the wires on the alternator. Tight and clean. I had forgotten, but there are only two wires to this alternator - the heavy red load wire, and a fine blue wire coming from the dash cluster. None of the various wiring diagrams I have collected show only two wires to the alternator.
As for the intermittent charge warning light, it illuminated the first time I started the car this morning, voltage at ECU plug was only 12.1 while running. Then voltage went up to 14.2 V and light went out and stayed out all day long with 6-8 restarts as I ran errands. Hard to diagnose something that is working. It will likely wait until I am 100 miles from home to fail again.

Had a look on the system ( I work at Bmw) yours has the a BSD line ,(the blue one) that’s processed by the engine management (DmE) Then the warning is illuminated or not, if you have the wrong rectifier fitted that’s going to confuse the system. I can get you the correct part number if you know what make and output the unit is.

The alternator that came in my 2004 325iT is a Bosch 120 A, BMW part number 0124515105. The BMW parts book lists this as a Z4 alternator.
The voltage regulator that came on my car was a Bosch part number F00M145435. According to Bosch technical support helpline, this part number is discontinued and there is no replacement. Bosch helpline said my only solution is a whole new alternator.
RealOEM shows three alternators for my car. The oval plug Bosch alternator that they list is BMW part number 12317519620. RealOEM lists no voltage regulator for that alternator under e46, they list only a rectangular plug voltage regulator that fits the other e46 Bosch alternator. However, when I look up that same 12317519620 oval plug alternator in RealOEM under Z4, they list a voltage regulator 12317515319 (Bosch part F00MA45A45219). If I replace the oval plug Mercedes (Bosch F00M144147) voltage regulator that I am using now, I will get a Z4 F00MA45A45219 regulator.
Do you think I would be wise to go ahead and get a Z4 voltage regulator?

The alternator it lists in the parts catalogue is either Bosch 120 with the oblong plug or a valeo 120, the oval plug is a BSD controlled unit listed under 2.2 Z4 which we’re BSD so I’d say you’re right a Z4 rectifier should do the trick

Thank you for looking in to this for me!
At the first hint of a charge light again, I will get a Z4 voltage regulator.

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Your welcome

My saga, it appears, continues. About 1000 miles with the Mercedes VR and the charge light reappeared. I pulled off the VR and found one brush melted in to its holder. Apparently the brushes failed to seat in the worn slip rings. Now one of the slip rings was obviously damaged from all the arcing.
I polished the slip rings as best I could (the closest shop that could turn them on a lathe is 50 miles away) and installed a new Z4 regulator and crossed my fingers.

250 miles on the new voltage regulator, tonight on the way home from work, first the radio went dark, then the Brake and ASC lights illuminated, then the antilock light lit, then the HV/AC display went dark and heater fan shut down, then the charge light came on…

Oddly, however, it was still charging. In fact, charging voltage reported by the dash menu indicated 14.7 volts. This car normally charges at 14.2 volts.

The radio and HV/AC went off and on periodically the rest of the way home. The brake and ASC lights went out and stayed out. The charge light went off and on.

I suspect that there is a lot of sparking going on around those brushes, and it is driving all my computers crazy. I made it home but I won’t drive it again until I get this sorted out.

I am ready to get a different alternator unless someone has a better suggestion…

Sensible idea. Voltage spikes from a faulty alternator could damage expensive to replace in-car electronics. Some auto parts stores may offer a pre-purchase test of the replacement alternator as well. Good idea to select oem alternator, or a rebuilt oem.

I would see if a local auto electric shop will work on it. Chances are better to get careful and precise work at a good auto electric shop, compared to buying a rebuilt or an aftermarket new unit.