Battery Not Charging

hyllus · Post by **hyllus** » 28 Aug 2017, 20:44

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?

lummert · Post by **lummert** » 29 Aug 2017, 15:45

Next thing to check is the ground wire running from alternator case to engine block. When turning key on does the battery light come on with the rest of the indicator lights?

hyllus · Post by **hyllus** » 30 Aug 2017, 13:11

Sometimes the battery light on the dash comes on and sometimes it doesn't. It seems to be intermittent.

hyllus · Post by **hyllus** » 30 Aug 2017, 15:42

Additional - When the battery dash light was on, it was charging at the battery at 14.2 v. I connected the D+ of the alternator directly to the + terminal on the battery and I'm only getting 13.8 v. It does go up to 13.99v when engine is revved.

Post by **Roger_850T** » 30 Aug 2017, 19:51

You should be able to get over 13.5V at idle. (Probably closer to 14.) I would put a load on it (lights on, AC on, blower on, rear defrost on,) then test in the following order:

Basic alternator check
1. Check voltage between alternator case and B+ terminal (basic alternator check.) Should be 14 to 14.5V.

Ground Check
2. Check for voltage between alternator case and engine block. Should be close to 0.
3. Check for voltage between engine block and negative battery terminal, should be close to 0.

Positive Check
4. Check for voltage between alternator B+ and battery plus. Should be close to 0.

Somewhere in there you will likely find an excessive voltage drop, which will be what you need to replace.

If it ends up being the alternator, I would dig further before replacing - on these Bosch alternators I have been able to just replace the brushes and keep it running. (Have to remove the regulator and solder new brushes, but it's pretty easy to do if you are comfortable soldering.)

Roger

hyllus · Post by **hyllus** » 31 Aug 2017, 04:00

Thank you. What gets me is how intermittent the problem is. 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?