Alternator Exciter Wire

[Jackle30]

Gentlemen,

Has anyone dealt with an issue where battery voltage is present at the exciter wire when it is disconnected from the alternator, and then has the voltage drop to approx. 1.5VDC when you connect the alternator wire to the alternator?

Any input would be appreciated.

[Gregs///M]

You should check out the ETM manual, you can download by google searching for it.

That wire (blue) is the alternator charge wire and is connected to switched ignition. If you see a voltage drop of that much, you are either hooking it up to the wrong terminal on the alternator or your alternator is bad.

The wire, same color, is also connected to the diagnostic port.

[Jackle30]

Greg,

Thanks for the info. I have the ETM for my 88 already, thanks to this forum. I checked the voltage with the Exciter wire disconnected from the D+ terminal on the alternator.

With it disconnected it shows a strong 12.6V at both the wire termination and Pin 12 on the diagnostic port with the key in turn. The voltage drops to approx. 1.5 when it comes back into contact with the D+ terminal.

I was assuming that was indicative of a internal problem with the alternator, but just checking here incase it's one of those, "Hey what about this (insert obscure detail forgotten by most)" situations.

Guess not. Yay, time for a new alternator...

[OnMy2ndE30]

I don't know if this matters or not, but I had a somewhat similar situation with my car.

My 87 325i wouldn't start. Turn the key and had power, but no start. Like a moron, I bridged across the starter solenoid and screwed up the charging system. I just didn't realize it yet. I had no clue I was running on the battery until it acted like the alternator was bad. I replaced the alternator and still no dice. I used to have to "excite" the alternator with a test probe on the small post on the back of the alternator to get it to charge the battery. I had no clue at the time that the freaking battery light on the cluster is what excites the alternator into charging state. It actually completes the circuit from the ignition switch through the cluster, through the bulb, out to the alternator. It actually wasn't my bulb, but a fried circuit on the cluster from when I bridged across the starter solenoid. I swapped in another cluster and all was good in the hood.

Like I said, I don't know if that tid bit of info helps you, but there is my experience with the alternator exciter wire...

[jlevie]

I've never measures the voltage at the exciter terminal, but what you describe sounds about right. When the alternator isn't turning the exciter coil loads the exciter wire, which is powered through the alternator warning light bulb. Causing the bulb to light. When the alternator is working the generated back emf stops the flow of current through bulb and the light goes out.

[Jackle30]

OnMy2ndE30 and jlevie, thanks for your input. After my last update I had my alternator rebuilt at a local shop, right around the corner from the equipment shop I worked at. They load tested it for me and did a full rebuild, although they noted that the bearings were all that were shot. Much to my dismay the shiny rebuild didn't do the trick.

As I understood it, the warning light bulb issue was a problem with earlier E30s, but I've got a few clusters around so I can give that a try. I'll be back to let you know if I figured it out or not. Thanks again guys.

[spdracrm3]

what you describe in your first post sounds normal ,there will be 12v with it disconnected as that is the voltage potential coming thru the cluster bulb (or by-pass resistor,allows alt to charge if bulb is burned out) it will be a full 12V because no work is being done , once you connect it to the Alt work is being done and current is flowing generating the magnetic field in the windings and also lighting the bulb in the cluster if its good ,so there will be a voltage drop depending where you are testing the in circuit , @ the "D+" you are measuring with the circuit complete is now the ground side thru the windings so voltage will be low.

that said it doesnt mean there cant be a problem there are three connections c101 c206 and the cluster between the IGN switch and ALT all all being a potential for issue .if your having charging issues you can run a jumper wire/test light from the main starter lug (batt voltage) to the D+ and see if it starts charging (test light will go out once it starts charging)if so then the charge circuit definately has an issue , if not problem could still be in the alternator (internal regulator?) also make sure your ground wire to the ALT body is good ,very common for that to be left disconnected or missing .

[Jackle30]

Spdracrm3, thanks for the additional info. I had it running and was getting the alternator to charge, however, the battery light was staying on and the alternator washing charging at all until after a few minutes of some very crappy idling.

It sputtered as if it wanted to die and then revved right back up to normal idle RPM and when I checked the alternator output it was at a strong 14.1. Turned the car off, took it off the jackstands, went to start it to put it in it's parking spot and it wouldn't start. Cranks like a champ but wouldn't fire. Yay for tomorrow's project...

[spdracrm3]

Spdracrm3, thanks for the additional info. I had it running and was getting the alternator to charge, however, the battery light was staying on and the alternator washing charging at all until after a few minutes of some very crappy idling.

It sputtered as if it wanted to die and then revved right back up to normal idle RPM and when I checked the alternator output it was at a strong 14.1. Turned the car off, took it off the jackstands, went to start it to put it in it's parking spot and it wouldn't start. Cranks like a champ but wouldn't fire. Yay for tomorrow's project...

you know if the idle is low after start-up it wont charge(a very common question on this site is why the charge light stays on at start-up until engine is reved) ,you have to rev engine over about 1000rpm to get the Alt spinning fast enough to create enough voltage to self excite the windings at which point the light will turn off with battery voltage applied to both sides of bulb and it will stay charging ,sounds like everything your seeing is normal and the ALT is charging ok if you've got 14.1v once reved up. just need to solve the running/idle issue now