Charging problem.

[D. Clay]

I have had the alternator and battery checked separately and both are good. The problem is the alternator does not charge the battery in the car. I have checked the ground from the alternator to the engine and from the right motor mount to the frame rail. I'll have 11.6 volts across the battery, start the car and it stays at 11.6. It runs until the battery gets too low to run the car. Are there any fuses or circuit breakers that could be bad? Any ideas at all on what the problem is or how to troubleshoot it? There only two wires that go the alternator - one large and one small - and a ground. As the little wire will not go on the larger terminal, I'm sure it's wired right.

[Nomadic]

I've got the same problem on my 1991 318is m42.   I'd love to get an idiots brief on the e30-m42 charging system and possible sore spots.

I'm wondering if these alternators have built in regulators?  I thought they did.  If not, where does the regulator live?  RealOEM lists a diode board for the alternator @ ~$180, but I suppose that could be for only the rectifier function, not the regulator.  

I pulled the alternator and had it tested a automotive chain store and tested fine.  Battery *could* be the problem.... but  has kept the car running and starting... for a while..  just at low voltage.   (Now that i think of It I'll try swapping a know good batt.. )   I wouldn't have known there was a problem, except the all the dash warning lights went nuts.    

The saga continues.

[rob_e30]

In order for the alternator to output voltage, you have to excite the field with a separate line from the battery.  It's the smaller post and runs via the  alternator light in the dash.  I know there was an earlier problem that if the light burned out that the alternator would not charge.  I believe it's been fixed by the M42/E30's but check that you're supplying +12 to the smaller terminal on the alternator.

[D. Clay]

Will check 12volt to small terminal. The wiring diagram shows it going to the ALT. light.

[rob_e30]

It does go to the light, or perhaps more accurately from it.  +12 flows through the light and to the alternator where it stimulates the field so the alternator will output voltage.  Once the alternator begins generating voltage there is no difference across the light so it goes out.

[Nomadic]

This older thread suggests that a bad or missing Service Interval Board (SI) will prevent the alternator from charging.    This makes sense if the smaller  12v+  lead still comes from the batt warning light.  

I'm still wondering if the small 12v+ is providing the voltage "regulator" function?   (I'm guessing it is NOT... )  This  Starting Circuit PDF suggests that the small 12v+ is not regulated and that the alternator has an internal regulator...  but I've no idea what year or model the diagram is for.  

 If it is a constant 12V+ signal (ignition on) then it can be bypassed... right?  

This would be a good to know for a possible emergency situation/solution.

[monko141]

Have you checked the terminal block in the engine bay on the passenger side?