Alternator Excitor

[Kevin Bird]

I have an '87 UK spec M535i. The alternator exciter only works (battery/charge light goes off) when the engine is cold if revved to over 1,200 rpm. With the exciter wire connected to the alternator and the ignition on, the voltage at the alternator is 1.6V. If the car is started and the engine is not revved, the battery/charging light stays on, and the voltage is still 1.6 V at the alternator. If you then rev it over 1,200 rpm, the battery/charging light goes out, and the voltage is then 14.3 V. With the ignition on, the car not running, but the exciter wire is disconnected from the alternator, it shows 12.1V, the battery voltage. With it disconnected and the engine running, revving the engine will not excite the alternator, so there is no charge. Reconnect and rev; all good. I have fitted a new alternator and tried a new voltage regulator, but still the same. I have also run direct earth the alternator, but still the same.

When the car has been driven for 5 mins and stopped and restarted, the exciter works immediately.

Why??

[vinceg101]

Perhaps it is one of those UK/USA English things, but can you define what an 'Exciter Wire' is in this system?
Can you show point to it in a RealOEM parts diagram or post a picture?

Otherwise, I'm taking pot-shots here:
Have you checked your main battery wire connections?
Does your new (or the previous) alternator have a separate ground strap bolted to the back of it? Or is this one of the newer versions that ground through the bracket? Are those connections clean and solid?
Are you experiencing a dead battery (battery not re-charging when running)?
How old and/or do you know the condition of your coil?
Have you replaced the Capacitor on the back of the Alternator? (these are NLA so you will have to source an aftermarket one or known-good one)
How old and tight is the Alternator belt?

It sure sounds like it's a bad wire connection (either positive or ground) somewhere.

[Kevin Bird]

Perhaps it is one of those UK/USA English things, but can you define what an 'Exciter Wire' is in this system?
Can you show point to it in a RealOEM parts diagram or post a picture?

Otherwise, I'm taking pot-shots here:
Have you checked your main battery wire connections?
Does your new (or the previous) alternator have a separate ground strap bolted to the back of it? Or is this one of the newer versions that ground through the bracket? Are those connections clean and solid?
Are you experiencing a dead battery (battery not re-charging when running)?
How old and/or do you know the condition of your coil?
Have you replaced the Capacitor on the back of the Alternator? (these are NLA so you will have to source an aftermarket one or known-good one)
How old and tight is the Alternator belt?

It sure sounds like it's a bad wire connection (either positive or ground) somewhere.

Hi Vince, The alternator has two connections: the main wire from the battery and the exciter wire from the dash.

All connections are good. I have fitted a new battery, alternator, alternator voltage regulator, and battery earth lead. The battery shows 12.4V. I have also run an earth from the battery direct to the alternator, but it's still the same.

It only needs to run for a minute then it's fine until the next cold start.