We had to replace the Ball Joints on my daughters 1989 240 DL today ..and afterward the car acted like the battery was dieing ...we looked by the alternator and there was a small black wire that came up toward the alternator in the same basic wiring harness as the large Red wire and the smaller red wire ...but the black one isnt connected to the alternator ..or anywhere else .thought maybe the mechanic knocked it loose when he was working on it ..the large red one is connected to the alternator and the small red wire is also connected to it via a small blue wire ....but for some reason the car wont charge ..any ideas on where this small black wire might go ..? Also there is a wire that goes into I suppose what is the oil pressure switch very close to the alternator on the side of the engine ..and it isnt hooked up to anything either and now the oil light on the dash is on ...I need some help ...that light wasnt on before today either ..and this car had no charging issues before today ..so if you could please shoot some ideas my way i would appreciate it..I mean does that black wire coming up toward the alternator hook into the wire for the oil pressure switch? ..thanks

 

The black wire must be the ground. Check to see if there is a small connector on the alternator that is missing a wire. I am trying to find a good picture of the alternator but I am not having much luck.

 

That black wire goes to the oil pressure sender. The alternator has two red wires, and a blue for the ground connection. With the key on (not started) make sure the battery light comes on and you have 12V on the small red wire connected to the alternator ... if you don't it won't charge.

 

If the OP has an internally-regulated alternator, as my 240 does, then it is only supposed to get the +12 volts on the field (small) wire when the key is in the "start" position. With the key in the "run" position, such an alternator gets its field energized from the internal regulator. If the field wire on an internally-regulated alternator were to receive 12 volts all the time the engine is running, then the alternator would charge all the time.

 

This is correct. The black wire is for the oil sender, the small red for the field wire (energizes the rotor to start the charging process). The alternator should have a small blue wire with red shrink insulation on the end that bolts to the bottom of the alternator and goes to the motor mount. The wiring harnesses on old 240s tend to go bad and those two wires are the first victims. Check that wire for power as stated above. If it doesn't have power (key on car off) then you need to go to the connector on the firewall and see if you have continuity from the wire at the connector to the end at the alternator. If you do have power you need to start the car and check the voltage output at the alternator itself to see if it is working or not.

 

I guess I was wrong. Thanks for the information.