Alternator problem

Has anyone had experienced the problem that I'm having with alternators ? I am on my third alternator (and third battery). Apparently my car is running on the battery voltage. I just installed a brand new Bosh alternator that was purchased from a reputable dealer. When I check the output of the alternator, I get a reading of 11.9 V. The battery voltage is 12.6 V. The electrical connections are corrosion free and tight. I run a 10 gauge wire to the starter solenoid (voltage 12.6 V.)and then use the harness wire supplied by Vintage to the alternator(voltage 12.6 V.car not running). After the car has completely warmed up and idles smoothly, I pull the positive battery cable and the engine dies. Before I again take off the alternator and have it tested, I would like to know what if anything I have done wrong.

Richard,
There should be a 1/4" spade lug terminal on the alternator. This is where the voltage supply to the regulator comes from. With the ignition on there should be ~12v available on the wire that goes to that terminal. If there isn't, I suspect thats the source of the problem.

As a check you can take a 12v test light and hook it from a 12v source to the spade lug terminal. With the ignition on and car not running, the light should be on. Start the car and blip the throttle and the light should go out. If it does, you have a problem "up front" I don't have a VS, but i assume there is an alt. light. It may be burned out or the 12v supply to it aand the wire going to the alternator may be disconnected.

The test light thing will confirm you alternator is capable of working.
HTH
Bill

It is absolutely essential that the D+ terminal on the alternator be connected to a functioning "Alt" warning light in the instrument cluster. If this light is missing or defective, the alternator will NOT charge the battery!

The system is very simple, but absolutely critical that you get it right. The D+ terminal on the alternator MUST connect to a functioning warning light in the instrument cluster.

There should be just one wire (blue) from the D+ connector on the alternator to the button on the bottom of the normal indicator light in the dash. The body of the bulb is connected to ground via the light holder (which also provides the ground connection for the other dash lights.)

The alternator must get a feedback current through this lamp so it can sense the battery voltage; it uses that as part of the alternator's internal circuitry needed to charge the battery. In other words, with the ignition on but engine off, the indicator light sees 12v from the battery and glows, but with the engine running, it sees 2v (14v minus 12v) running the other way, from the alternator. It doesn't glow (needing more than 2v to do that), but the alternator still "sees" the connection to the battery.

An LED light won't work for this purpose. LED's are diodes and will not allow current to flow in the opposite direction. With the LED, it would see the 12v, but the reverse flow 2v would be stopped by the diode nature of the LED, so that wouldn't work.

richard, you need more than batt voltage in order to charge. you should read >14 volts
during charge.

Richard:

Jeff is on the right track, but his wiring description is a little wrong.

Yes, it is essential to connect the D+ lead from your alternator to one lead of your alternator dash lamp. But the OTHER lead from that dash lamp MUST go to +12 volts, usually connected to your ignition lead. Again, one lead from the lamp goes to the D+ on the alternator, and the other lead goes directly to +12 volts.

If you ground one side of that dash lamp, the alternator charging circuit will not work!!

The reason for this Jeff stated: The alternator needs a battery reference voltage to know when to charge or not. It gets this reference voltage through the dash lamp to +12 volts in the system. When your battery drops below a specified voltage (somewhere between 11.6 and 12.4 volts) the charging circuit in the alternator starts to work. (There is a detailed description of this somewhere else on the forum. do a search on "alternator"). If it sees no reference voltage at all, chances are you'll not be charging at all, either.

Also, when the alternator is spinning and the engine is running over 2,000 rpm, the alternator output voltage should be somewhere around 13.9 volts (some run a bit higher, but I think Bosch is 13.9) That way there's enough extra voltage there to quickly (but not too quickly) charge the battery.

Gordon
One of the "Speedstah Guys" from Beaufort, SC

Sorry about the little blip in the wiring description. I pulled it from a VW site which obviously focuses on the Beetle. Maybe this illustration will more accurately describe the fiberglass bodied Speedster.
1800vw.bizhosting.com/alternatorwiring.htm

Alan, does it make much difference in amps if the pulley is the smaller 356 type?....

An altenator will produce only "so many amps" no matter how fast it turns depending on the internal windings

Alan:

D+ through a lamp to ground? I can't see how the field winding would ever see a reference voltage.

Where is Brian Hawthornthwaite when we need him?

gn

I can't thank you all enough. I believe that the charge light aspect is the problem. It has never indicated anything. It just did not make sense that three alternators were all bad. Right now, I'm at the office;bur as soon as I get home I will go through the charging light circuit.
Thanks again. What a great forum!

DO NOT disconnect the battery when the engine is running.

As was stated above, 12v switched igniton goes to one pole of the lamp and D+ to the other. When the alternator is at rest(not turning), D+ provides ground, lighting the lamp(with the voltage from the ingition switch). When the alternator spools up, that ground ceases as voltage appears thus cancelling out the lamp. The regulator will then read the voltage from this circuit and charge accordingly.
As George stated don't run the engine with the battery disconnected, it will damage the internal regulator and diodes.

BD

Go to cbperformance.com click on HOW TO GUIDES.... then click on ALTERNATOR WIRING GUIDE.

On the CB how to picture, it shows two wires at the bulb, the positive from the bulb goes to the ignition. I get that. Now the wire from the bulb to the D+. Is that connected to the negative side of the bulb?

There really isn't a "positive" side of the bulb...unless you are using the chassis as a ground. In auto electrics you need the + and - to make the bulb light. On a single wire socket the outter casing of the socket gets grounded by the chassis and the wire supplies the 12 volt power. On a fiberglass Speedster you need two wires, and since the fiberglass does not conduct a ground, you can wire the bulb either way and it won't make a difference.

I believe the bulb is in series...

jeff you dont need two ires- you can just ground all the guages.

Scott:

Absolutely wrong.....the alternator dash bulb must be isolated from chassis "ground". There are two types of bulbs used in VW/Porsche/Bosch dash clusters - one has a bronze shield tube that makes "ground" contact with the gauge can and the bulb has a single tab for electrical connection. The other type has a plastic shield with two lead tabs protruding from it. On this type, one lead from the bulb (doesn't matter which one) goes to the ignition side of the ignition switch (which feeds it with +12 volts when the "key" is "on"). The other tab from the bulb goes to D+ on the alternator (the small, 1/4" tab connection on the top of the alternator, marked "D+").

If you use a regular, single tab, bronze shielded dash bulb holder, the charging circuit will never work right, nor will the bulb ever light correctly.

Follow the circuit diagram that CB performance provides (see post above). They've got it right.

gn

Ahh sweet success. 13.8 Volts at the alternator at idle. Your knowledge and the CB Performance diagram were the keys. It did seem strange to connect a wire fron the accessory terninal to the other spade on the alternator light since both spades are connected to each other. But it did work.
What ticks me off is that the VS wiring diagram does not show the wire from the other spade to the ingintion switch. In fact the VS diagram only shows one spade on the alternator light (connected to the D+ on the alternator). What gives with VS?
Again thanks for your help, there was no way I could have solved the problem without your knowledge.

From the alternator the D+ wire is ran to the dash idiot bulb and from the the bulb connected on the acc. side of the ignition switch...Correct alt output is 13.8 volts

Gordon,

I stand corrected, I don't have any idea as to how my info was off that much.