OK, this should be a simple thing to solve but I'm striking out. Here are the particulars:


* 1994 940 (B230F)
* Battery is not charging - about 11 V at terminals when running at 2k rmp
* Took battery into shop - tested fine
* Took alternator into shop - tested fine
* Checked voltage at B+ terminal of alternator - full battery voltage


What's the next thing I should check?

 

Alternator could be putting out somewhere between 13.5 and 14.5 volts, depending on the vehicle. If it's putting out less than 13.5 volts, it's probably the voltage regulator. The voltage regulator, if I remember correctly, is built in the back of the alternator. That can normally be changed out with some ease, if one is available.
You also want to check any and all connections between the battery and alternator to see if there is a loose connection or corrosion. Those items have the potential to be the issue.

 

Does the battery light on the instrument cluster come on when your key is turned to the II position prior to starting the car? Without this bulb functioning, the battery will not charge even though the alternator is good. If the battery light is not functioning, check the bulb and the small red wire on the alternator to ensure a good connection.

 

OK, here is the odd thing - no the battery light wasn't coming on when the key was in the II position. I checked the thinner red wire going into the alternator (I believe it's called D+) and found no power either. I pulled that wire off to inspect. When I put it back on, and turned the key again, the battery light appeared on the dash. After starting the car, the battery was getting 14.3 volts. I think I now need to track down a loose wire or something.

 

the thin red wire is the 'exciter' that tells the alternator to start charging, so you are correct to re-check that. some people have said that revving the engine up to 2500rpms does the same thing but i am not certain about that? anyone know for sure?
anyway it does appear that both your alternator and battery are good.

 

Thank you. Where is the terminal point of the 'exciter'? What does the battery light on the dash have to do with it? BTW - I just check it again today and the battery light is off and battery not charging again.

 

you should go here :

https://www.brickboard.com/FAQ/700-9...torNotCharging

there is also a good writeup there about the exciter and what it does

 

That was a great link - thank you. After a short period of seeing the battery light, and watching the alternator charge the battery at about 14.4v, the battery light is off again and no charging. I took off all the wires I could and cleaned the connections - nothing. I ran a line directly from the battery + to the exciter terminal on the alternator - success! The battery was charging at around 14.0v. What are your thoughts about just hard-wiring the exciter like that and calling it a day?

 

i think they discuss the pros and cons of doing that in the articles.
i would say that it may be a good temporary fix to the problem, but meantime you could look into the root cause of why the exciter wire was failing in the first place. might be something as simple as worn insulation etc. by tracing the wire you might find the culprit.
i suppose having the 'idiot' light working properly does have its advantages
other than the exciter function.