FAQ E28 charging system

[Mike W.]

While dedicated to E28s, all of this should apply directly to E23s and E24s. Mostly to everything from 2002s thru E34s, but there may be some detail differences. Get into the rolling computers that are E39s and things may change a bit.

70s-80s BMW (Bosch) alternators hardly ever fail.
I’ve put 600K on them and the closest I’ve come to a failed alternator is one where the front bearing was so bad it would intermittently seize then spin. Felt like a quarter of an inch of play in the bearing and it would still charge when it was spinning! Regulators fail, brushes fail, but the alternator very rarely. And most of what applies below assumes you have a good battery, yeah, sounds simple but not always as simple as it sounds. To me replacing the alternator because the light comes on is like replacing the engine because it won't start.

You need a meter, and the ability to at least sort of know how to use one. If you can’t read voltages you’re like Blinkin from Robin Hood, Men in Tights. You know, the blind guy who stands guard saying I guess nobody’s coming... If it’s a digital you need it set at 20V, DC. Do not use the 10A terminal.

Does battery light in cluster work
No? Not always, but usually it’s needed for the alt to work. (sometimes there’s a jumper inside the cluster, but do not assume there is) First make sure the small blue wire (started as blue, decades later it might not appear to be blue) is attached to the back of the alternator, and in the right place on the D+ terminal, the small one with an insulator on it, it’s not uncommon to get broken off over the years. If it is attached, remove it and try grounding it, the light should light up, if not the bulb in the cluster may be burned out (most likely), or a wiring problem between the cluster and the alt (unusual, but it happens). This may or may not be the problem, but you need to fix it first to easily tell if you have a problem.

If the battery light lights up when grounded, but not when attached to the alternator replace the regulator. The light should work with a defective regulator, but it doesn’t always.

If the light flickers, sometimes comes on, sometimes goes off, suspect the regulator, or more specifically the brushes in it, which some of us do actually replace.

If the battery light comes on with the key off, you probably have a bad diode in the alternator and need to actually replace the alternator itself. In that case, immediately disconnect the ground cable when you stop driving if you want the car to start in the morning.

Yes? If it comes on with the key, does it go off when you start the car? If it stays on, replace it, if it goes off, check voltages, see further down.

The regulator.
It’s attached with 2 screws to the back of the alternator. Often it’s rebuildable with new brushes, although once in a while they’ll just smoke in which case they may or may not even light the battery light on the cluster. But to call it a 5 minute job is to overstate it on an E28, it’s super quick and easy. I’ve even been known to replace the brushes before trouble because I was going on vacation and they were getting short. They're cheap, a spare is a good idea for a long road trip.

OK, now we get a little more complicated. The light comes on when it’s supposed to, goes off when it’s supposed to, but the battery keeps going dead.

Voltages.
You need to measure them. A good, charged battery, should be 12.55 to 12.75 volts. That’s engine off, everything off, just sitting there. Running, with a light to moderate load it should read 13.5-14.7. Definitely under 15 unless you like bright headlights and replacing them and the battery frequently. If it’s down to ~13 even running, at the battery, it might start in the morning and it might not. But it better start quick because it’s not going to crank long. If the voltage at the battery is higher running than off, it suggests the alternator is working, perhaps a bad regulator, but more likely wiring problems. Some E28s have rubber or urethane mounts on the alternator to minimize vibration. Those have to have a separate ground wire/ground strap between the alternator body and the engine or alt bracket. You also need the ground strap between the body/subframe and the engine on the passenger side bridging the motor mount, but it’s probably not going to start if that’s missing.

