W Terminal low voltage reading

[overdrivegear]

Hi Everyone, I'm attempting to use the Dakota Digital DSL-1 signal converter for my 87 Cabriolet diesel swap (out of an 86 Jetta).  I'm running into a problem with the voltage off the W Terminal.  The signal converter is meant to take in an AC voltage of 1-20VAC and my W-Terminal is only putting out 0.2-0.4 VAC @ ~200-250 Hz (idle).  Obviously the frequency is proportional to the engine speed but the voltage shouldn't be.  I was expecting to see somewhere around 10VAC.  My alternator works great so I don't see what the issue is.  I've checked and cleaned connections to the spade off the back of the alternator and that's not the issue.

I don't know if it is related but I modified this diesel alternator (http://forums.vwvortex.com/showthread.php?4308948-Re-volting-idea-for-regulators) and I'm getting 14.80 VDC at the battery.  It's great.  I cannot imagine that this is related to the low voltage at the W-terminal.  Hopefully not because I like having my voltage indicator pegged almost all the way to the right.

Can anyone help explain what is going on here?

[92EcoDiesel Jetta]

The W terminal output is taken before rectification by the diodes. I get different readings on the w terminal depending on the volt meter I use. Try a different meter.

[bajacalal]

Voltage varies with engine speed... hence the need for a voltage regulator.

And, what I think is happening, is that your alternator is producing a "square wave." Your meter is expecting something more like a sine wave and doesn't know what to make of a square wave. I think car alternators produced something like a square wave to prevent voltage "ripple" after rectification, or else you would have a DC voltage that varies according to the frequency of the AC input... this is what you get with household appliances that use AC/DC adapters.

[92EcoDiesel Jetta]

Yes, voltage should increase with increasing RPM. Dumb question: Did you have your meter set on the AC scale? Check house voltage to make sure it is working.

[bajacalal]

Another dumb question... did you ground your meter?

[overdrivegear]

Another dumb question... did you ground your meter?

The voltage only marginally changes with RPM and the frequency is the thing that changes with RPM.

I know the signal is a square wave and I do have it set on AC voltage and toggle over to check frequency.  Yes my multi-meter is grounded and it's a rather expensive Fluke. 

[overdrivegear]

I just saw this thread:

http://www.vwdiesel.net/forum/index.php?topic=31316.0

Says voltage does not change with RPM.  Vincent was getting 8 VAC. 

[bajacalal]

Another dumb question... did you ground your meter?

The voltage only marginally changes with RPM and the frequency is the thing that changes with RPM.

I know the signal is a square wave and I do have it set on AC voltage and toggle over to check frequency.  Yes my multi-meter is grounded and it's a rather expensive Fluke.  

I'm going to have to take a look at this with my meter then, this has got my curiosity going.

I just saw this thread:

http://www.vwdiesel.net/forum/index.php?topic=31316.0

Says voltage does not change with RPM.  Vincent was getting 8 VAC.  

That kind of makes sense with these alternators now that I think about it, the voltage regulator works on the un-rectified a/c voltage I would think. 8 volts sounds about right, considering it's a 3 pole system, right, so √3 x 8 = 13.86 volts which is where it should be... unless my understanding of this stuff is off.  

[RabbitJockey]

Voltage varies with engine speed... hence the need for a voltage regulator.

And, what I think is happening, is that your alternator is producing a "square wave." Your meter is expecting something more like a sine wave and doesn't know what to make of a square wave. I think car alternators produced something like a square wave to prevent voltage "ripple" after rectification, or else you would have a DC voltage that varies according to the frequency of the AC input... this is what you get with household appliances that use AC/DC adapters.

They use capacitors to filter out the noise,  not sure what type of signal the w terminal should have but square or sine would read the same because it just reads the low to the peak difference.  I'm thinking maybe the alternator isn't excited or u have 1 dead leg but I would think your alternator wouldn't charge properly then

[bajacalal]

OK, I'm reading 7.8-7.9 volts and 200-800 hz (going from 800 to about 2500 rpm).

Fluke meter Model 175. Incidentally, my Fluke will not zero at a/c open circuit but will read like 0.02-0.05 at ranges under 6v and 0.2-0.5 at ranges above... This behavior, according to my Fluke manual, is normal.

I think what you have here is an open circuit.

edit: I should mention that I read nothing until the alternator excites...

[overdrivegear]

OK, I'm reading 7.8-7.9 volts and 200-800 hz (going from 800 to about 2500 rpm).

Fluke meter Model 175. Incidentally, my Fluke will not zero at a/c open circuit but will read like 0.02-0.05 at ranges under 6v and 0.2-0.5 at ranges above... This behavior, according to my Fluke manual, is normal.

I think what you have here is an open circuit.

edit: I should mention that I read nothing until the alternator excites...

My Fluke behaves the same also (not zero at open circuit).  But I don't think I have an open circuit when connected to the W terminal because I'm still reading 'a' voltage (just not the right amount) and the frequency changes as it should with RPM.  I'm going to tear down this alternator tomorrow in search of a dirty connection internally.

But yeah, my alternator charges is pumping out 14.80 V at the battery with no load so it's charging my system very very well.  I really hope it's just a bad connection because I can't stomach replacing a whole alternator over a faulty W terminal...

[92EcoDiesel Jetta]

If you're able to read frequency how can it be a bad connection? Something weird is going on.

[overdrivegear]

If you're able to read frequency how can it be a bad connection? Something weird is going on.

There could be a bad internal connection where there is potentially a significant voltage drop but what little signal that does exist can have it's frequency read.

[92EcoDiesel Jetta]

If you're able to read frequency how can it be a bad connection? Something weird is going on.

There could be a bad internal connection where there is potentially a significant voltage drop but what little signal that does exist can have it's frequency read.

 If that's the case no need to take the alternator apart yet. Measure the resistance of the W terminal (diodes) to the D+, B+ and ground terminas and compare to a good diode from Radio shack (1N4001 etc). Should be high resistance (meg ohms) one polarity and around 3 k ohms the other polarity.

[overdrivegear]

 If that's the case no need to take the alternator apart yet. Measure the resistance of the W terminal (diodes) to the D+, B+ and ground terminas and compare to a good diode from Radio shack (1N4001 etc). Should be high resistance (meg ohms) one polarity and around 3 k ohms the other polarity.

Here is what I'm working with:





I have taken apart the alternator and cleaned up everything, re-tested and still have the same result. 

Unless I'm missing something, I don't have a D+ terminal

Here it is with the back off:





Then my modified voltage regulator to get me an extra half volt: