now that the above is settled, I have a related query:

when you swap a newer motor (say from a Carrera) into an early, longhood car, what conversion do you have to do?

the later motors have internal regulators

do you need to add/remove any resistors?

Disassembled the whole thing yesterday, and one of the 2 brushes is very worn - less than half of what the other one is. I took it in to an auto electrics place today and the guy said 5 bucks to replace the brushes. Would've done it on the spot if I was in a hurry, but at that price I said no hurry, I swing by later. He said no guarantee it's the brushes - they're not caked on the end, so they're still making contact, but very well could be them. For $5, it can't hurt to try.

have him test/lube/replace the bearings

and test the diodes

A little hijack but I hope it is OK Christien.

I was out driving on Saturday, but after an hour or so the car stalled and died with a depleted battery. Been on a charger since then and today I started the troubleshooting.

The battery was at 12.6 and I had a static drain around 30mA with the ignition off so all that seems OK to me.

As the battery light had not been on I started by pulling the oil gauge out and found that the blue wire from the bulb was disconnected. Reconnected that, ignition on and the light came to life.

When I started the engine the light got a bit dimmer and measuring the voltage at the battery at idle showed 13.0V. (Never measured at higher revs because I was alone, but revving did not change the glow of the battery light).

As my belt seems a bit loose I will change it, but is it the likely culprit?

Bonus questions:
1) I'm not questioning that it is correct, but how can the alternator be affected by a loose belt? If the fan turns so does the alternator, right? To me it feels more like an on/off affair. Again, I'm a 911 newbie so if someone could explain the theory I'd be very grateful.

2) I wanted to check all the wires and earth straps but I could not really find them. The only ones I found was at the VR. Can B+ and the rest be inspected without removing the alternator? And, where is the earth strap for the engine? The picture in the Bentley manual is a bit to close to really show where it is. Is it possible to see it without the car on a lift?

Cheers, Björn

Björn,

Remember that the fan blade itself draws about 3hp to cool the engine, and a 1260 watt (90 amp) alternator will require 1260 watts (about 1.7 hp) to produce current when the field is energized. That means that the fan assembly resists turning at high rpm with around 4.5hp.

Imagine trying to stop a 4.5 hp lawnmower engine or electric motor and you get a sense of the force pushing back on the belt. The belt tension is usually sufficient, but if not properly tensioned (with six shims present) the belt can easily slip, all at a time that the engine's ignition system is demanding quite a few amps (about 1 amp per 1000 rpm for an MSD box, for example).

The alternator connections are on the back of the alternator which must be removed. Be CERTAIN to remove the battery negative before attempting any work back there.

Other connections are on the 14-pin connector, pin #11 if memory serves

Wow, I did not suspect that the fan draw as much as 3 hp. What I still have a hard time getting my head around is that slipping is not something you can see by looking or listening and that the alternator is so sensitive to slipping that even if the fan is spinning the alternator is not producing enough juice.

Thank you very much for taking time to elaborate on this, very interesting!

While I wait for the pulley spanner I ordered to arrive I'll go over my 14-pin connector, it looks a bit beaten up so could probably benefit from some tlc.

Cheers, Björn

To get an idea of how much slip there is you could use an adjustable timing (strobe) light, mark the fan pulley and the belt and watch the marks move. Note that there would be slip at both pulleys, but more on the smaller of the two.

I suspect that there would be some slip even with a properly tensioned belt.

Tim K

Hey, that's a really good idea. Will try it next time I'm in the garage!

Cheers, Björn

Installed the brushes tonight, no difference. Time to rebuild the alternator? Or should/can I test the VR first? If so, how? Couldn't find anything on a search.

Christien,

Before you rebuild it, try the "full field test."

* * * WARNING * * * ACHTUNG * * * ATTENZIONE * * * PELIGRO !
The full field test involves running a jumper wire from the +12v terminal of the battery directly to the DF connection on the alternator. This will cause it to go to MAXIMUM output immediately. The battery MUST be connected, don't try this on the bench. Only operate like this for a couple seconds, long enough to verify the alternator output at the battery.

If it's the VR, the full-field test will show up something like +15V.

Don't do it if you aren't comfortable, you can easily blow the alternator up! DISCONNECT your nav system, your norelco shaver, your cd player, your electric cat on the rear parcel shelf with eyes that light up with the brake lights, etc. High voltage will zap them!

Any more detail on that procedure? What's the DF connection? Stupid question perhaps, but I'm assuming this is with the engine NOT running, right? (don't know how I'd access the alternator with the engine running)

Thanks!

Oh yeah, and if it's NOT the voltage regulator, what kind of #s will I see?

Christien, there are dozens of threads on it if you want more, but here's basically how the charging system works.