Regardless, if voltage at the battery is low, check the voltage at the alternator itself. Between the big red B+ wire and the alt case. If the voltage at the alternator itself is more than a tenth or two above the voltage at the battery, you have wiring problems. I had an E21 one time where the battery kept going dead. Turned out I only had 12.9/13.0 at the battery but 14.3 at the alternator. I added a wire between the alternator and the battery and all was good. If you turn everything on, A/C on high, high beams, etc, it should at least come close to keeping up even at idle. Voltage may drop to 13, but pick up a bit with some revs. If you have a 6 pack of aircraft landing lights on the roof that can be seen 3 zip codes away, the preceding does not apply to you.

So you check for smaller voltages between the alternator and the battery. Crank things up with the A/C on high and high beams on and voltage will drop a bit especially at idle, but it shouldn’t be more than a few tenths. Check from B+ to the positive terminal. Check from the body to the negative terminal on the battery. If all of the sudden you get a half a volt or something, that’s your problem. Wires don’t usually fail, but the ends or terminals will. And or clean everything. I’ve seen tight battery terminals with a voltage drop from them to the battery post because of resistance. You can see a dirty connection but you can’t see resistance. Sometimes it looks good and clean but electrically it isn’t. But if you get a drop on the positive side, that’s where you troubleshoot. On E28s the hot side goes from the alternator to the starter terminal, to the battery. So you have several different places to inspect. Again, you can’t see resistance or voltage drop, clean or dirty, you need to check with the meter.

Worry about voltages, not amps. Amps follow volts, volts are easier to measure and amps will be ok if volts are ok.

Now you can have a phantom draw/loss due to something being on when it shouldn’t be, but that’s beyond the scope of this. This is just the charging system.

[Click]

So far I've had 2 E30s and an E28 and have probably put 150-200k on them. Alternators have been solid for the most part but my one E30 had a bum battery within a year or two of replacing it. I also noticed the headlights were a bit dim and found the alternator was only putting 12-12.5V out. I replaced the battery and voltage regulator (with a Hella unit for what it's worth) and there was no improvement! I did some research and found these adjustable regulators for our alternators. I bought and installed one of the internally adjustable ones and cranked it to the max. I get like 13.5+V now and has been OK for the couple thousand miles I have put on since.

https://www.prancingmoose.com/AdjustableVoltage.html

[tig]

I've been meaning to dig into the circuitry that causes this



On Vlad (87 535is M30B35 swap) this sometimes happens after starting and a blip of the throttle fixes. I believe it's because the alternator belt is not tight enough and it is slipping. The blip causes it to bite. The fix is obvious, but I have been lazy...

On Minerva (85 Euro 528i with S54 swap) this has happened after long drives (50+ miles). Once it happens it won't go off until after I let the car completely cool down. This can not caused by the alternator belt slipping because serpentine belt. It has not happened in a while, so it may have been cured by other forces.

Regardless, of *cause* I want to understand precisely what circuitry is involved in DETECTING it. Any insight there?

[Mike W.]

I've been meaning to dig into the circuitry that causes this

On Vlad (87 535is M30B35 swap) this sometimes happens after starting and a blip of the throttle fixes. I believe it's because the alternator belt is not tight enough and it is slipping. The blip causes it to bite. The fix is obvious, but I have been lazy...

No, I don't think it's the belt. I think the tiny amount of current that provides the initial excitement is in adequate at low speeds. Remember it's going thru the light, which I believe is a 2W bulb, which translates to .17A of current going to the field to initiate charging. Contrast that to the potential (off rotor resistance, not counting the inductive factor) of 3 to 4 amps when it's running. Now I have seen some with what must be residual magnetism that were self exciting, but I don't recall at what speed they started to generate output. I bet if you wired in say a 12 ohm resistor in parallel it would start charging at cranking speed, but if you did that the indicator light wouldn't work.

On Minerva (85 Euro 528i with S54 swap) this has happened after long drives (50+ miles). Once it happens it won't go off until after I let the car completely cool down. This can not caused by the alternator belt slipping because serpentine belt. It has not happened in a while, so it may have been cured by other forces.

Regardless, of *cause* I want to understand precisely what circuitry is involved in DETECTING it. Any insight there?

