BrianG

John. I looked over the schematic for my 86 car and also found that the keyed ignition switch is the issue in your assertion about a persistent "exciter circuit". Not that a failure isn't possible, but it would imply that the "charge warning" light would stay lit, also. Is that the case?

Normy

This is way interesting-

-At Embry Riddle I was taught that an alternator is the same as a generator, only that an alternator 1. produced AC power [and had as such, "diode rectifiers" as described by the infamous and hugely informative but computer-like and sometimes recalcitrant Wally], and 2. That it used a coil winding instead of a permanent magnet.

Old British cars had generators. If they were sitting in traffic too long they died- they produced less power at idle engine speed than the car required, and as a result the battery would drain and when it eventually ran out of power, the car's ignition would fail.

However, an alternator was used on later cars. An alternator got its magnetic field from current passing through wire instead of from a permanent magnet. This adds a dimension- field current. Field current is determined by a component called a voltage regulator, which measures voltage and schedules field current accordingly.

Six Diodes?

-I've had numerous German vehicles with Bosch alternators, and they have all [including my 928] had the same problem: Occasionally they start up and if the RPM has not gone above 1800, they will not "light off" and all your dashboard gauges stay lit.

Only revving the engine [great thing when it is cold~] will alleviate this problem and make the system run right.

-Aircraft generators have permanent magnets and field coils. In flight, if a generator for some reason begins acting up [ie: excessive or low voltage, or incorrect frequency], the GCU will trip off the generator and we have to retrieve THE BOOK and do what it says. This generally involves opening a covered switch and disconnecting the unit itself. There is a way to separate the rotating portion of an aircraft's generator from its associated engine from the cockpit.

[If you do a generator disconnect you better be thinking ahead; You cannot reconnect it- the mechanics have to physically grab the components and and reconnect it for you. On a "whale" [747] this is not an issue. If it is on an Airbus or a "Guppie" [Boeing 737], since these are twins, and as such only have two engines, then it can be...]

Again, my understanding was that a permanent magnet initially excites the alternator. Eventually the voltage regulator comes online and adds or deletes field current so as to maintain system voltage at 13.7 volts. The situation where my dashboard lights stay on upon startup is a result of insufficient primary voltage [magnet voltage], a common problem with Herr Bosch's Alternators.

Normy!
'85
S2-
n' I do the shiftin'

BrianG

Perhaps this will put this thing into perspective.

<img src="http://www.momentoffame.com/uploads/0e4ad4c81e23abcbcc02bb10f398e50baltfig1[1].jpeg" alt=" - " />

For more see:

<a href="http://www.vtr.org/maintain/alternator-overview.html" target="_blank">http://www.vtr.org/maintain/alternator-overview.html</a>

BrianG

And, Normy, my Comanche has an ordinary old Delco alternator, with a couple of FAA tags on it, of course. But it's nothing near as complex as your turbine generators!!

Erik - Denmark

Brian,
Great diagram and link.
John,
This diagram explain 'my test lamp proposal' and Wally's explanations.
When you turn the ignition key, the current flow from the battery via the ignition key via the alternator warning lamp to the rotor (field current supply) in the diagram
The lamp (bulb) in the 928 is very small - too small to deliver enough current supply (Ampere), therefore Porsche placed the resistor parallel to the lamp to secure enough current to the field supply.
What I do with my test lamp proposal I placed one more parallel supply to help the generator to start up
I were told (I am not an electrician) during repair - or shortcuts) the rotor can loose there natural magnetic force and therefore need help for starting up. I fact that is excellent explained in Brians link as follows

Quote:
ALTERNATOR ROTOR
We'll start our tour of the alternator where it all starts in the alternator itself - at the alternator rotor. The rotor consists of a coil of wire wrapped around an iron core. Current through the wire coil - called "field" current - produces a magnetic field around the core. The strength of the field current determines the strength of the magnetic field. The field current is D/C, or direct current. In other words, the current flows in one direction only, and is supplied to the wire coil by a set of brushes and slip rings. The magnetic field produced has, as any magnet, a north and a south pole. The rotor is driven by the alternator pulley, rotating as the engine runs, hence the name "rotor."
quote over

When I, back in 1995, bought my first 928, there were some problem with the power supply from the alternator.

The seller (A VW dealer) has given me some guaranty and I 'was new', so the car was sent to a Bosch garage - I shoves-up the alternator was ok in his test bank but not in the car.
There were two problem and he solved them as follows:
1. Need high RPM before start charging - He placed a small bulb (2 W) at the el-board connected to terminals O6 - O8 (!5 and D+)
2. The charging voltage was too low - He installed a diode between the generator and the regulator - This to increase the charging voltage - (I was told it would increase with 2,4 Volt)

Now all this is history for me, but it was working well for several years, until I learnt how to bring the car back to the original, step by step

OK, he worked on a guaranty matter so he want to do it cheap, I was 'new' and he was an 'old fox' so he found cheap functional solutions.

Greg86andahalf

Hi Wally,

I'm glad your'e explaining the resistor circuit in more detail. When I changed my resistor, I saw an improvement in the volt meter. Almost one volt higher reading when running the engine. I thought the new resistor was the cause of this improved volt reading.

Now I must assume that somewhere during R/R of the pod, I must have improved a connection or something to create the one volt improvement on the volt reading. I have a new instrument PCB ready to install. I hope that ann good solid connections at the PCB will nail down the gauges.

Greg