okay. to start: 1989 245. I have an ongoing charging system problem. It will sometimes charge on start up, but with a few minutes, the charging rate drops, until it is discharging (<12 volts, checked on 2 meters). it usually charges better when it is cold out. or first thing in the morning.

I have voltage to the alternator. I have a good clean ground. my battery light always comes on and then goes out. I have swapped two other alternators (one a rebuilt one) and no improvement.
If its an engine harness issue: where does the big red wire go? If I follow it out, it looks like it goes to the fuse on the fire wall, but the wire gauge is much smaller.

 

the big red wire on the alternator goes to the even bigger red wire on the starter then to the battery +

take your volt meter, when the charging voltage is low at the battery, put one meter terminal on the battery - and the other on the alternator metal body... if you see more than about 0.1V, you got a ground problem. if so, then try engine block to battery - and engine block to alternator body to see which ground.

 

I checked for grounding issues. There were none. By turning my other 245 into a test mule, I found that I had three bad alternators (the original, one used one of unknown history and one rebuilt one that was about a month old(not from that car)). Hopefully the new rebuilt one will last.

By the way, should the battery/charge light illuminate when the car stops charging? Mine would come on until it was running, then go out, even though the alternator wasn't charging.

Thanks for your help!

 

if the light goes out, the alternator innards /should/ be making juice, so if its not charging the battery, I'd suspect the internal diodes are bad (the light is on a seperate set of diodes).

check the voltage at the back of the alternator, from the alternator body to the +B big red wire terminal, and from the engine block ground to the big red terminal, both should be like 13.something volts if the engine is running, a good volt higher than when the engine is off and hte battery has sat an hour to dissipate the surface charge.