This one is much tougher since I don't have a schematic of either the alternator, down to the diodes inside, or the car itself and being a transplant, you may have one, whoever did it may have one, or it may have been someone knew how to make it work with no documentation of that detail.

Seat of the pants guess would be the regulator

[tig]

I'm not being clear enough.

When this happens, ALL of those lights come on. I am trying to find in the ETM the circuitry that triggers that.

[RossDinan6]

Excellent write up. I have a couple comments based on my own experience.

First, I do not agree with the mention of 15 volts. Depending on battery chemistry some will be happy with 14.5 or so. Most common batteries should charge at 13.7 to about 14.2. At least these voltages are what I have worked with for years. 15 will cause electrolyte loss in most batteries, unless an exotic is used.

Second, my E24 never had adequate voltage for modern batteries. My battery vendor told me '80s batteries had a different chemistry. New ones are designed to run the 14 volts I mentioned above. I tried multiple different regulators over the years trying to get the voltage up and extend the life of my batteries, including the adjustable regulator. I pulled that when monitoring the voltage and saw some large voltage spikes. Likely too short to harm, but didn't like what I saw.

I ran the low voltage output by my motor/alternator guy. He supplied an aftermarket regulator, Regitar VRB195 15v. It runs at 13.8 to 14 volts. It has been there about 2 years, so so far so good. I do not like the made in China part.

Last, regarding the battery light. I had a no charge/no light with a good bulb and alternator. The fix was pulling and disassembling the cluster, cleaning and tightening lamp holder contacts, terminals and sockets.

[Mike W.]

I'm not being clear enough.

When this happens, ALL of those lights come on. I am trying to find in the ETM the circuitry that triggers that.

I don't see it there either. What I do see is a blue from the battery light going to the ABS control unit with a splice and two additional blues coming off it. Where they go I don't see. My 85 would light the ABS light along with the battery light until I blipped it to start charging. But not the two additional lights. The 85 ETM only shows one additional blue off the ABS input not two, but doesn't show where that goes either.

Did it do this before the engine swap? What cluster are you using? What tach? I see an ///M tach, is it E28 or different? Kind of looks like it has the hole for the SI reset like later ones have.

[Mike W.]

Excellent write up. I have a couple comments based on my own experience.

First, I do not agree with the mention of 15 volts.

I said it was too high, are you saying it is too high, not high enough, or the max limit should be lower than the ~14.7 I referenced? An older big blue BMW manual says 14.5 is max but I think a tenth or two higher is ok. Not half a volt higher.

[dsw99a]

Im thinking STICKEY?!

Great work as usual Mike.

[Blades74]

Purchased my e28 about 6 months ago. I've apparently never had charge from the alternator, and realize now that I've also never seen the alternator indicator light on. My old alternator was misaligned to the pullies, so i went ahead and changed it. New one ooks great. My indicator light still does not light up with the ignition. It is getting 12 plus volts when the ignition is on, and will light up if I ground the D wire. But that's the only time. I'm awfully confused.

[tig]

I'm not being clear enough.

When this happens, ALL of those lights come on. I am trying to find in the ETM the circuitry that triggers that.

I don't see it there either. What I do see is a blue from the battery light going to the ABS control unit with a splice and two additional blues coming off it. Where they go I don't see. My 85 would light the ABS light along with the battery light until I blipped it to start charging. But not the two additional lights. The 85 ETM only shows one additional blue off the ABS input not two, but doesn't show where that goes either.

Did it do this before the engine swap? What cluster are you using? What tach? I see an ///M tach, is it E28 or different? Kind of looks like it has the hole for the SI reset like later ones have.

Still struggling to figger this one out. I've been driving Minerva a bunch and this symptom happens only when the car is warmed up. It doesn't happen all the time, but when it does, it's after I've been driving. I can turn it off, wait 5 minutes, and those lights will still light. I put the car to bed overnight and they are off when I drive again.

