Alternator question

[ischmitz]

Early_S_Man, what is the red heat sink on my alternator. I am guessing it carries the B+ diode trio, right?
It seems there is no way around to taking the culprit out of the car. Well so it be. I am with you in that the guy at the shop was confused.

autobonrun, do you think the spike on my D+ scope trace is normal? I thougt that looked like one dead D+ diode. I would be dying to see a scope trace on another car.

[Early_S_Man]

Ingo,

I am not sure which heat sink you are referring to ... rebuilders often spray or dip everything inside the case with red Glyptal compound. I think the heat sinks were bare aluminum from the factory.

Here are a couple of pages that may help:

[ischmitz]

Early_S_Man, the alternator pictured in your scan looks awfully similar to what I currently have. When I had mine out some 6 month ago I noticed the heat-sink for B+ in red. It was bolted to the back with two nuts.

Is that the originally equiped alternator for a 74 or has someone replaced this with an earlier version?

[Early_S_Man]

I don't remember if the 770 Watt Marchals had the isolation diode or not ... it has been years since I had mine out to replace the brushes!

The power output was upgraded in 1975 to 840/980 Watts, and those don't have the isolation diodes.

[Sean Hamilton]

Originally posted by Early_S_Man
Quote from Warren's pages............"It is therefore always important that the generator control lamp is not burned out or of improper wattage"...........

Well, learn something new "critical" everyday.
Is it the dash lamp referred to here?

[Jim Sims]

Yes, it's the red "alternator" light usually located in the lower center of the oil temp/oil pressure gage. It should glow red with the ignition on but with the engine not running. Cheers, Jim

[Sean Hamilton]

I've just been told by my Auto electrician, when replacing the alternator, after first reconnecting the wire to the back of the alternator, always connect the POSITIVE lead to the battery first.
Okay, Does that suggest that "both" Pos' and Neg' leads must be removed prior to disconnecting the wires from the back of the alternator - NOT just the Positive lead?

[jk911]

I recently had my alternator rebiult with new diodes.

I have an wierd different make external regulator.

Problem is that when the car is idleing below 2000 rpms the voltage is 12.4 to 12.6. But above 2500 rpms its 13.4 to 14.0.

Can a bad regulator cause this problem ???

The wiring on my alternator is like this (note I have a similar motorola but mine is a 1975 model 911 2.7L)

[autobonrun]

The charging voltage is a function of the field excitation which is subsequently a function of the need of the battery. The VR is designed to allow 13.5 to 14.5 volts to be applied to the battery if needed during normal charging. The key function for the VR is not to allow the alternator to overcharge the battery. The faster the alternator spins (eg. higher rpm) the greater the capability for the output of the alternator (B+) to rise up to the limit of the VR. This is why when you apply a full 12v to the DF terminal of the alternator with the VR out of the circuit, you'll see the voltage at the battery go up to the maximum output of the alternator; somewhere around 16 volts.

Here are excerpts from a couple articles.

1)
REGULATOR

The regulator has two inputs and one output. The inputs are the field current supply and the control voltage input, and the output is the field current to the rotor. The regulator uses the control voltage input to control the amount of field current input that is allow to pass through to the rotor winding. If the battery voltage drops, the regulator senses this, by means of the connection to the battery, and allows more of the field current input to reach the rotor, which increases the magnetic field strength, which ultimately increases the voltage output of the alternator. Conversely, if the battery voltage goes up, less field current goes through the rotor windings, and the output voltage is reduced.

2)

With the engine running at high idle (1500 rpm), measure the voltage at the battery terminals using a volt/ohm meter (VOM); digital is more accurate, but for this test an analog needle will work fine. On high idle, the voltage at the terminals should be between 13.6 and 14.4 volts. Outside that range indicates bad or dirty connections or defective alternator.


PS.
By "outside that range", I'm sure they mean under. Substantially over that amount would more than likely mean a bad VR and could result in overcharging the battery.

[jk911]

As for my wiring on the alternator should the D- (brown) connection be also connected to the earth ?

So I guess a faulty regulator can cause low charge at low rpm's! or am I wrong ?

[304065]

An "Old School" alternator should have four connections:

B+ (two red wires, this is the battery positive voltage)
D- (ground, this is a brown wire that goes to the 14-pin connector)
DF (Dynamo Field, this is a black wire that goes to the 14-pin and into the voltage regulator)
D+/61 (blue wire, this is the initial excitation/warning circuit)

Most folks also have a braided ground strap between the alternator case and one of the top engine case bolts. This gives you a nice solid ground, preventing the alternator from using the belt as a ground and burning out.

JK, I think the connection between the D- and the ground is probably redundant, if you look at the D- terminal, it is not isolated from the frame of the alternator-- the bolt touches the metal.

Just FYI, I have an 88 Carrera alternator (in an updated fan shroud) in my '71, and when I start it, idle comes initially to 600 rpm, and the charge light glows brightly. After I have oil pressure, I rev the motor to about 1200 RPM, and the light goes out, then the idle drops back down and the light remains out. If you aren't getting the light to go out when you first start the motor, follow Warren's advice, and wire in parallel with your warning light circuit a 68-ohm, 5W wire-wound resistor, which will increase the current flow through your D+/61 circuit, and cause the field to be excited with more juice from the battery initially, which will induce a greater current in the stator windings, which will be rectified by the "trio" and fed back into the field, "Boostrapping the alternator!"

[bigchillcar]

I've had the same symptoms lately in my '74 911 with Marchal alternator. My battery has been extremely weak of late (750 amp battery from wal mart bought new just over 2 years ago), so I replaced with a similar model (still under warranty).

Typical good starts now with the new battery, but still need around 2000 rpm's to excite the diodes and dim the light. However, at idle speeds of ~ 1000 rpm's, I'm noticing the faint glow is still around. This leads me to believe that the reason the battery went south after just 2 years was probably due to alternator or external VR trouble.

I know how to remove the alternator, but before I get into that fun again, is there a way to bench check a VR? I bought it new also 2 years ago...believe it is actually a Bosch part...

Btw, hi to all the fellas...I've been away from the board for probably 6 months...
Ryan

[RoninLB]

Originally posted by bigchillcar
is there a way to bench check a VR?

it's easier for me to just bring it to a shop and ask for the max amp output.

[bigchillcar]

hi ron,
long time no see of me...any bigbigride's in the last 6 mos.? and btw, when you say 'shop', do you mean an alternator shop specifically or does your average gas station/garage shop have what it takes to measure max alternator output?
ryan

[Lorenfb]

"However, at idle speeds of ~ 1000 rpm's, I'm noticing the faint glow is still around."

This is a common problem. It may result from;

1. a voltage drop from the rear to front
2. a bad connection at the alt. B+ (need to solder the crimp connector)
3. a bad plus diode (open) will cause the alt. light to glow
4. a regulator adjusted to too high a voltage
5. a too low of an excitation current from the alt. light (add a 50 ohm 3 watt resistor
across the light)

[bigchillcar]

thanks, loren...i suspect my VR the most.
ryan