Alternator Wiring?

[LowLight]

I have an 86 325e. I'm having problems with the alternator not charging. I've had the alternator tested twice, passed. New belts and tensioned properly. The battery light on the dash does come on. Battery is good.
I was looking at the wiring from the alternator (Physically and wiring diagrams) and it looks like it goes to the starter solenoid. Is this true or am I crossed/backwards. If it is true, could goofed wiring at the starter solenoid be keep the alternator from charging the battery? It is a manual trans.

Thanks
LowLight

[richardodn]

Start by charging the battery.

There are two connections to the alternator.

The wire that provides initial voltage to the alt connects to the D+ and is the smaller blue wire. (Actually there are 2 wires here. One to the ignition and one to the diagnostic connector. ) This should show voltage when the ignition is switched to on but not yet started. The cluster light will be on now too. You can read this out at the diagnostic connector at pin 12.

The other heavier red wire runs to the starter on the same terminal as the main cable to the starter. This is the wire that actually does the charging. (There's also a smaller red wire that goes to the diagnostic connector pin 14.) Start the car and check for voltage at pin 14. This should read around 13.5 volts and the cluster light should be out. If it's reading significantly lower, then the car isn't charging and you'll see the cluster light on. If it is reading right, you'll want to check all the alternator and engine ground connections and make sure they're clean and tight. Check the battery cable connection at both cable ends.

[LowLight]

The wire that provides initial voltage to the alt connects to the D+ and is the smaller blue wire.

That's the exciter wire, right?

[richardodn]

Yessir!

[thejlevie]

Check to make sure that the alternator is grounded by measuring the resistance from the alternator body to a chassis ground. And as mentioned check for exciter power to the alternator when the ignition is on.

[LowLight]

Ok, I tested pin #12 with the ignition ON but engine off and got a little over 1.xx volts while the battery said 12.5, is this normal? And I test the resistance of my ground to the alt and it was .7.

[thejlevie]

Check the voltage at the alternator exciter wire (the small wire) when the ignition is on. It should be battery voltage.

[LowLight]

Check the voltage at the alternator exciter wire (the small wire) when the ignition is on. It should be battery voltage.

And if it's not? Mine is only showing 1.6 volts with the ignition on.

[Rigmaster]

And if it's not? Mine is only showing 1.6 volts with the ignition on.

sounds like a bad/loose connection

just back-track a bit and see where the voltage drops from 12.xx and you'll have it licked in no time.

[LowLight]

Thanks for the help guys. I'll have to do some investigating tomorrow between classes.

LowLight

[thejlevie]

And if it's not? Mine is only showing 1.6 volts with the ignition on.

Then there is a fault in the exciter power supply circuit.

[LowLight]

My next step will be to check pin #2 on the C200 connector for power (see below). If there is power move down the chain, if there is not then I move up the chain. If I am wrong, someone please correct me.


Thanks
LowLight

Moving on... I went to connector C101 on the driver side of the firewall. With the ignition on and the connector DISCONNECTED the instrument cluster side (bottom half) read the same voltage as the battery. When the it was reconnected the same spot I tested a few seconds ago read 1.6x volts.

Could this be a short from the alt back to that connection? From the wiring diagram above there are no other connections between them.



Any ideas?

[richardodn]

Disconnect the exciter wire from the alt and measure C101 again.

[LowLight]

I removed the exciter wire from the back of the alt. Then tested the wires there and pin #12 on the diagnostic connector and got about the same voltage that was shown at the battery.

Would something inside the alt be causing a major drain?

[richardodn]

I'm no expert on the alternator internals, but I'd hazard a guess on the voltage regulator.

[heifetz17]

I'm no expert on the alternator internals, but I'd hazard a guess on the voltage regulator.

+1, either the voltage regulator or the alternator internals.

[LowLight]

I just replaced the VR and the alternator tested good on a bench tester...twice! This car is kicking my butt!

[heifetz17]

I just replaced the VR and the alternator tested good on a bench tester...twice! This car is kicking my butt!

Remove the exciter wire and replace it with a jumper wire from a 12 volt source (like the battery) and I think you'll see that the alternator begins charging.

It sounds like your problem is in the exciter wire. The alternator will not charge until it get's that 12 volts from the exciter. Since the voltage drops when it's connected to the alternator, and you've replaced the VR, I'd venture to say that your alt. has an internal fault.

[LowLight]

Thanks. I'll have to try the jumper tomorrow after work. The PO said he had replaced the alt twice trying to fix this issue but I am beginning to believe this guy less and less each time I look at the car.

[nickgriffin1]

Start by charging the battery.

There are two connections to the alternator.

The wire that provides initial voltage to the alt connects to the D+ and is the smaller blue wire. (Actually there are 2 wires here. One to the ignition and one to the diagnostic connector. ) This should show voltage when the ignition is switched to on but not yet started. The cluster light will be on now too. You can read this out at the diagnostic connector at pin 12.

The other heavier red wire runs to the starter on the same terminal as the main cable to the starter. This is the wire that actually does the charging. (There's also a smaller red wire that goes to the diagnostic connector pin 14.) Start the car and check for voltage at pin 14. This should read around 13.5 volts and the cluster light should be out. If it's reading significantly lower, then the car isn't charging and you'll see the cluster light on. If it is reading right, you'll want to check all the alternator and engine ground connections and make sure they're clean and tight. Check the battery cable connection at both cable ends.

he said to check pin 14, you then said you checked pin 12. either he is wrong, and you are right. or he is right and you are wrong.

[LowLight]

Pin 12 (exciter circuit)with the ignition on, engine off. Pin 14 (alt. output) while the engine is running. At least I think that's how it goes.

[thejlevie]

Is the replacement alternator a factory (Bosch or Valeo) rebuilt, or one of the cheap generic remans? If the latter I'd first suspect the alternator and replace it with the right part.

[LowLight]

I'm pretty sure the VR that came out of it was Bosch but I'm not sure about the alt itself. The case is pretty boogered up and there are no legible stickers.

I'll see if I can get my hands on a right one for sure.

Thank you everyone who has helped me thus far. I probably would have pulled all my hair out by now if I were alone on this.

[heifetz17]

The case is pretty boogered up and there are no legible stickers.

If this is the case, I suspect the PO lied about replacing the alternator...

[richardodn]

Pin 12 (exciter circuit)with the ignition on, engine off. Pin 14 (alt. output) while the engine is running. At least I think that's how it goes.

This is correct and as described. Of course you can always take the same readings at the alternator itself. In fact, that would be my next step if I got bad readings at the port. The diagnostic port just make things a bit more convenient.