When I get this situation, the following lights, light:

- "ABS"
- "Brake"
- "Park Brake"
- "Charge"

Using the '85 ETM, let's start with the ABS indicator:

Power to the indicator comes from the ignition switch (RUN or Start) via C302 and a .75 GN wire (M):


This .75 GN goes directly to the ABS blub. The other side of the bulb is either a .75 GY/RED from pin 1 of the ABS Hydraulic Unit or a .75 BK/GY from pin 29 of the ABS ECU:



So, according to the above the ONLY way the ABS indicator can light is if EITHER the ABS pump OR ECU says so.

Now let's look at "Brake":

Power to the "Brake" warning indicator comes from Fuse 6 (hot in Run or Start) .5 GN/WT to pin 7 of the instrument cluster printed circuit board. There is also circuitry. This same lead powers the "Brake Lining Warning" (which is off in my case, although there's a possibility the bulb is dead):



So, according to this, the ONLY way the "Brake" indicator could be lit is if the brake fluid level switch is closed.

Sigh. Now on to "Park Brake":

Again, power comes from Fuse 6, via a .75 GN/WT to pin 7 ("15S") on the Instrument Cluster printed circuit board. NOTE that above the ETM says this GN/WT wire is .5, but here it says it's .5. Sigh. See here:



On the right side of the diagram see "15S" below.


So, according to this, the ONLY way the "Parking Brake" indicator could light is if the park brake switch was closed. Da fuq?

Finally, in case you're not already exhausted and confused, let's do the "Charge" indicator...

As you can see below, that indicator gets power from the ignition switch (hot at all times) via a .5 GN into pin 22 of the Instrument Cluster circuit board ("15U"). The other side (ground) is connected to "CHARGING SYSTEM" via a .5 BU via pin 21.


Further details show that .5 BU goes to the C302 accessory connector pins 9 and 20 which are not "Not used". It also gets bigger (.75 BU) and goes to the diagnostic connector pin 12 and to the alternator.


Of course, we have this interesting note in the bottom right corner.

Nevermind that, because, according to the above the ONLY way the "Charge" indicator could light is if the D+ terminal on the alternator is pulled to ground by the regulator, indicating bad voltage.

Since this is a "Charging System" thread, I'm going to try to stay focused here...

We also know that in the case of Minerva (the photo above) my alternator is an E46 M3 (S54) alternator.

I have yet to test the voltage on the alternator when these lights are lit (I keep forgetting), but I have tested when they are off and my alternator is working fine. EVEN IF the alternator were not generating enough voltage, and the "Charge" indicator got lit, those other indicators have no reason to be on.

As I noted, I've seen similar behavior (all these same indicators come on) on Vlad (M30B35 swap) where I believe my alternator pully is a bit loose and slips when the car is first started sometimes. A blip of the throttle causes the belt to grab and they all go off.

I just play an electrical engineer on a streaming service. I'd appreciate someone who knows what the fudge they are doing to tell me how any of these 4 indicators are related!

[tig]

Note, I can't find an E46 M3 ETM, but here's the approrpate page from the Z3M (I have a Z3M harness, so...):



I am not sure if we wired the output of pin 1 of the MSS54 ECU to the indicator or not. Even if we did, it still doesn't explain all those other lights....

[Mike W.]

Wow. There's a lot there. I see some possibilities. Maybe. I know where the E28 links are, but how about one for the Z3 harness you're using. I'm guessing a Z3 ///M?

[Mike W.]

OK, on the parking brake light I have no idea, it appears to be a simple dumb circuit.

However, on the ABS and even the brake wear light, they are smart circuits. I know on the ABS but I hadn't heard on the brake lining, that less than ~13.? volts will cause the light to go on. The brake lining might be the same thing given that it looks like a solid state relay is involved and might be voltage sensitive.