The alternator consists of two basic parts: the Rotor, which is the part that "rotates", connected to the car via the brushes, and the Stator, which is the part that remains "static," which is connected to two sets of output diodes. One set of output diodes goes to the B+ terminal-- it's function is to "rectify" the alternating current induced in the stator into an overlapping series of direct current pulses that charges the battery. The other set of output diodes (called the "trio" even though in motorola alternator's its a single diode) takes some of the alternator output and routes it to D+/61 (the "blue wire). This is used both to illuminate the warning lamp and also as a reference for the voltage regulator-- the voltage regulator compares the voltage coming off D+/61 to the "set-point" voltage and varies the DF voltage (DF is "dynamo field") that is flowing to the brushes and the rotor to achieve the set-point value.

That's a long sentence-- but basically:

key on engine not running, current flows out of battery, through warning lamp, to VR. VR senses that voltage is around 11v after the voltage drop from the bulb and wires, that is below the "set-point" of 14v. So the VR sends that 11v through the rotor to ground. The rotor is now acting like an electromagnet, inducing a flux in the stator.

engine starts. now the rotor begins moving, and the magnetic flux comes and goes through each of the three parts of the stator. To the stator, the passing by of the electromagnet looks like a sine wave of flux, which in turn creates a sine wave of current in the stator coils. This current goes through the output diodes and the trio diodes as a stream of direct current.

a coupel seconds later, the voltage coming from the trio is greater than +14v, so the regulator shuts off the DF current and the voltage begins to fall. When it falls below the "set-point" the DF current switches back on. This happens many, many times per second (the old electromechanical voltage regulator looks like a big relay). The average voltage from the DF turning on and off is equal to the set point voltage.

Of course all this fluctuation has to be dampened out-- that's the battery's job.

When you full field you are basically jumping from D+/61 in the VR cannon plug to DF. That takes the VR out of the loop.

If it's not the VR you will see the same charging voltage.

Thank you for that explanation.

This is OT, but if you don't mind:

What in the alternator gets damaged when the battery is disconnected with the engine running?

Tim K

The output diodes. The battery acts like a brake on the current. When you take the battery out of the circuit, all the electrical "pressure" that was previously held back by the battery is released through the diodes and they fry.

Look at it another way: take the old SWB 911, it has 12 fuses. Only 10 are operating at the same time, and they are 8 amps apiece. So that's 80 amps if you are running all the current consumers. Suppose you have a 300 amp-hour battery, it has enough current capacity to supply that 80 amp load for 3 hours and 45 minutes before it's at 100% discharge.

So the battery has enough current to supply all the loads. Now you take the battery out of the circuit, and the wheezy little 35 amp alternator has to supply 80 amps of load! The first thing it does is go to MAXIMUM output, which heats up all the wires, diode plate, bearings, everything. The longer this goes on, the hotter the diodes get until eventually they reach their thermal limit and fail.

This is why you should not jump start your 911 except in a life or death emergency. Your alternator was designed to bring the battery back to 100% charge after a start and replenish the ordinary current losses of operating the car's systems. Call it a 5% drop in capacity. If you run the battery down to 25% of its capacity, at 12.2v, the poor alternator has to try to bring that all the way back to 12.75 volts. This is the job of a battery charger in your garage, not the alternator-- it will soon overheat and die.

"But I jump-start my Chevy all the time and the alternator is OK!"

Your Chevy does not have the alternator as the main shaft of the engine cooling fan located three inches from the engine case and hot, AIR-cooled cylinders!

Thank you,

Tim K

So what's the consensus regarding driving the car as it is right now? I have a short trip to make tomorrow - 1.5 hrs highway driving, return trip and won't have time to pull the alternator again and get it fixed before then. I certainly don't want to get stranded.

Just wanted to describe what I have figured out with the problems I have. I measured the input to the VR through the blue wire (D+). With the engine running it was only about 10.3 volts which seems to indicate a broken diode in the alternator (could be bad wiring too).

Made a temporary fix by shorting the warning light bulb to get the D+ voltage up and thus minimizing the risk of ruining the battery by overcharge. Will report back when I have the alternator out but I have decided to postpone that until after my upcoming registration inspection.

A little bit longer description of what I did is in my blog - Owning and driving a 911 SC

Cheers, Björn

Ok, so I took it out for a test drive today. At first, the dash light was on full bright at idle, then decreased to 1/4 bright under load. After about 5 minutes of driving, the light stayed on full bright at any rpm. My first thought was broken fan belt, so I pulled over and checked it, it's fine. And it's turning the alternator/fan no problem. No visible slipping - certainly, it's turning at what looks like the appropriate rpm and increasing with engine speed increase, proportionately, as it should. So the belt's fine. On my way home, I notice that the bulb will go dim around 1000-1100 - sometimes. I couldn't find a consistent spot where it would dim. At one stop light, idling it was just glowing at 1/4 brightness, the next one it was full bright. And accelerating/decelerating I did seem to find a spot just over 1000 rpm where it the brightness would diminish, but again, not consistently. Certainly at 3000-5000 it's on full bright.

New brushes last week, fan belt retensioned last night and it's where it should be. Just pull it and have it rebuilt? Test the VR first?

As a last ditch effort ...

Are you sure the alternator is spinning? Positive that the woodruff key is properly aligned and seated?