You've just really got to break out a meter when those lights start coming on, I have the feeling the alternator is not working, just as it's telling you.

[cararecools]

Hello, I know this discussion as not been used in a While but i desperately need help: My D+ is at 5v when the key is in the first position. My battery light from the Clutch is the original one and work juste fine. Now I know that my alternator work (tested it + new), but at the end of the day, when I try to make it work in my car (by using an external power supply or jumping the D+ to the battery) it just do nothing. Any idea, even simple one could potentially help me get out of this 4 months nightmare. 

Thanks.

[Mike W.]

Hello, I know this discussion as not been used in a While but i desperately need help: My D+ is at 5v when the key is in the first position. My battery light from the Clutch is the original one and work juste fine. Now I know that my alternator work (tested it + new), but at the end of the day, when I try to make it work in my car (by using an external power supply or jumping the D+ to the battery) it just do nothing. Any idea, even simple one could potentially help me get out of this 4 months nightmare. 

Thanks.

Welcome to the group! A new thread would have been better, but we can go from here.

One possibility is some of the alternators have rubber mounts in them effectively insulating them from the ground of the engine. Even if it's the all metal case, a ground is supposed to be there. Start it up and while running, check voltage at the alternator, the B+ terminal with the heavy wire to the body/case of the alternator itself. Not to the ground on the battery, not to the body of the car, the alternator case itself.

[austin8753]

Hello, I know this discussion as not been used in a While but i desperately need help: My D+ is at 5v when the key is in the first position. My battery light from the Clutch is the original one and work juste fine. Now I know that my alternator work (tested it + new), but at the end of the day, when I try to make it work in my car (by using an external power supply or jumping the D+ to the battery) it just do nothing. Any idea, even simple one could potentially help me get out of this 4 months nightmare. 

Thanks.

Welcome to the group! A new thread would have been better, but we can go from here.

One possibility is some of the alternators have rubber mounts in them effectively insulating them from the ground of the engine. Even if it's the all metal case, a ground is supposed to be there. Start it up and while running, check voltage at the alternator, the B+ terminal with the heavy wire to the body/case of the alternator itself. Not to the ground on the battery, not to the body of the car, the alternator case itself.

this, absolutely this. my M5 hasn't charged very well (if at all) the entire time i've owned it. just realized a few weeks ago, it has an alternator with isolator bushings... and a missing ground strap. voila, problem solved. don't ever over look that!!!

A

[///M]

I noticed on this that @tig question was not answered. The lights are connected to the microprocessor on the Service interval board. The oil light is also connected. Most of the warning indicators  are connected to the processor.
There could be a fault there or a mismatch in the wiring on the conversion car. The cluster may not match the car loom. On the ETM mentions early and late production. There is a diode between the alternator and the light on some models and not on others from diagrams I looked at, unlike in other simpler systems where when initially D+ is actually a ground so the light comes on and when the alternator charges D+ is positive and should have battery voltage.
The bulb then has 12v+ on both sides so the light goes out. It also was the case that a failed bulb meant no charging. By including a resistor fixed that problem.
   
 

[Chicagotrader92]

On Vlad (87 535is M30B35 swap) this sometimes happens after starting and a blip of the throttle fixes. I believe it's because the alternator belt is not tight enough and it is slipping. The blip causes it to bite. The fix is obvious, but I have been lazy...

Did you ever figure this out? Mine has always done the same thing. (All lights stay on after starting and goes away after the first touch of the throttle.)  

[tig]

On Vlad (87 535is M30B35 swap) this sometimes happens after starting and a blip of the throttle fixes. I believe it's because the alternator belt is not tight enough and it is slipping. The blip causes it to bite. The fix is obvious, but I have been lazy...

Did you ever figure this out? Mine has always done the same thing. (All lights stay on after starting and goes away after the first touch of the throttle.)  

Nope. Still happens. Haven't spent any time fixing it, but want to!